Estimating the Massflux from

Airborne Doppler Lidar

Measurements

A diploma thesis submitted to the

Institute of Meteorology and Geophysics,

University Innsbruck

for the degree of

Master of Natural Science

presented by

Martin Großhauser

supervised by

Ao. Univ.-Prof. Dr. Georg Mayr

April 2009

Abstract

Alpine pumping is a thermally induced circulation between the Bavarian foreland

and the Alps, which develops on summer days with strong insolation. One goal of the

VERTIKATOR (Vertical exchange and orography) field campaign was to quantify

the mass flux of air towards the Alps during Alpine pumping. An airborne Doppler

lidar system measured the horizontal wind field in six vertical cross-sections during

four intensive observations periods (IOPs) in July 2002, from which mass fluxes

were computed. Estimated mass fluxes, integrated vertically up to the boundary

of the inflow layer, ranged from 880–1200 kgm−1s−1 with depths of the inflow layer

between 1250–2125 m. Mass fluxes in the northern cross-sections were smaller than

in the southern ones. Maximum measured wind components in the boundary layer

towards the Alps were around 5 ms−1. The results are compared to an existing

VERTIKATOR case study from another IOP. The mass fluxes in this work are

about half the magnitude of the ones in the previous study. The lower values are

presumably caused by (1) suboptimal synoptic conditions for the development of

Alpine pumping, and by (2) carrying out the measurements during the premature

stage of the thermal circulation in the course of the day. An analysis of each vertical

column shows increased mass fluxes towards the valleys in the cross-sections which

are located closer to the Alps. In the cross-sections further away from the mountains

no correlation between orography and mass fluxes was found.

iii

Zusammenfassung

Alpines Pumpen ist eine thermische Zirkulation, die sich an sonnigen Sommertagen

zwischen dem Bayrischen Alpenvorland und den Alpen ausbildet. Die quantitative

Bestimmung des Massenflusses zu den Alpen hin war eines der Ziele des Feldexpe-

riments “Vertikaler Austausch und Orographie (VERTIKATOR)”. Ein flugzeugge-

tragenes Doppler-Lidar lieferte sechs Querschnitte des horizontalen Windfeldes an

vier verschiedenen Intensiv-Messtagen im Juli 2002, aus denen die Massenflusse

berechnet wurden. Die abgeschatzten Massenflusse, integriert bis zur Obergrenze

der einstromenden Schicht, betrugen zwischen 880–1200 kgm−1s−1, die Machtigkeit

der einstromenden Schicht reichte von 1250–2125 m. Massenflusse in den nordlichen

Querschnitten waren geringer als in den sudlichen. Maximale gemessene zu den

Alpen gerichtete Windkomponenten betrugen 5 ms−1. Die Ergebnisse wurden mit

einer VERTIKATOR Fallstudie von einem anderen Intensiv-Messtag verglichen. Die

in dieser Arbeit berechneten Massenflusse sind ungefahr halb so groß wie die in der

vorangehenden Studie. Als Ursachen dafur werden (1) ungunstige synoptische Be-

dingungen fur die Ausbildung von Alpinem Pumpen, sowie (2) die fruhe Tageszeit

der Messfluge, als die thermische Zirkulation noch nicht voll ausgebildet war, ver-

mutet. Eine Analyse aller einzelner Lidar-Windprofile zeigt in den sudlicheren Quer-

schnitten verstarkten Massenfluss zu den Talern hin. In den nordlichen Querschnit-

ten ist dieser Zusammenhang zwischen Massenfluss und Orographie nicht sichtbar.

v

Contents

Abstract iii

Zusammenfassung v

Contents vii

List of abbreviations ix

1 Introduction 1

1.1 General Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.2 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 VERTIKATOR project description . . . . . . . . . . . . . . . . . . . 2

1.3 Investigation area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3.1 DLR Falcon flight pattern . . . . . . . . . . . . . . . . . . . . 3

1.4 Time data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.5 Instrumentation and Measurements . . . . . . . . . . . . . . . . . . . 5

1.5.1 Surface in-situ measurements . . . . . . . . . . . . . . . . . . 6

1.5.2 Wind-Temperature Radar . . . . . . . . . . . . . . . . . . . . 6

1.5.3 Radio soundings . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.6 Literature Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.6.1 Thermal evolution of the valley atmosphere . . . . . . . . . . 7

1.6.2 Thermal circulations in mountainous terrain . . . . . . . . . . 7

1.6.3 Flow over and around mountains . . . . . . . . . . . . . . . . 17

2 Lidar Measurements – Data Acquisition and Processing 21

vii

2.1 Doppler lidar basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.2 Determining the Wind . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.2.1 Velocity azimuth display technique (VAD) . . . . . . . . . . . 24

2.2.2 Sine-wave fitting . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.2.3 Data quality checking . . . . . . . . . . . . . . . . . . . . . . . 29

2.3 Characteristics of the Doppler lidar instrument WIND . . . . . . . . 29

2.4 Determining the mass flux . . . . . . . . . . . . . . . . . . . . . . . . 30

2.5 Determining the mass flux error . . . . . . . . . . . . . . . . . . . . . 31

3 Case studies 33

3.1 5 July 2002: Southwesterly flow . . . . . . . . . . . . . . . . . . . . . 35

3.2 8 July 2002: Short wave ridge . . . . . . . . . . . . . . . . . . . . . . 41

3.3 9 July 2002: Southwesterly flow . . . . . . . . . . . . . . . . . . . . . 47

3.4 12 July 2002: Southwesterly flow . . . . . . . . . . . . . . . . . . . . 53

3.5 Mass fluxes and orography . . . . . . . . . . . . . . . . . . . . . . . . 59

3.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.6.1 Dependence on time of flight . . . . . . . . . . . . . . . . . . . 63

3.6.2 Dependence on weather situation . . . . . . . . . . . . . . . . 63

3.6.3 Pressure difference as a measure for Alpine pumping . . . . . 64

4 Summary 67

Bibliography 69

Acknowledgments 73

Danksagung 75

Curriculum Vitae 77

viii

List of abbreviations

AFO 2000 Atmospharenforschung 2000 (Atmospheric research)

AGL Above ground level

ASL Above sea level

ATR Along-track resolution

AVHRR Advanced Very High Resolution Radiometer

AWS Automatic weather station

CET Central European Time (UTC + 1 hour)

CEST Central European Summer Time (UTC + 2 hours)

CNES Centre National de la Recherche Scientifique (National

Center for Scientific Research)

CNRS Centre National d’Etudes Spatial (National Center for

Space Studies)

CTR Cross-track resolution

DLR Deutsches Zentrum fur Luft- und Raumfahrt (German

Aerospace Center)

DWD Deutscher Wetterdienst (German weather service)

IOP Intensive observations period

LASER Light amplification by stimulated emission of radiation

LIDAR Light detection and ranging

LO Local oscillator

MAP Meso-scale Alpine Program

NOP Number of points (used by sinewave-fitting algorithm)

RADAR Radio detection and ranging

RMS Root-mean-square

ix

SNR Signal-to-noise ratio

TAWES Teilautomatisiertes Wettererfassungssystem (semi-

automatic weather acquisition system)

TAF Topographic amplification factor

UTC Temps Universel Coordonne (Coordinated Universal

Time, same as Greenwich Mean Time)

VERA Vienna Enhanced Resolution Analysis

VERTIKATOR Vertikaler Austausch und Orographie (Vertical Ex-

change and Orography

WIND Wind Infrared Doppler Lidar

WTR Wind Temperature Radar

ZAMG Zentralanstalt fur Meteorologie und Geodynamik

x

Chapter 1

Introduction

1.1 General Introduction

1.1.1 Motivation

This thesis discusses mass fluxes towards the Alps, derived from airborne Doppler

lidar measurements during the VERTIKATOR campaign, carried out in 2002. One

goal of VERTIKATOR was to quantify the mass flux from the Bavarian foreland into

the mountains during summer days with strong insolation (Lugauer et al. 2003).

Through differential heating of the atmosphere in the mountains and over the plain,

a hydrostatically induced pressure gradient drives a circulation, which during day

advects boundary layer air from the foreland into the mountains and, under favorable

conditions, into the free atmosphere. Accompanying transport of water vapor into

the free atmosphere can lead to deep convection, and further on to thunderstorms.

During night the circulation reverses, and air from the Alps drains into the foreland.

Due to the oscillating nature of this circulation it is often referred to as Alpine

pumping. Evidence for this circulation was already provided in the first half of

the 20th century by climatological analysis of pilot balloon soundings (Burger and

Ekhart 1937). Lugauer and Winkler (2005) discussed Alpine pumping based on data

from surface observations in the German federal state Bavaria, with respect to the

amount of incoming short-wave radiation and large-scale weather situation.

The vast number of in-situ and remote sensing measurements during the in-

1

tensive observations periods (IOPs) during VERTIKATOR allowed to analyze the

temporal and spatial structure of Alpine pumping more in detail. Vertical cross-

sections of the wind field in the Bavarian foreland on 19 July, derived by VAD

technique (c.f. Chapter 2) from measurements of a 2-µm Doppler lidar onboard the

DLR Falcon research aircraft were already discussed by Weissmann et al. (2005).

The authors’ method to derive mass fluxes from Doppler lidar data will be used as

a working model for this thesis and will be applied to data recorded by the WIND

lidar on four other IOPs on 5, 8, 9 and 12 July 2002. The five cases, including

Weissmann et al.’s case, will be compared to each other, with respect to synoptic

characteristics and differences in the evolution of Alpine pumping.

1.1.2 Outline

Below in this introduction, the VERTIKATOR project and its investigation area,

along with the different measurement platforms used in this thesis, will be intro-

duced, followed by an overview of existing literature on this thesis’ topic, with an

emphasis on mountain-plain circulation. Chapter 2 provides basic understanding

of lidar measurement and data processing, and how the mass fluxes are computed.

Chapter 3 discusses the four case studies, for which data from the WIND lidar on-

board the DLR Falcon research aircraft is available. Chapter 4 will summarize the

results of this thesis.

1.2 VERTIKATOR project description

The VERTIKATOR (VERTIKaler AusTausch und ORographie—Vertical Exchange

and Orography) project took place in southern Germany in 2002 and was funded

by the German Federal Ministry of Education and Research in the framework of

the AFO 2000 (Atmospharenforschung 2000—Atmospheric research) program. The

measurements making up the base for this thesis were part of Sub Project 8: Deep

Convective Processes (Hochreichende Transportprozesse) (AFO2000 2004) which is

also related to Sub Project 6: Alpine Pumping. The goal of Sub Project 8 was

“to investigate the vertical exchange processes and its resulting precipitation events

2

in deep convective systems, which are triggered and influenced by orographically

determined processes”(AFO2000 2004). This should be achieved by collecting data

from a variety of in-situ and remote sensing measurement platforms (see Section

1.5) and by simulating these processes in numerical models. With the data gained,

the handling of convective processes in numerical models should be evaluated and

improved, but it should also lead to a better understanding of deep convection over

complex orography.

1.3 Investigation area

The investigation area roughly encompasses the Bavarian Alpine foreland and the

Eastern Alps, bordered by the river Rhine and Lake Constance in the west, the river

Danube in the north, the river Salzach in the east and the Alpine main ridge in the

south (Fig. 1.1). The terrain slowly rises from 400 m ASL at river Danube up to 700

m ASL at the northern edge of the Bavarian foothills (inclination 1:400). In the west-

east direction the terrain first rises from Lake Constance up to the Allgau and then

slopes slowly downward (inclination 1:500) into the Bavarian Foreland. This west-

east slope causes a secondary thermal circulation (Lugauer and Winkler 2005). The

mountainous part of the investigation area coincides with the geologically defined

Northern Limestone Alps with a crest height about 2500 m ASL in the vicinity

of Inn Valley and reaching over 3000 m ASL at Alpine main ridge. The different

mountain ranges are separated by the main valleys of the rivers Rhine, Iller, Lech,

Loisach and Isar. The Inn valley is special among the other valleys as it is the

deepest (1700–2000 m), longest (130 km) and largest with its major tributaries. It

has also been a large observatory even since the first investigations in mountain

meteorology.

1.3.1 DLR Falcon flight pattern

Fig. 1.1 shows locations of the both flight legs, along which the DLR Falcon research

aircraft measured vertical cross-sections of horizontal wind. The flight legs are 15 km

apart. Data from six vertical cross-sections on four different days (5, 8, 9 and 12

3

Austria

Germany

Bavaria

Switzerland

Italy

Danube

Rhin

e

Lake Constance

Ille

r

Lech

Lois

ach

Isa

r

Inn

Inn

Inn

Wip

p

Alpine

main ridge

Figure 1.1: Investigation area with topography (shaded, units: m ASL), flight legs

(lines), instrument sites (diamonds, c.f. Tab. 1.1) and valley names.

4

July 2002) are available for this thesis.

1.4 Time data

Time in this thesis is mostly given in UTC. Mean local time for the investigation

area is obtained by adding 45 minutes. Central European Time (CET) is obtained

by adding one hour to UTC. Local Standard Time in summer is Central European

Summer Time (CEST), which is UTC plus two hours.

1.5 Instrumentation and Measurements

The investigation area was equipped with a large number of measurement platforms.

In this section, the systems used for this thesis and meaningful for the mountain-

plain circulation will be described (Tab. 1.1). The lidar platform will be discussed

in detail in Chapter 2. The position of the measurement sites can be seen in Fig. 1.1.

Token Name Instrument Coordinates Height

/ m ASL

AUG Augsburg-Muhlhausen AWS 48.43◦ N, 10.94◦ E 461

ELL Ellbogen AWS 47.17◦ N, 11.43◦ E 1080

GAP Garmisch-Partenkirchen AWS 47.48◦ N, 11.10◦ E 719

HP Hohenpeißenberg AWS 47.80◦ N, 11.01◦ E 977

IBK Innsbruck AWS, radio sounding 47.27◦ N, 11.39◦ E 574

LI Lichtenau WTR 47.88◦ N, 11.23◦ E 610

MUC Munich AWS 48.16◦ N, 11.54◦ E 515

OSH Oberschleißheim radio sounding 48.25◦ N, 11.55◦ E 484

VIL Vilgertshofen AWS 47.95◦ N, 11.92◦ E 708

WEN Wendelstein AWS 47.70◦ N, 12.01◦ E 1832

WAN Wank AWS 47.51◦ N, 11.14◦ E 1780

ZUG Zugspitze AWS 47.42◦ N, 10.99◦ E 2960

Table 1.1: List of in-situ measurement sites discussed in this thesis.

5

1.5.1 Surface in-situ measurements

Data were available from automatic weather stations (AWS) of the operational sur-

face station network of the German Weather Service (DWD) and the German Army

(Bundeswehr) – 38 stations in total. Measured parameters were wind direction and

wind speed at every station and additionally pressure, temperature and humidity at

most of them. Tab. 1.1 and Fig. 1.1 list the stations which appear in this thesis.

Additionally, data from stations at Innsbruck airport, operated by ZAMG (Zen-

tralanstalt fur Meteorologie und Geodynamik – Central Institute for Meteorology

and Geodynamics), and Innsbruck University and Ellbogen, both operated by the

Institute of Meteorology and Geophysics Innsbruck were used.

1.5.2 Wind-Temperature Radar

The Wind-Temperature Radar (WTR) of the Institut fur Meteorologie und Kli-

maforschung at Forschungszentrum Karlsruhe is able to measure the profiles of

temperature and wind vector from the combined backscatter of radio and acoustic

waves (Emeis et al. 2004). Vertical resolution is 60 meters and averaging interval

is 30 minutes. The accuracies are given by 0.5 ms−1 for horizontal wind speed,

0.1 ms−1 for vertical wind speed, 3◦ for wind direction and 0.2 K for temperature

(cited from internal project documentation). The WTR was located in Lichtenau,

along the northerly flight leg (Fig. 1.1). Only wind information data measured by

the WTR is discussed in this thesis.

1.5.3 Radio soundings

Radio soundings were processed from Innsbruck and Oberschleissheim near Munich

(Fig. 1.1). Operational soundings were performed every night at Innsbruck Airport

at around 01 UTC and an additional sonde was launched at approximately 11 UTC

during IOPs. Soundings at Oberschleissheim were launched every 3 hours from

06 UTC to 18 UTC and at 00 UTC during the IOPs. Measured parameters were

pressure, wind vector, temperature and moisture.

6

1.6 Literature Survey

In this chapter the synoptic basics for understanding the mountain-plain circula-

tion will be given. First, thermal circulations will be explained, which act at the

mesoscale up to about 100 km. In the next subsection, an overview of the inter-

action of the mountain-plain circulation with the large-scale synoptic flow will be

given. Emphasis will be put on the situation at daytime in summer, as this is the

time when the thermal circulation is best developed, and when the measurement

campaign was carried out.

1.6.1 Thermal evolution of the valley atmosphere

In the morning, the valley atmosphere is generally stably stratified, with a mid-valley

inversion produced by catabatic winds, which are the nocturnal component of the

along-slope wind system. Observations from the Alps show, that this inversion is not

necessarily destroyed in the course of the day (Rotach and Zardi 2007). Whiteman

(1982) describes the processes involved in potentially destroying the inversion as

convective mixing from the ground, and sinking of the mid-valley inversion through

up-slope transport of warm air from the ground into the upper cell (Fig. 1.2), or a

combination of both. By performing numerical simulations with an idealized valley

topography, Rampanelli et al. (2004) found that warming by convective mixing

from the valley ground plays only a minor role, instead the downward transport of

warm, stably stratified air in the valley center warms the whole valley atmosphere.

Additionally, as also observed during the VERTIKATOR campaign, vertical mixing

can be promoted by Fohn. In the VERTIKATOR case studies, both scenarios—

vertical mixing and preservation of stable stratification—were observed and will be

discussed in Chapter 3.

1.6.2 Thermal circulations in mountainous terrain

The mountain-plain circulation is at the large scale end of a whole “spectrum” of

thermal circulations with different time and length scales. It is driven by a pressure

gradient, which is produced by differential heating of the mountain atmosphere in

7

Figure 1.2: Cross-section of cross-valley circulation with two circulations cells due

to an inversion in the valley atmosphere (Vergeiner 1982).

contrast to the foreland atmosphere. Slope-winds are on the small scale end, where

the absorbed part of the incoming solar radiation is converted into sensible and

latent heat. This heat is communicated first to the slope, then to the valley volume,

and next to the whole mountain atmosphere.

Up-slope winds

Up-slope winds in steep terrain are at the smallest scale. Solar energy is transformed

into heat at the slope surface, which is communicated to the valley atmosphere by

up-slope winds. Up-slope winds are produced by heating of an air parcel near a

slope surface, so its temperature is higher compared to an air-parcel further away

from the hill at the same level. This temperature excess drives buoyancy forces,

advecting the parcel up-slope, but since the parcel is still heated on its way up, the

parcel will never reach equilibrium with the adjacent air, if slope length is infinite.

This phenomenon was described by Prandtl (1942) in his classic one-dimensional

steady-state slope wind model in a stably stratified atmosphere. Since then this

model has been expanded to two dimensions with along-slope variations (e.g. Egger

1981), and numerical simulations were performed (e.g. Schumann 1990). A more

8

detailed yet clear overview of slope winds is given in Vergeiner and Dreiseitl (1987).

Typical values for up-slope flows are 1–5 ms−1 wind speed, where the maximum

is located 10–20 m above the slope. The total thickness of the layer is 50–150 m,

and it is deeper on the up-slope end (Whiteman 2000).

Up-valley winds

Up-valley winds are thermal circulations advecting cooler air into the mountains,

which are caused by a pressure gradient between the inner parts of a valley and

an adjacent plain. The pressure gradient is driven hydrostatically by overheating

of the mountain atmosphere, compared to the atmosphere over the plains. Daily

mean barometric temperature variation inside the Inn Valley is twice as high as over

Munich in the same atmospheric layer (Nickus and Vergeiner 1984).

One contribution to overheating of the valley atmosphere is the topographic

amplification factor (TAF), which was first proposed by Wagner (1932) and applied

to Inn valley by Steinacker (1984). As the air volume to be heated in the valley is

smaller than a volume over the plains, with equal horizontal area and thus equal

incoming solar radiation, TAF describes the larger diurnal temperature variation of

the valley atmosphere. Whiteman (1990) gives a good overview of literature on the

topographic amplification factor.

The air volume to be heated is not only determined by the valley side-walls, but

also by the presence and height of a mid-valley temperature inversion (Steinacker

1984), as shown in Fig. 1.2. This explains, why the climatological ratio between

valley and plain barometric mean temperature variation is lower in summer when

deep convection is possible.

Another contribution to the heating of the valley atmosphere is the downward

mixing of potentially warmer area which compensates the up-slope flow in the valley

cross section, as described above. This effect would be able to heat even a valley

with vertical sidewalls (Rampanelli et al. 2004), where TAF is of no relevance.

Temperature and thus pressure also vary inside the valley due to different

area-height distributions and variations in the energy balance along the valley axis

(Whiteman 2000). Freytag (1985) found in case studies in the Inn Valley a tran-

9

sition zone at the valley mouth with a temperature step of approx. 1 K. Further

up-valley, the temperature increases by 0.03 Kkm−1.

Cross-valley circulations

Asymmetries in the valley can produce cross-valley circulations, which differ from

the theory given above, with ascending air along the slopes and compensating sub-

sidence in the valley center. By different exposition, and thus different insolation of

the opposite slopes, up-slope wind is observed over the sunlit slope, while down-slope

wind is observed over the less insolated or shaded slope (Whiteman (2000), White-

man et al. (1989)). Additionally, Weigel and Rotach (2004) showed that valley

curvature can induce a cross-valley circulation through the centrifugal force acting

on the up-valley flow. This circulation can have an additional effect in stabilizing

the valley atmosphere by downward mixing of potentially warmer air.

Mountain-Plain Winds

Mountain-plain winds are at the large-scale end of the spectrum of thermal circu-

lations in mountainous terrain. They are driven by a pressure gradient which is

established between the plain and the mountain atmosphere, where at day time

low pressure is in the mountains, due to overheating in the valleys relative to the

plain. This pressure gradient drives a flow in the boundary layer which is directed

towards the mountains. In the thermal low pressure center inside the mountains

rising air supports the formation of convective clouds and thunderstorms. Above

the boundary layer the air flows back into the Alpine foreland, where it subsides due

to continuity. The upper branch of the mountain-plain circulation is superimposed

on the synoptic flow, which might be some magnitudes stronger, so the return flow

can often only be seen by a divergent zone above the mountains (Burger and Ekhart

1937).

The pressure field in the vicinity of mountains shows a diurnal and an annual

variation. In the seasonal mean, in winter high pressure is located in the mountains,

and in summer high pressure is located in the foreland. In the daily mean there is

a high pressure center in the mountains in the morning and a low pressure center

10

Figure 1.3: Mean sea level pressure for all 0900 UTC analyses of January 1989-

2001 (top) and all 1200 UTC analyses of May 1989-2001 (bottom)

derived from VERA (Vienna Enhanced Resolution Analysis). Contour

interval is 0.5 hPa. (Bica et al. 2007)

11

at noon. Fig. 1.3 shows the climatologic mean of the pressure fields for January 09

UTC and May 12 UTC. These are the both extreme phases where the diurnal and the

annual effect are accumulated. The May 12 UTC pressure field should represent the

mean summer-noon condition for the thermal mountain-plain circulation. The mean

pressure gradient between Innsbruck and Munich in May at 12 UTC is approximately

2 hPa.

The existence of a closed Alpine mountain-plain circulation was already proven

by Burger and Ekhart in 1937 by climatological analysis of pilot balloon soundings

in and around the Alps. They found that air is advected towards the mountains by

valley winds and large-scale slope winds — slope winds caused by the gentle sloping

terrain in the northern Alpine foreland. They also identified the compensation flow

above the inflow layer, which is weaker because it is not channeled by topography,

as a layer of divergent flow. The neutral layer of the thermal system, which divides

the inflow and the outflow branch, was located at approximately 1400 m ASL in 150

km distance from the Alpine Main Ridge, from where it rises up to 2 to 3 km ASL

in the mountains.

An interesting picture also yields the ground flow pattern from climatological

analysis of surface station observations in the Bavarian foreland from the DWD

(Lugauer and Winkler (2005), Lugauer and Winkler (2002a)). The observations from

days with strong insolation (total global radiation ≥ 20 MJm−2d−1) were analyzed,

For the days fulfilling this criterion—this are 42 % of all days between April and

September 1996-2000—the authors found a thermal circulation developing in the

Bavarian foreland. The mean pressure gradient maximum for such days between

Innsbruck and Munich is about 2 hPa per 100 km and is reached at 14 UTC. For

the radiation class 10–20 MJm−2d−1 it is ∼1 hPa per 100 km. The mean onset

of northerly flow (inflow into the Alps) begins at 7 UTC at valley stations (Fig.

1.4) and about two hours later at Augsburg, 80 km away from the Alpine rim. The

thermal system also grows from the plain to elevated stations: inflow starts at 8 UTC

at observatory Hohenpeißenberg, which is located on a hill elevated 300–400 m

above the surrounding plain. Due to this elevation the flow on Hohenpeißenberg

is representative for the boundary layer flow in the Alpine foreland. The mean

12

HP AUG ZUG VIL

Figure 1.4: Daily variation of v-component at four characteristic stations. Aver-

aged over 340 days with daily total global radiation ≥ 20 MJ m−2 in

the years 1996-2000. Time is in CET. (Lugauer and Winkler 2005).

maximum northerly wind component is reached between 14 UTC and 17 UTC and

is about 2 ms−1. Close to the ground in the plain, the northerly flow component is

weaker due to surface friction. Inflow gets also weaker at stations located more to

the north, further away from the Alps. The observatory at Zugspitze (2960 m ASL)

is already in the reverse flow branch of the thermal wind system and shows an

inverted signal of the north-south wind component. A secondary thermal wind

system develops in the vicinity of the elevated Allgau plateau, driving an easterly

wind component in the flow pattern of the Alpine foreland (Fig. 1.5).

Radiation input is largest for weak or southerly synoptic flow, due to suppression

of cloud formation by subsidence in the Alpine region. In the first case subsidence

is caused by high pressure systems. In case of southerly flow, subsidence occurs

dynamically in the lee of the Alps. This is mostly associated with Fohn, i.e. a

pressure gradient which is directed from Northern Italy across the Alpine main

ridge. Even in this case, the thermal circulation is well developed (Fig. 1.5, lower

right panel). For this situation the authors located the pressure minimum, i.e. the

convergence zone, in the Northern Alps, while they expect it south from the Alpine

main crest for northerly synoptic flow.

Further interesting features are the northerly flow components at the peaks of

Wendelstein (1832 m ASL) and Wank (1779 m ASL). The maximum north compo-

13

Figure 1.5: Mean flow pattern, 08 CET - 20 CET (07 UTC - 19 UTC) on 340 days

with daily total global radiation ≥ 20 MJ m−2 in the years 1996-2000.

Top panel is averaged over all 340 days, for bottom panels averaging

was performed for typical weather conditions separately (Lugauer and

Winkler 2005).

14

nent at Wank is 2.4 ms−1 and thus significantly higher than at Wendelstein with

only 1.3 ms−1. Lugauer and Winkler do not address this issue—perhaps the dif-

ference is caused by the fact that Wendelstein is closer to the Alpine rim or by a

general east-west pattern in the Alpine mountain-plain circulation. More trivial, it

could also be caused by local characteristics of the measurement sites.

Amongst the research flights of the DLR Falcon during the VERTIKATOR

campaign was one on 19 July 2002 with the new DLR 2-µm Doppler-LIDAR on-

board. This case was analyzed by Weissmann et al. (2005). The flight strategy was

similar to the flights with the 10-µm WIND LIDAR onboard: three vertical cross

sections parallel to the Alps were sampled. The weather situation for this day was of

the weak-gradient type—it was a sunny day with about 10 hours of sunshine in the

investigation area. The temporal evolution of the boundary layer flow sampled by

WTR in Lichtenau is shown in Fig. 1.6. Onset of northerly flow (inflow towards the

Alps) was at 0930 UTC and reached its mature stage at ∼15 UTC with an inflow

layer depth >1000 m with winds from northeast. After 20 UTC the depth of the

inflow layer decayed until there was no more northerly flow after 23 UTC.

The Falcon flight took place between 15 UTC and 17 UTC, in the mature stage

of Alpine pumping. Fig. 1.7 shows a north-south cross-section from the Alpine

foreland towards the Alps. The top of the boundary layer (thin solid line) can

be identified by a strong aerosol backscatter signal. It is nearly parallel to the

topography at the Alpine foreland of 2000 m ASL (1500 m AGL) and rises up to

4200 m ASL (1500 m above the highest peaks). The top of the inflow layer also rises

from ∼800 m AGL in the foreland near Munich up to 1200 m AGL at the northern

rim of the Alps.

Fig. 1.8 shows vertical cross-sections of wind speed and wind direction in the

northern and southern flight leg. Wind speeds of approx. 1–4 ms−1 from northerly

directions occur up to ∼1900 m ASL. Above the boundary layer at 2000-3000 m

westerly flow with a southern component prevails, which is identified by Weissmann

et al. as return flow. By applying the continuity equation to the cross-sections, a

convergence between the two flight legs is computed, which corresponds to a net

rising motion of ∼0.05–0.1 ms−1. Between the two northern cross-sections (north-

15

Figure 1.6: Profiles of horizontal wind on 19 July 2002, measured with WTR at

Lichtenau (Weissmann et al. 2005).

Figure 1.7: North-south cross-section of the foreland and mountain atmosphere.

Thin solid line and square indicate the boundary layer from a strong

aerosol backscatter signal from airborne LIDAR and ground-based

backscatter LIDAR at Lichtenau, respectively. Horizontal bars show

upper boundary of northerly flow from different profiling instruments.

Two “x”s show the top of convective boundary layer (Weissmann et al.

2005).

16

Figure 1.8: West-east cross sections of horizontal wind speed and direction mea-

sured with the 10 µm Doppler lidar onboard the DLR Falcon on 19

July. (a) and (b) are from the northern cross-section, (c) and (d) are

from the southern one. Solid line is the ground height derived from

lidar ground return. (Weissmann et al. 2005).

ernmost not shown), a divergence was computed, which yields subsidence in this

region and thus closure of the thermal circulation. Total cross section mass fluxes

of 4.96 · 108 kgs−1 and 3.92 · 108 kgs−1 with an accuracy of 18 % were evaluated for

the 223 km long northern flight leg and the 212 km long southern one, respectively.

1.6.3 Flow over and around mountains

Flow regimes

The mountains do not only affect the life of the boundary layer through thermal

processes, but also have an influence on the meso-scale flow, as the mountains are a

barrier against the approaching air masses. Schar (2002) developed a regime diagram

for idealized flow past obstacles depending on dimensionless mountain height

ε = NH/U, (1.1)

17

Figure 1.9: Regime diagram for idealized uniform flow past an isolated obstacle.

Bold lines show the boundaries for the different flow regimes. Ellipses

show typical parameters for several mountain ranges. (Schar 2002)

where N is Brunt-Vaisalla frequency, H is mountain height and f is Coriolis param-

eter, and Rossby number based on the mountain width

R0 = U/(Lf) (1.2)

with U denoting upstream velocity and L is mountain half width. N and U are

uniform in the upstream profile. For large Rossby numbers (R0 ' 0.5) the critical

dimensionless mountain height, separating the flow-over regime from the flow-around

regime, is εcrit ≈ 1.2. Theoretically, with the idealized assumptions, the Alps are

mostly in the flow around regime (Fig. 1.9), but in practice Fohn – which is flow

over the mountains – is a common phenomenon, and both regimes can exist at the

same time, which can be seen in the 09 July 2002 VERTIKATOR case below.

Alpine wake

Fig. 1.10 displays the idealized case of a southerly air mass that flows over and

around the Alpine arc as numerically modeled by Schneidereit and Schar (2000).

The mountain ridge works as an obstacle for free air motion which leads to a west

18

component on its north-western side and implies a slight east component on its

north-eastern side. The east component is much weaker due to the geometric shape

of the Alps. At the downstream side of the obstacle the circumfloating branches

meet and create an Alpine wake, analogue to a river floating around a bridge-pylon.

An increase in wind speed (shaded areas) is visible on the lee-side of the Alps, which

shows the potential of the idealized model to simulate Fohn-winds.

Figure 1.10: Flow 30 m AGL in the vicinity of idealized Alps in a numeric model.

Shaded areas denote wind speed (ms−1) and arrows wind direction.

(Schneidereit and Schar 2000)

19

Chapter 2

Lidar Measurements – Data

Acquisition and Processing

As shown by Weissmann et al. (2005), airborne Doppler lidar measurements yield

good data for deriving the mass flux towards the Alps. In contrast to in-situ measure-

ments, which capture the atmospheric parameters only at single locations, airborne

Doppler lidar measurements provide a snapshot of the wind field below the flight

track with an extent of 100-200 km within a short enough time for the wind field to

remain nearly stationary.

In this chapter the basic principles of Doppler wind lidar measurements will be

briefly explained, followed by a more detailed description of the WIND instrument

onboard of the DLR Falcon, and how the horizontal wind can be derived from the

measured data. Finally the procedure of determining the mass flux and its error

will be described.

2.1 Doppler lidar basics

Lidar (light detection and ranging) is an active remote sensing method. Active

means that radiation - in this case light - is emitted by a laser (light amplifica-

tion by stimulated emission of radiation) device, is backscattered by aerosols and

clouds (echo) and received by a telescope. This work will focus on the monostatic

transceiving method, which means that sender and receiver are in the same loca-

21

tion, in contrast to bistatic transceiving, where the sender is locally separated from

the receiver. With different lidar applications many atmospheric parameters such as

aerosol and trace gas concentrations, temperature and finally wind can be measured.

Doppler effect - after the Austrian physicist Christian Doppler (1803-1853) -

describes the phenomenon that the received frequency is shifted in comparison to

the transmitted one if the relative velocity between source and receiver is non-zero.

The Doppler shift is defined by the difference between the transmitted frequency f0

and the frequency observed by the receiver f1.

An everyday example of this phenomenon is the sound of the siren on an am-

bulance car passing the observer: the observed frequency is higher when the car

approaches and lower when the car moves away.

For waves which are advected by a medium, like sound which is advected by

air, we have to distinguish whether the source or the receiver or both are moving.

But since light is not bound to a medium, the magnitude of the Doppler shift only

depends on the relative velocity between source and receiver for a known frequency

of the laser light f0. The frequency of the shifted laser radiation received by particles

which are advected by the wind is given by:

f1 = f0

(1 +

v

c

)(2.1)

When the light is scattered by the particle, and a portion is received by the

lidar telescope, the frequency is shifted again relative to f1. So the frequency f2

received by the telescope is:

f2 = f1

(1 +

v

c

)= f0

(1 +

v

c

)2

≈ f0

(1 + 2

v

c

)(2.2)

So the observed frequency shift ∆f is the difference between the transmitted

and the twice Doppler shifted, backscattered frequency:

∆f = f2 − f0 (2.3)

The frequency shift observed by a stationary lidar transmitting 10.6 µm radia-

tion, which is scattered back by a particle moving with 1 ms−1 away from the lidar,

thus would be 0.189 MHz. Since this frequency is too high to be detected with good

22

accuracy – modern detectors can only detect signals with a lower frequency – the

signal is mixed with the signal from another laser, called the local oscillator (LO).

This mixing gives a beat signal which consists of two oscillations: the first compo-

nent represents the difference between the LO frequency and the received, Doppler

shifted frequency, and the second component represents its sum. So the detector

measures the AC current iAC (Weitkamp (2005)):

iAC = ρ{√

2PLOP (x, λ) cos[2π(fLO − (f0 + ∆f))

]

+√

2PLOP (x, λ) cos[2π(fLO + (f0 + ∆f))

]} (2.4)

where ρ is detector sensitivity, PLO is the power of the reference laser and P (x, λ)

is power of the backscattered radiation. The second component of the mixed signal

is too high to be detected by the detector and thus is equal to zero. The first

component, which is called beat frequency, is low and can be measured with good

accuracy. If fLO 6= f0 it is called heterodyne detection. This technique has the

advantage over homodyne detection, where fLO = f0, that the sign of ∆f can be

determined.

2.2 Determining the Wind

By measuring the Doppler shift, as described above, only the radial component of

the wind can be determined. This is the component with the same direction as the

laser beam and is often referred to as line-of-sight (LOS) component vLOS (Fig. 2.1).

In the two-dimensional case, vLOS is given by vLOS = v cos(α), where α is the angle

between the laser beam and the wind vector. In the three-dimensional case, if the

lidar viewing direction is defined by the azimuth angle θ, clockwise from north, and

the elevation angle ϕ, as the deviation from nadir, the LOS velocity is given as a

function of u, v, and w (west-east, south-north and vertical wind components):

vLOS = −u sin θ cos ϕ− v cos θ cos ϕ− w sin ϕ (2.5)

23

2.2.1 Velocity azimuth display technique (VAD)

There might be applications, where only the line-of-sight component of the wind

is of interest. But for the evalution of mass fluxes and the interpretation of wind

fields, the u and v components have to be known—vertical wind w is computed, too,

but not discussed here. Therefore a method has to be found, to evaluate the three-

dimensional wind from line-of-sight measurements. In lidar practice, the velocity

azimuth display technique (VAD) has become a standard (Browning and Wexler

1968). The name illustrates the idea, to measure the line-of-sight velocities during a

conical scan around the nadir axis—i.e. the elevation angle ϕ is fixed, and azimuth

angle θ runs from 0◦ to 360◦—and to extract three-dimensional wind information

from a characteristic diagram of line-of-sight velocity vs. azimuth angle θ.

In theory, at least three linear independent line-of-sight components would have

to be measured at three different azimuth angles (Fig. 2.2). Then Eq. 2.5 would

expand to a system of three equations with three unknown variables, which could

be solved.

In practice, a lot more than these three values are measured, since, as explained

laser transmitter

laser beam

wind vector

LOS componentα

Figure 2.1: 2D-schematics of the line-of-sight (LOS) component, which is mea-

sured by a Doppler lidar.

24

more in detail later in this subsection, the measurements may not be representative

for the mean wind in the measurement volume, e.g. when turbulence or strong wind

shear is present, or when from a measurement with a low signal-to-noise ratio (SNR)

a wrong value is obtained. Therefore the line-of-sight velocity vLOS is measured

while performing a conical scan (Fig. 2.2). Afterwards the measured values of one

scanner revolution can be plotted as a velocity vs. azimuth diagram. If the wind

in the measurement volume is stationary and homogenous, and no measuring errors

are present, this will result in a sine curve (Fig. 2.3), from which the magnitude of

the horizontal (the amplitude A) and the vertical wind (the vertical displacement

∆vLOS), and the wind direction (displacement of phase ∆θ) can be determined.

But in reality, the measured wind is seldom homogeneous or stationary. This

is true for the wind at an arbitrary location, e.g. above a ground-based Doppler

lidar, which is scanning conically with an elevation angle of 30◦. An example for

non-stationary wind could be a gust of several seconds which is measured by the

lidar during an averaging interval of 60 seconds, which is “disturbing” the desired

smooth lidar signal. Furthermore, 5 km above the lidar the diameter of the circle

nadir

ϕ

θ

vLOS,1

vLOS,2

vLOS,3

vLOS,4

Figure 2.2: Schematics of a VAD scan with fixed elevation ϕ and variable az-

imuth θ

25

described by the scanning laser beam is already ∼6 km. In this catch the wind

might be very heterogenous, e.g. when the site is embedded in complex orography,

like a valley junction, where winds of different speed and direction lie close together

at the same height level.

The wind measured by an airborne Doppler lidar is even less uniform. When

the measurements are taken from a flight level of 5 km AGL with an aircraft ground

speed of 200 ms−1, with accumulating 3 scanner evolutions, where each one takes 20

seconds, the along-track resolution is 12 km, and cross-track resolution is 6 km on the

ground (Fig. 2.4). In this measurement volume of 12 km by 6 km, the disturbances

of the wind from homogeneity and stationarity are even more extreme than in the

ground-based case. Additionally, the aircraft ground speed has to be determined

with very high accuracy, since it is of approximately two orders of magnitude higher

than the wind speed.

From such a real life wind field, which is non-stationary and inhomogeneous,

a measured VAD scan might look like Fig. 2.5. The measurements do not show a

perfect sine curve anymore. To estimate the three-dimensional wind, a sine-wave

fitting algorithm is applied to the measured data.

2.2.2 Sine-wave fitting

The sine-wave fitting algorithm tries to find the sine curve which matches the mea-

surements best in a sense of root-mean-square (RMS) deviation of the data points

from the estimated sine curve in the VAD diagram by harmonic analysis (Browning

and Wexler 1968). From three conical scans 200 LOS measurements are obtained,

by accumulating the three values for each azimuth. Beginning from a first guess

with all 200 measurement data included, the algorithm approximates the wind in

three iterations by omitting data points, which deviate from the estimated sine curve

more than an empirically determined threshold. If the estimated values of the al-

gorithm do not satisfy empirical quality requirements, e.g. a minimum number of

points used for the last approximation (NOP) or RMS error, the measurement is

discarded. For a total of 2170 VAD measurements used in this thesis, the sine-wave

fitting algorithm discarded 564 values (26 %) (Tab. 2.1).

26

Figure 2.3: Schematics of a VAD scan, assuming homogenous wind.

Figure 2.4: Scan pattern of the lidar laser beam at the ground (top) and at a height

of 5 km AGL (bottom) for a flight level of 10 km AGL, 200 ms−1,

30◦ nadir angle and a scanner rotation period of 20 s. Taken from

Reitebuch et al. (2001)

27

Figure 2.5: Example of a measured VAD scan, with the horizontal wind obtained

from the sinewave-fiting algorithm.

Total measure-

ments

discarded by sine-

wave fitting

discarded by NOP

< 35

5 July, south 329 115 (35 %) 9 (2.7 %)

8 July, north 381 78 (20 %) 5 (1.3 %)

8 July, south 375 134 (36 %) 4 (1.1 %)

9 July, south 355 66 (19 %) 4 (1.1 %)

12 July, north 317 67 (21 %) 3 (0.9 %)

12 July, south 413 104 (25 %) 9 (2.2 %)

Total 2170 564 (26 %) 34 (1.6 %)

Table 2.1: Overview of the number of VAD measurements and how many where

discarded, due to quality requirements.

28

2.2.3 Data quality checking

Despite quality checking in the internals of the sine-wave fitting algorithm, still some

computed wind estimates seem subjectively unrealistic, e.g. when very high winds

are calculated near the surface – perhaps due to contamination of the lidar signal

from ground return echo, while the neighboring range gates show moderate winds.

A minimum NOP value of 35 as subjective criterion was found to discard these

measurements. Applying this criterion to all data, 34 (1.6 %) out of a total 2170

measurements are discarded.

2.3 Characteristics of the Doppler lidar instru-

ment WIND

The development of the Wind Infrared Doppler Lidar (WIND) is a cooperation of the

two French institutions Centre National de la Recherche Scientifique (CNRS) and

Centre National d’Etudes Spatiales (CNES) and the Deutsches Zentrum fur Luft-

und Raumfahrt (DLR) (Werner et al. 2001). At the time of its validation flight

on 12 October 1999, it was the first airborne research Doppler lidar to measure the

complete wind profile below the aircraft down to the ground (Reitebuch et al. 2001).

The WIND instrument is a heterodyne lidar, emitting radiation at 10.6 µm

wavelength via a pulsed CO2 laser (Reitebuch et al. 2001). The laser beam leaves

the aircraft through an optical scanner which deflects it 30◦ off-nadir and performs

a vertical scan around the vertical axis. A small portion of the emitted radiation is

backscattered from aerosols and clouds into the telescope of the instrument where

the signal is mixed with the radiation from a continuous wave CO2 laser, which acts

as local oscillator. The mixing signal is finally detected and digitized for processing.

While performing a conical scan 200 LOS measurements for each atmospheric layer

of 250 m thickness are obtained. If the SNR is too low, three scanner evolutions can

be accumulated to get a better SNR and, as a consequence, a better coverage over

measurements along the flight path, but of course the along-track resolution gets

worse. In experimental validation, the lidar showed an agreement to radio soundings,

wind temperature radar and to numeric weather prediction models of better than

29

0.5–1.5 ms−1 for wind speed and better than 5◦ for wind direction (Reitebuch et al.

2001). The characteristics of the WIND lidar are summarized in Tab. 2.2.

2.4 Determining the mass flux

Weissmann et al. (2005) showed that Doppler lidar data are a good source for

estimating the amount of air advected towards the Alps. In their case study of

Alpine pumping on 19 July 2002, the authors quantified the total mass flux into the

Alps as nearly the entire air in the lowest kilometer between Munich and the Alpine

rim. The authors method should be used as a working model for this thesis. Data

from the WIND instrument onboard the DLR Falcon are available from six vertical

cross-sections on four different IOPs during the VERTIKATOR project. From these

data, mass fluxes are computed to extend the study from Weissmann et al. with

additional cases from different weather situations.

The computation of the mass fluxes is orientated towards Weissmann’s (2005)

procedure:

1. Starting from the cross section of wind direction and wind velocity, the com-

ponent perpendicular to the Alps v350 is computed. This component is tilted

Wavelength 10.6 µm

Vertical resolution 250 m

Range Ground to 500 m below flight level

Temporal resolution 60 s (3 scans accumulated)

Wind speed accuracy 0.5–1.5 ms−1

Wind direction accuracy 5 ◦

ATR 12 km (3 scans accumulated)

CTR 6 km

Table 2.2: System specification of the Doppler lidar WIND. ATR and CTR com-

puted with flight level 5 km AGL and aircraft speed 200 ms−1 (Reite-

buch et al. 2001)

30

10◦ counterclockwise from the northward pointing vector since the Alps are

slightly inclined on the west-east direction.

2. Data close to the ground is often missing due to contamination of the signal

by ground return. Values are extrapolated from the bottommost value after

resolution is doubled to 125 m to get a better resolution close to the ground.

The wind profile in the boundary layer is assumed to be constant above 125

m AGL and to have a power-law shaped profile below 125 m AGL with an

exponent of 1/7 (Hsu et al. 1994, Peterson and Hennessey 1978):

u(z) = u (ze)

(z

ze

) 17

(2.6)

where ze is the reference height, 125 m AGL. Computing the mean wind com-

ponent u in the lowest layer yields

u =1

ze

∫ ze

0

u(z)dz =7

8u(ze) (2.7)

3. An average v350-component for each height level in the cross-section is com-

puted.

4. The air density profile, as well as partial water vapor density, is computed

from the radio sounding Oberschleissheim.

5. By scalar multiplying mean v350-profile by air density profile and partial water

vapor density, specific mass flux and specific water vapor mass flux, respec-

tively, are obtained. These parameters are a measure of mean inflow towards

the Alps, and as they are independent of the length of the flight leg, they allow

intercomparison of the individual cross-sections.

6. Total cross section mass fluxes are obtained by multiplying specific mass fluxes

by the length of the flight leg. They represent the whole mass flux through

the cross-section, derived from lidar measurements.

2.5 Determining the mass flux error

The error margins for the WIND lidar are given by ∆v = 0.5–1.5 ms−1 for wind

velocity and ∆ϕ = 5◦ for wind direction (Reitebuch et al. 2001). As v350 is a function

31

of both wind velocity and wind direction, the error margin for v350 is obtained by

∆v350 =

∣∣∣∣δv350

δv

∣∣∣∣ ∆v +

∣∣∣∣δv350

δϕ

∣∣∣∣ ∆ϕ (2.8)

For each lidar measurement two errors are computed, one with ∆v = 0.5 ms−1 and

one with ∆v = 1.5 ms−1. For the extrapolated values close to the ground a relative

error of 100% and 200%, respectively, is assumed.

Analog to computing the mass flux, the error margins for each height level

are averaged to obtain a vertical profile of the error margin. This profile is scalar

multiplied with the air density profile. Dividing the result by the specific mass flux

yields the relative error.

32

Chapter 3

Case studies

The climate in the investigation area during July 2002 was about normal, with

amount of precipitation and mean temperature comparable to the long-term average,

but 10% less sunshine duration (Lugauer and Winkler 2002b). Between 5 July and

19 July the synoptic situation was variable with several frontal systems passing

through the investigation area. No stationary high-pressure situation developed.

Therefore the best conditions for IOPs were during southerly flow, downstream of

approaching troughs (5, 9 and 12 July), a short-wave ridge (8 July) and a weak

gradient situation (19 July, discussed by Weissmann et al. (2005)).

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20−4

−3

−2

−1

0

1

2

3

4

Day in July 2002

Pre

ssur

e di

ffere

nce

/ hP

a

Figure 3.1: Pressure difference between Munich–Innsbruck ∆pMUC−IBK (black)

and Munich–Garmisch-Partenkirchen ∆pMUC−GAP (gray) in July 2002.

Positive values indicate higher pressure at Munich. DLR Falcon flight

days are bordered by solid lines.

33

In Fig. 3.1, the pressure difference between Munich—Innsbruck and Munich—

Garmisch-Partenkirchen is shown, where positive values indicate higher pressure at

Munich. Pressure measurements at Innsbruck and Garmisch-Partenkirchen were

reduced to Munich barometer height (526 m ASL). In the absence of synoptic dis-

turbances on a day with strong insolation, the pressure difference, which is driving

the mountain-plain circulation, is expected to be positive in the afternoon and to

change sign at night. All days when DLR Falcon research flights took place (bor-

dered by solid lines) show this pattern. Of course, the pressure difference is affected

by a other phenomena as well, e.g. the large pressure difference on 10 July is caused

by a cold front passing Munich.

34

3.1 5 July 2002: Southwesterly flow

Figure 3.2: ECMWF analysis of 300 hPa geopotential height for 5 July 2002 at

12 UTC. Iso spacing is 4 decameters, shaded areas show jet streaks.

Synoptic situation

On 5 July the investigation area was in a southwesterly flow downstream of a trough

moving eastward from the British Isles (Fig. 3.2). Satellite pictures of the AVHRR

instrument (Fig. 3.3) show orographic clouds south of the Alpine main ridge and a

clear sky to the north, where subsidence suppressed convection, causing a sunny day

with 12 hours of sunshine in Innsbruck. Southerly synoptic flow and thus turbulent

mixing from above, along with convective mixing from the Inn valley floor, were

possibly the factors leading to a Fohn break-through in Innsbruck.

Boundary layer

The Innsbruck radio sounding from 11 UTC (Fig. 3.4) measured a shallow, cold and

stably stratified layer with up-valley wind close to the ground and a warm and mixed

Fohn layer above, reaching up to 2000 m ASL. Around crest height (2500 m ASL)

and above, the atmosphere was stably stratified and the wind shifted to the large-

scale southwesterly flow. Flow up the Inn valley started at 10 UTC and lasted

35

Figure 3.3: Satellite composite images from channel 1 and 2 of the AVHRR in-

strument on 5 July 2002 at 1235 UTC and 1510 UTC.

until 13 UTC. Afterwards Fohn broke through in Innsbruck and stopped up-valley

flow. The Oberschleissheim sounding at 11 UTC measured a layer with easterly

flow in the lowest 1000 m with a potential temperature 6 ◦C colder than the Inn

valley atmosphere (indicating different air masses). Easterly flow in the boundary

layer in the Bavarian foreland is often observed during south Fohn events. A strong

inversion separates the boundary layer from the free troposphere. The 17 UTC

sounding showed a weak northerly component in the lowest 500 m, which was not

measured by SYNOP stations at the same latitude. The maximum reduced pressure

difference between Munich and Innsbruck was 2.0 hPa between 12 UTC and 14 UTC,

around the break-through of Fohn in Innsbruck.

Temporal evolution of Alpine pumping

Northerly flow at Lichtenau started at 7 UTC in a shallow layer close to the

ground (Fig. 3.5). At time of flight (10 UTC) inflow was measured at around

500–700 m AGL, but close to the ground inflow components were weak. Reduced

pressure difference between Munich and Innsbruck was 0.9 hPa at that time. Alpine

pumping at Lichtenau was best developed at around 14–15 UTC, with flow towards

the Alps up to 1000 m AGL, but decayed quickly afterwards, possibly associated

with the Fohn break-through in Inn valley. The observation, that southerly flow

36

20 25 30 35 40 45 50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Potential temperature θ / ° C

Hei

ght /

km

AS

L0 5 10 15

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Mixing ratio / gkg−1

0 90 180 270 360

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind direction / °

Hei

ght /

km

AS

L

0 5 10 15 20

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind speed / ms−1

20 25 30 35 40 45 50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Potential temperature θ / °C

Hei

ght /

km

AS

L

0 5 10 15

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Mixing ratio / gkg−1

0 90 180 270 360

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind direction / °

Hei

ght /

km

AS

L

0 5 10 15 20

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind speed / ms−1

Figure 3.4: Radio sounding from Innsbruck (top) and Oberschleissheim (bot-

tom) on 5 July 2002, launched at 1113 UTC and 1047 UTC, respec-

tively. Potential temperature (solid line) and water vapor mixing ratio

(dashed line) are shown in the left column, wind speed (solid line) and

wind direction (x) are shown in the right column.

37

prevails in Innsbruck, while northerly flow is present in the Alpine foreland, is con-

sistent with Lugauer and Winkler’s (2005) theory, that during southerly synoptic

flow, the thermal low and thus the convergence zone of Alpine pumping is located

north from the Inn valley. Though, with the data available, the low pressure cen-

ter could not be located. The pressure difference Munich–Garmisch-Partenkirchen

showed the same pattern as Munich–Innsbruck (Fig. 3.1).

Spatial structure of Alpine pumping

DLR Falcon research flight took place in the morning at around 10 UTC, at the

onset of flow towards the Alps at Lichtenau. At the same time flow up the Inn valley

started at Innsbruck. Only a few range gates below 750 m AGL in the western part

of the cross-section, and below 500 m AGL in the central and eastern part, showed

Time / hours UTC

He

igh

t / m

AG

L

20 ms-1 225° SW

Figure 3.5: Vertical profiles of horizontal wind on 5 July 2002 measured with WTR

at Lichtenau.

38

9.5 10 10.5 11 11.5 12 12.50

1000

2000

3000

4000

5000

6000

Longitude / °E

Hei

ght /

m A

SL

20 m/s SW

−5

−4

−3

−2

−1

0

1

2

3

4

5

Figure 3.6: Vertical cross section of horizontal wind (arrows) and v350-component

(color, units are ms−1, red/negative values denote flow towards the

Alps) measured with WIND lidar onboard the DLR Falcon on 5 July

2002, approximately at 10 UTC, along the southern flight leg.

flow towards the Alps, with v350-components of at most 3 ms−1.

Mass flux

The WIND Lidar measured flow towards the Alps only in a few range gates close to

the ground. In this phase of Alpine pumping outflow from the mountains predomi-

nated, thus a total cross section mass flux towards the Alps was not computed.

39

3.2 8 July 2002: Short wave ridge

Figure 3.7: Same as Fig. 3.2, but for 8 July 2002 at 12 UTC.

Synoptic situation

The trough lying over Great Britain on 5 July passed over the investigation area and

caused heavy precipitation in the Alpine foreland on 6 July (Lugauer and Winkler

2002b). On 8 July a shortwave ridge came to lie over the Alps (Fig. 3.7). Above

ridge height, moderate winds from westerly to southwesterly directions prevailed.

Warm, dry and stably stratified air suppressed convection. Warm air advection and

a maximum possible sunshine duration of 14 hours caused the maximum diurnal

temperature range for July 2002 of 19.0 ◦C in Innsbruck (all-time record in summer

was 24 ◦C on 11 July 1908, long-term seasonal mean for summer is 12 ◦). The

mountain-plain circulation was shaping up well due to the absence of synoptic dis-

turbances, and due to the large amount of insolation. Shallow convection developed

only along the Alpine main ridge, as warm-air advection enhanced stability in the

investigation area.

41

20 25 30 35 40 45 50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Potential temperature θ / ° C

Hei

ght /

km

AS

L

0 5 10 15

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Mixing ratio / gkg−1

0 90 180 270 360

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind direction / °

Hei

ght /

km

AS

L

0 5 10 15 20

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind speed / ms−1

20 25 30 35 40 45 50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Potential temperature θ / °C

Hei

ght /

km

AS

L

0 5 10 15

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Mixing ratio / gkg−1

0 90 180 270 360

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind direction / °

Hei

ght /

km

AS

L

0 5 10 15 20

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind speed / ms−1

Figure 3.8: Same as Fig. 3.4, but for 8 July 2002. Innsbruck sonde launched at

12 UTC and Oberschleißheim at 14 UTC.

42

Boundary layer

The pressure gradient between Munich and Innsbruck reversed between 10 UTC and

11 UTC. Up-valley flow was measured at Innsbruck from 11 UTC until 19 UTC,

while onset of up-valley wind in the Wipp Valley was already at 7 UTC and lasted

until 19 UTC.

Innsbruck radio sounding at 11 UTC showed a shallow convective mixed layer

up to 400 m AGL (Fig. 3.8) and up-valley winds up to crest height. Above, a

local minimum in wind speed occured and the wind direction shifted to synoptic

westerly flow. Despite the large supply of incoming solar radiation vertical mix-

ing of the valley atmosphere up to crest height was not reached. Possible factors

that enhanced stability—and thus inhibited convection—might have been warm-air

advection and thermally induced subsidence inside the valley (c.f. Section 1.6.2).

The Oberschleißheim sounding at around 14 UTC (Fig. 3.8) showed a well-mixed

boundary layer up to 1600 m ASL, that is 700 m deeper than the Inn Valley atmo-

sphere two hours before. At 11 UTC Oberscheleissheim (radiosounding not shown)

was already unstable stratified up to 1500 m ASL. Northerly flow in the lowest

300 m AGL at 14 UTC indicated that the mountain-plain circulation reached as far

north as Oberschleißheim. Above the wind shifted to the large-scale westerly flow.

Temporal evolution of Alpine pumping

Northerly flow at Lichtenau started lately: WTR Lichtenau measured the first range

gates with northerly flow at 1130 UTC, but one hour later the inflow layer was

already 600 m deep (Fig. 3.5). At 1600 UTC the inflow layer reached a maximum

depth of 1200 m and also its maximum wind magnitudes, afterwards it was decaying

until 20 UTC.

Spatial structure of Alpine pumping

The flight of the DLR Falcon took place between 1200 UTC and 1330 UTC, after

northerly flow had started at Lichtenau (Fig. 3.5). Profiles of both flight legs are

available, and northerly flow is already well-developed at the time of the flight.

In the western part of the northern flight leg (Fig. 3.10, top), flow towards

43

Time / hours UTC

He

igh

t / m

AG

L

20 ms-1 225° SW

Figure 3.9: Same as Fig. 3.5, but for 8 July 2002.

the Alps was measured below 2000 m AGL, with maximum v350-components of at

most 3 ms−1 at around 10.5 ◦E. In the eastern part, two layers with northerly flow

exist: one in the boundary layer below 2000 m AGL and one above. The latter is

assumed to be a dynamic feature of southerly synoptic flow passing the Alps (Alpine

wake—c.f. 1.6.3). The turning of wind direction from southeasterly to northeasterly

directions towards the eastern part of the cross sections is also present in numerical

simulations performed for the 8 July case (Reitebuch et al. 2003). In the mean

v350-profile, horizontally averaged over longitude, the maximum inflow towards the

Alps is 1.2 ms−1 at 1300 m ASL, and mean inflow is measured up to 2000 m ASL.

In the western part of the southern flight leg, northerly winds are measured up

to 2200 m ASL (ca. 1800 m AGL) (Fig. 3.10, bottom). Maximum v350-components

are approximately 4 ms−1 at 10.25 ◦E and 11.25 ◦E. Analog to the northern flight

leg, a second layer above the boundary layer with flow towards the mountains exists

44

9.5 10 10.5 11 11.5 12 12.50

1000

2000

3000

4000

5000

6000

Longitude / °E

Hei

ght /

m A

SL

15 ms−1 southwest

−5

−4

−3

−2

−1

0

1

2

3

4

5

9.5 10 10.5 11 11.5 12 12.50

1000

2000

3000

4000

5000

6000

Longitude / °E

Hei

ght /

m A

SL

20 ms−1 southwest

−5

−4

−3

−2

−1

0

1

2

3

4

5

Figure 3.10: Same as Fig. 3.6, but for 8 July at 12–13 UTC. Top panel shows

northern, bottom panel shows southern flight leg.

45

in the eastern part. Maximum mean v350-component in the cross section is 1.4 ms−1

at 1300 m ASL. Mean inflow towards the mountains is measured up to 2200 m ASL.

Therefore, inflow is deeper and stronger in the southern cross section, as assumed

by the theory of an inclined boundary layer and accelerated flow towards the Alps

(c.f. Section 1.6.2).

Mass flux

Computed specific mass fluxes for northern and southern cross sections are 880 kgm−1s−1

and 1200 kgm−1s−1, respectively. These values are comparable to those of the follow-

ing cases of 9 and 12 July, but lower than those of 19 July (discussed by Weissmann

et al. (2005)). But it should be kept in mind—since the flight was in the strength-

ening phase of Alpine pumping—that the mass fluxes were likely to have increased

in the course of the day.

The difference of specific mass flux between north and south yields divergence

in the enclosed area, which is presumably compensated by subsidence.

46

3.3 9 July 2002: Southwesterly flow

Figure 3.11: Same as Fig. 3.2, but for 9 July 2002 at 12 UTC.

Synoptic situation

A short-wave ridge moved rapidly eastwards, and Europe came under the influence

of a cyclone which reached the British Isles. The investigation area was in a south-

westerly synoptic flow, downstream of the upper level trough, promoting Fohn in the

lee of Alpine main ridge. A cold front with a pre-frontal squall line lay over France,

Figure 3.12: As Fig. 3.3, but on 9 July 2002 at 1151 UTC and 1514 UTC.

47

20 25 30 35 40 45 50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Potential temperature θ / ° C

Hei

ght /

km

AS

L

0 5 10 15

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Mixing ratio / gkg−1

0 90 180 270 360

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind direction / °

Hei

ght /

km

AS

L

0 5 10 15 20

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind speed / ms−1

20 25 30 35 40 45 50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Potential temperature θ / °C

Hei

ght /

km

AS

L

0 5 10 15

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Mixing ratio / gkg−1

0 90 180 270 360

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind direction / °

Hei

ght /

km

AS

L

0 5 10 15 20

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind speed / ms−1

Figure 3.13: Same as Fig. 3.4, but for 9 July 2002. Innsbruck sonde launched at

1101 UTC and Oberschleißheim at 1046 UTC.

48

both reaching the Bavarian Foreland on 10 July. Sky was clear in the morning of 9

July (Fig 3.12), so the investigation area was provided with maximum insolation .

In the afternoon deep convective cells developed in the upper Inn valley, which were

advected by the south-westerly flow into the Bavarian foreland, leading to severe

thunderstorms with hail and wet downbursts in the afternoon (a detailed case study

of this downburst event can be found in Dotzek and Friedrich (2008)).

Boundary layer

At 11 UTC the Inn valley atmosphere was well-mixed by Fohn up to crest height,

only a shallow layer near the ground showed up-valley wind (Fig. 3.13). Fohn broke

through at Innsbruck at 12 UTC and lasted until 17 UTC.

At 11 UTC, the convectively mixed boundary layer at Oberschleißheim was

800 m deep, with easterly and south-easterly winds. The easterly flow can be in-

terpreted as eastern branch of an Alpine wake. This assumption is supported by

the picture of the u-component in the lidar cross section (Fig. 3.16, bottom), where

the western part showed westerly wind components, whereas east from the 11.5 E

meridian mostly easterly wind components occurred. The analysis of the 850 hPa

flow field from the ECMWF agrees with the observations (Fig. 3.14). Determin-

ing the parameters for the regime diagram (c.f. 1.6.3) from the upstream Milano

sounding, yields a dimensionless mountain height ε = 3.0, and thus the flow around

regime for these synoptic conditions.

Temporal evolution of Alpine pumping

Northerly flow at Lichtenau started after 10 UTC and prevailed until 18 UTC (Fig.

3.15). The wind field was unsteady until 15 UTC and reached up to nearly 1000

m AGL. After this time, wind grew stronger and wind direction shifted to north-

west. At the same time thunderstorm cells were entering the investigation area from

southwest. Strong winds near the ground after 18 UTC might be interpreted as thun-

derstorm outflow. At 14 UTC the Oberschleißheim sounding already showed wind

components towards the Alps, and a growing boundary layer, which was already

1800 m deep at 17 UTC.

49

20 m/s

longitude (deg E)

lati

tud

e (

de

g N

)

0 5 10 15 20

42

44

46

48

50

52

Figure 3.14: ECMWF 850 hPA analysis of horizontal wind with streamlines on 9

July at 12 UTC

Time / hours UTC

He

igh

t / m

AG

L

20 ms-1 225° SW

Figure 3.15: As Fig. 3.5, but for 9 July 2002.

50

Spatial structure of Alpine pumping

The DLR Falcon flight took place at around 1030 UTC, shortly after inflow started

at Lichtenau. Measurements along the southern flight leg are available. Around

11.5 ◦E the lidar was not able to measure, as the aircraft was flying a curve. Flow

towards the mountains with westerly components are measured up to a height of

approximately 1800 m ASL on the western part of the cross-section. Maximum

v350-components around 5 ms−1 were measured at 11.5 ◦E near the ground. In the

eastern part the inflow layer is less deep and the wind is weaker with an easterly

component (Fig. 3.16). Mean inflow in the cross-section towards the mountains

is measured up to 1600 m ASL, with a maximum v350-component of 1.6 ms−1 at

1000 m ASL.

Mass fluxes

Specific mass flux along the southern cross section on 9 July 2002 was 1100 kgm−1s−1

and thus slightly lower compared to those on 8 July and 12 July. Specific water vapor

mass flux was with 10.5 kgm−1s−1 the largest of all cross sections. The presence of

latent heat in the boundary layer, which was advected into the Alps by the mountain-

plain circulation, might have been an important factor for the development of deep

convection and severe thunderstorms later on.

51

9.5 10 10.5 11 11.5 12 12.50

1000

2000

3000

4000

5000

6000

Longitude / °E

Hei

ght /

m A

SL

20 ms−1 southwest

−5

−4

−3

−2

−1

0

1

2

3

4

5

9.5 10 10.5 11 11.5 12 12.50

1000

2000

3000

4000

5000

6000

Longitude / °E

Hei

ght /

m A

SL

20 ms−1 southwest

−5

−4

−3

−2

−1

0

1

2

3

4

5

Figure 3.16: Top panel as Fig. 3.6, but on 9 July 2002. Color coding of the bottom

panel shows the u-component (west-east) in the same cross-section.

52

3.4 12 July 2002: Southwesterly flow

Figure 3.17: Same as Fig. 3.2, but for 12 July 2002 at 12 UTC.

Synoptic situation

An upper-level depression remained stationary over Great Britain, and the Alps

were under the influence of a southwesterly flow (Fig. 3.17). Subsidence in the lee

of the Alps caused break-up of clouds in the Bavarian foreland in the morning, so the

12 July was another sunny day with a diurnal temperature variation of 14 ◦C and a

total sunshine duration of ten hours at Innsbruck University, with partly shadowing

by cumulus clouds after 13 UTC. In the evening thunderstorms were observed in

the Alps with weak showers in Innsbruck. Warm air advection by southwesterly

synoptic flow suppressed convection in the Alpine foreland. Due to the absence of

a pressure gradient across the Alps, Fohn did not develop, and the thermal wind

system was shaping up well in the investigation area.

Boundary layer

At 11 UTC the Inn valley atmosphere was nearly well mixed up to 1500 m ASL,

with easterly wind components up to crest height (Fig. 3.17). Onset of up-valley

flow at Innsbruck was at 10 UTC and lasted until 21 UTC. Ellbogen had up-valley

53

20 25 30 35 40 45 50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Potential temperature θ / ° C

Hei

ght /

km

AS

L

0 5 10 15

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Mixing ratio / gkg−1

0 90 180 270 360

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind direction / °

Hei

ght /

km

AS

L

0 5 10 15 20

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind speed / ms−1

20 25 30 35 40 45 50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Potential temperature θ / °C

Hei

ght /

km

AS

L

0 5 10 15

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Mixing ratio / gkg−1

0 90 180 270 360

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind direction / °

Hei

ght /

km

AS

L

0 5 10 15 20

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Wind speed / ms−1

Figure 3.18: Same as Fig. 3.4, but for 12 July 2002. Innsbruck sonde launched at

1103 UTC and Oberschleißheim at 1047 UTC.

54

Figure 3.19: Satellite pictures from channel 1 of the AVHRR instrument on 12 July

2002 at 1309 UTC and 1557 UTC.

flow from 7 to 22 UTC. In the afternoon convection developed (Fig. 3.19) and in

the evening lightning was registered in the Northern Alps. Between 21 and 22 UTC

a shower was registered in Innsbruck.

At 11 UTC the mixed layer in Oberschleißheim (Fig. 3.17) was approximately

700 m deep and 4 K colder than the mixed layer at Innsbruck. At none of the ob-

servation times (soundings at 14 UTC and 17 UTC not shown) did Oberschleißheim

show northerly flow in the boundary layer, thus the mountain-plain circulation did

not reach that far to the north, although the vertical extent of the boundary layer

grew up to at least 1600 m ASL (from 14 UTC sounding).

The maximum pressure difference on 12 July between Munich and Innsbruck

was 2.8 hPa at 15 UTC, and thus was the largest during the IOPs in July 2002. The

pressure gradient between Munich and Garmisch-Partenkirchen was 1.1 hPa less, so

it can be assumed that the convergence zone of the thermal wind system is south to

Garmisch-Partenkirchen, maybe in the vicinity of Innsbruck or closer to the Alpine

main ridge.

55

Time / hours UTC

He

igh

t / m

AG

L

20 ms-1 225° SW

Figure 3.20: Same as Fig. 3.6, but on 12 July 2002.

Temporal evolution of Alpine pumping

In the boundary layer at Lichtenau, easterly winds prevailed throughout the day

(Fig. 3.20). Weak northerly wind components were measured between 10 and 21

UTC, except for a short period at 15 UTC with a pronounced flow towards the

Alps at 1000 m AGL. This was obviously the time when the thermal wind system

was best developed, as at 15 UTC also the pressure difference between Munich and

Innsbruck and the up-valley flow at Innsbruck University reached their maximum.

Spatial structure of Alpine pumping

The DLR Falcon flight took place at around 15 UTC, when Alpine pumping was well

developed. Cross sections of both flight legs are available (Fig. 3.21). Generally,

boundary layer flow towards the Alps is stronger in the southern cross section, and

in both cross sections it is more pronounced in the western parts. The negative v350-

56

components above 2000 m ASL, which are measured in the eastern parts of the cross

sections, but more developed in the northern one, are likely to be a dynamic feature,

not a thermal one, as this is already above the mixed layer, which is 1600 m ASL at

the 14 UTC Oberschleißheim radio sounding. Nevertheless this flow is included in

the computation of the mass flux as it is flow towards the Alps, too. In the northern

flight leg, v350-components up to 3 ms−1 are measured around 11.5 ◦E. Mean flow

towards the Alps was present up to 1600 m ASL, and a second layer extending up to

2600 m ASL. Maximum mean v350-component was 0.9 ms−1 at 1000 m ASL. In the

southern cross-section, flow towards the Alps was stronger, with v350-components

up to 5 ms−1 in the eastern part of the cross section at the Rhine valley exit and

in the Allgau, and up to 4 ms−1 around 11.5 ◦E. Mean inflow was measured up to

1800 m ASL, with a maximum of 1.6 ms−1 at 1200 m ASL.

Mass fluxes

Depth of the inflow layer was 1375 m in the eastern cross section, while it was 2125 m

in the northern one, as northerly flow was also measured above the boundary layer.

Specific mass flux towards the Alps was 1200 kgm−1s−1 in the southern cross section,

while it was 890 kgm−1s−1 in the northern one. This yielded divergence in the Alpine

foreland boundary layer which was likely to be compensated by subsidence. Mass

fluxes are comparable to those on 8 July and 9 July. But in contrast to these

cases the flight of the DLR Falcon on 12 July took place in the afternoon, when

the mountain-plain circulation was well developed. As pressure difference between

Munich and Innsbruck, and wind profiles at Lichtenau suggest, mass flux towards

the Alps was maximum at this time.

57

9.5 10 10.5 11 11.5 12 12.50

1000

2000

3000

4000

5000

6000

Longitude / °E

Hei

ght /

m A

SL

15 ms−1 southwest

−5

−4

−3

−2

−1

0

1

2

3

4

5

9.5 10 10.5 11 11.5 12 12.50

1000

2000

3000

4000

5000

6000

Longitude / °E

Hei

ght /

m A

SL

20 ms−1 southwest

−5

−4

−3

−2

−1

0

1

2

3

4

5

Figure 3.21: Same as Fig. 3.6, but for 12 July at 15 UTC. Top panel is northern

flight leg, bottom is southern one.

58

3.5 Mass fluxes and orography

Valley winds and large-scale slope winds contribute to the mountain-plain circula-

tion. Therefore the mass flux near the valley exits can be expected to be stronger

compared to mass fluxes towards mountain massifs. To investigate this hypothesis,

specific mass fluxes of the individual v350-profiles and their errors from all vertical

cross sections were computed (Fig. 3.22). Lacking additional data (e.g. vertical

mixing), the depth of the inflow layer was subjectively determined from the v350

cross sections. Often the thermally induced circulation in the boundary layer can

not be separated from the upper synoptic flow, as is the case for northern flight

leg on 12 July (Fig. 3.21). Profiles were discarded if no inflow towards the Alps

was present, or if values were missing in the inflow layer. Due to these criteria nine

profiles had to be ignored. Here height of the inflow layer was determined for each

profile individually, while in Section 2.4 a mean inflow layer height was determined

for the whole profile. Therefore total mass flux computed with the two different

methods is not the same.

In the western part of the northern cross section of 8 July 2002, the individual

specific mass fluxes were not larger at longitudes of valley exits. At Lech, Iller

and Isar, mass fluxes were even smaller than in between. Large mass fluxes at

the Inn valley cannot solely be interpreted as thermally induced, as flow towards

the Alps was also measured above boundary layer (Fig. 3.10), and such dynamic

features could not be clearly seperated from thermal circulation. Correlation in the

southern cross-section is better: mass fluxes are largest at Iller, Lech, Loisach and

Isar, with minima in between (profile west of Isar was discarded for individual mass

flux computation, but Fig. 3.10 shows weaker inflow). Inflow in the BL at the Inn

valley exit is also weak. This might be related to late onset of up-valley wind in the

Inn Valley at Innsbruck, which was at 11 UTC (mean onset in summer is at around

9 UTC (Vergeiner and Dreiseitl 1987)).

The southern cross-section on 9 July shows maximum mass fluxes at Isar and

Lech, and lower values at Iller. At the Loisach Valley exit the mass flux is small.

Weak mass fluxes at the Inn Valley exit might be associated with Fohn break-

through at Innsbruck and the corresponding blocking of the valley wind system.

59

0

2000

4000

6000

0

1000

2000

3000

08 July, north

0

2000

4000

6000

0

1000

2000

3000

08 July, south

0

2000

4000

6000

0

1000

2000

3000

09 July, south

0

2000

4000

6000

0

1000

2000

3000

12 July, north

0

2000

4000

6000

0

1000

2000

3000

12 July, south

10 10.5 11 11.5 12 12.50

1000

2000

Longitude / deg E

Sp

eci

!c

ma

ss "

ux

/ (k

gm

−1

s−

1)

In"

ow

laye

r d

ep

th /

mH

eig

ht

/ m

AS

L

Loisach InnIller Lech Isar

Figure 3.22: Upper five panels show individual specific mass fluxes (boxes) and

their errors (bars)—and inflow layer depth of the individual profiles

plotted against longitude. See Section 2.5 for computation of errors.

Bottom panel shows topography 15 km south of southern flight leg

with the names of the valley exits. Their location is marked with a

dashed line.

60

No significant large mass fluxes are measured around 11 ◦E, where convective cells

developed in the mountains later on.

Mass fluxes of the cross-sections on 12 July show a similar pattern as 8 July:

In the northern cross-section, mass fluxes are smaller at valley exits, and larger in

between. Mass fluxes in the southern cross-section are maximum at Lech, Loisach,

Iller, and east of Inn valley. The largest mass flux is the westernmost, which is

located near Lake Constance at Rhine valley exit.

While there seems to be a relation between mass fluxes in the southern cross

section and orography, in the sense of larger mass fluxes near valley exits, the height

of the inflow layer, as determined from the wind fields, does not show a regular

pattern.

3.6 Discussion

Six vertical cross-sections of horizontal wind field with corresponding mass fluxes

from four IOPs with Alpine pumping during the VERTIKATOR project were pro-

cessed. Table 3.1 gives an overview of the discussed cases, along with the mass

fluxes from Weissmann et al. (2005). 5 July is omitted in the table, as no mass flux

towards the Alps was measured, although it will be discussed in this section.

The estimated relative error for the evaluated mass fluxes is quite large (56 %

to 184 %), as measured wind components are in the same order of magnitude as

accuracy of the WIND Doppler lidar instrument, which is given by 0.5–1.5 ms−1.

Extrapolated values increase the error, as they are solely based on the bottommost

measured range gate and the theoretical assumption of a power-law shaped wind

profile in the lowest 125 m AGL. Relative errors of the mass fluxes computed by

Weissmann et al. (2005) for 19 July is only a third of relative errors computed in

this work. This is explained by the better accuracy of the 2µm Doppler Lidar, which

was assumed with 0.1 ms−1 for horizontal wind speed. Also the maximum relative

error for extrapolated values was assumed to be 50 %.

The similarity of the specific mass flux values on 8, 9 and 12 July is surpris-

ing. Nevertheless, no common meteorological parameters in these case studies can

61

8 July, 9 July, 12 July, 19 July ∗,

12–13 UTC 10 UTC 15 UTC 15–17 UTC

south north south south north south north

Weather situation Short wave ridge Southerly flow Southerly flow Weak gradient

∆pMUC−IBK,flight time 1.4 hPa 0.3 hPa 2.8 hPa 2.5 hPa∆pMUC−IBK,flight time

∆pMUC−IBK,max0.7 0.3 1.0 1.0

Inflow layer depth /m 1750 1500 1250 1375 2125

Length of flight leg

/km

185 189 195 189 150 212 223

Specific mass flux

/kgm−1s−1

1200 880 1100 1200 890 1850 2220

Specific water vapor

mass flux /kgm−1s−1

8.9 6.9 10.5 8.3 5.7

Total cross-section

mass flux /106 kgs−1

222 166 215 232 134 392 496

Total cross-section wa-

ter vapor mass flux

/106 kgs−1

1.7 1.3 2.1 1.6 0.86

Number of extrapo-

lated values

24 25 24 34 21

Relative error /% 68-158 77-163 56-126 63-129 95-184 18 18

Table 3.1: Overview of the estimated mass fluxes. ∗19 July case discussed by

Weissmann et al. (2005).

be found: Measurements were taken during different weather situations (short-wave

ridge and southerly flow) with individual flow structures, and varying pressure dif-

ferences between the plain and the mountains at the time of flight. Also the stage

of the mountain-plain circulation—in terms of time of flight, and also in terms of

relative pressure difference with respect to maximum pressure difference—was vari-

able in the case studies. Hence the similarity of the specific mass flux values are

assumed to be coincidental, and not a feature of Alpine pumping.

The mass fluxes are in the same order of magnitude as the ones discussed by

Weissmann et al. (2005), although they all were smaller, about 65 % and 40 %

of the mass fluxes on 19 July for southern and northern cross-section, respectively.

62

Following characteristics of the case studies can cause the observed differences:

3.6.1 Dependence on time of flight

Research flights on 5, 8 and 9 July were performed several hours before the climax of

Alpine pumping was reached. As pressure differences between plain and mountain

has grown later on, and also the flow towards the mountains measured by the WTR

profiler at Lichtenau has strengthened in all three cases, mass fluxes on these days

are expected to have grown until afternoon.

3.6.2 Dependence on weather situation

Lugauer and Winkler (2005) found from climatological analysis of surface observa-

tions, that for southerly synoptic flow, wind towards the Alps is weaker than during

weak gradient situations (Fig. 1.5). 12 July and 19 July research flights were both

performed during the mature phase of Alpine pumping, as identified from WTR

soundings at Lichtenau (Fig. 3.20 and Fig. 1.6). Mass fluxes on 12 July are only

65 % and 40 % of the mass fluxes on 19 July in the southern and northern cross-

section, respectively. This could be explained by different weather situations on

these days. 19 July was a weak gradient situation, the mountain-plain circulation

was able to shape up well, while on 12 July southern synoptic winds counteracted

on Alpine pumping, although no Fohn event was registered in Innsbruck. This

confirms the results of Lugauer and Winkler (2005), which found stronger daytime

surface flow in the Bavarian foreland during weak-gradient conditions, than during

southerly synoptic flow (Fig. 1.5). On 19 July Alpine pumping also reached as far

north as Oberschleißheim, while on 12 July northward extent was less. The conver-

gence zone on 19 July reached to the northern flight leg, while divergence was found

further north. On 12 July divergence was determined between the northern and the

southern flight leg. Southerly synoptic flow was also present on 5 and 9 July, with

Fohn break-through in Innsbruck on both days. Due to the early flight on 5 July,

when flow towards the Alps was only measured in a few range gates close to the

ground (Fig. 3.6), no statements on the vertical structure of Alpine pumping can

be made. On 9 July, when Fohn stopped up-valley flow in the Inn valley, also mass

63

0

500

1000

1500

2000

2500

5 July 8 July 9 July 12 July 19 July

Sp

ecif

ic m

ass f

lux / k

gm

-1s

-1

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

∆ p

/ h

Pa

Figure 3.23: Specific mass fluxes (boxes) along the southern (dark grey) and north-

ern flight legs (light grey) and pressure difference ∆pMUC−IBK at time

of flight (horizontal bars).

fluxes at the Inn valley exit are weaker compared to mass fluxes in the western part

of the cross section (Fig. 3.22).

3.6.3 Pressure difference as a measure for Alpine pumping

Fig. 3.23 shows specific mass fluxes and pressure difference between Munich and

Innsbruck for the case studies in this thesis. No quantitative correlation between

pressure difference and mass fluxes can be identified. The problem is that the

location of the thermal low pressure center is not fixed to Innsbruck (or Garmisch-

Partenkirchen), but depending on at least the weather situation (it is likely to depen-

dend on other factors as well, e.g. stratification of the mountain atmosphere, which

are not addressed in this thesis). While Munich can be used as a pressure reference

in the plain, Innsbruck does not necessarily represent the low pressure center. E.g.

on 9 July a pressure gradient across the Alps, with higher pressure in northern Italy

promoted Fohn break-through in Innsbruck, at the same time boundary layer air

was transported from the Bavarian foreland towards the mountains, by a thermally

64

induced pressure gradient. The resulting pressure difference ∆pMUC−IBK was thus

0.3 hPa, while mass flux was comparable to 8 July, when ∆pMUC−IBK was three times

higher. Pressure difference ∆pMUC−GAP was higher until afternoon, leading to the

conclusion that the convergence zone of the mountain-plain circulation is located in

the northern Alps, as already stated by Lugauer and Winkler (2005). Extracting

pressure differences from weather charts, e.g. VERA, would be a better method to

obtain pressure difference between plain and mountains.

65

Chapter 4

Summary

For this thesis, six vertical cross-sections from four different days with Alpine pump-

ing during the VERTIKATOR campaign were visualized, and corresponding mass

fluxes towards the Alps were estimated. For intercomparison of mass fluxes, the

parameter specific mass flux was introduced, which is independent of the length

of the cross section. Measured specific mass fluxes ranged from 880 kgm−1s−1 to

1200 kgm−1s−1, which are significantly weaker than in a previous study from (Weiss-

mann et al. 2005) for 19 July 2002. Possible reasons are early flight times on 5, 8 and

9 July, when Alpine pumping was not fully developed yet, and unfavorable weather

conditions on 12 July, where southerly synoptic flow counteracted the northerly

thermal wind from the Bavarian foreland towards the Alps. These observations are

in accordance with previous findings from Lugauer and Winkler (2005), that flow

towards the Alps is stronger during weak-gradient situations than during southerly

synoptic flow.

Mass fluxes of individual profiles from the lidar cross-sections along the southern

flight leg showed correlation with the orography: Mass fluxes were higher towards

major valley exits and lower towards mountain massifs. In the northern cross-

sections, this correlation was not observed.

Pressure difference between Munich and Innsbruck is not a good measure for

the mass flux towards the Alps, as the thermal pressure low is not always found in

the same location, depending on weather situation and other meteorological char-

acteristics. A better method would be to extract the maximum pressure variation

67

between plain and mountains from weather charts. But it is doubtful that such

an complex and variable phenomenon as Alpine pumping can be quantified by one

single numeric value.

68

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Acknowledgments

Many thanks go to Georg Mayr (University Innsbruck) as well as Martin Weissmann

and Oliver Reitebuch (DLR Oberpfaffenhofen) for supervising this diploma thesis.

Susanne Drechsel, Felix Schuller and Siegfried Vogt helped with providing me with

data. Thanks also to Jakob Abermann, Thomas Raab and Svea List for proof

reading.

I am deeply grateful for the first great 7 1/2 years in Innsbruck, which I spent

with my “fellow students”—especially Tom, Svea, Philip, Nitsch, Jakob, Gunther

and Esther—and my “cohabitants” in the Wetterherrenweg.

As I’m neither capable of drawing, nor of writing Latin, the realization of the al-

ternative Latin title page would not have been possible without the virtuosic work of

Klemens Maya (illustration—http://www.kmaya.de) and Dr. Florian Schaffenrath

(translation).

Thanks to Wolfgang Jentsch and the Climate & Weather working group of the

Deutschherren-Gymnasium in Aichach for awakening my interest in meteorology.

Last but not least I want to thank my parents for their support and their

patience during my years of study.

73

Danksagung

Als erstes mochte ich mich bei Georg Mayr von der Universitat Innsbruck und

bei Oliver Reitebuch und Martin Weissmann vom DLR Oberpfaffenhofen fur die

Betreuung dieser Diplomarbeit bedanken, sowie bei Felix Schuller, Susanne Drechsel

und Siegfried Vogt fur die Bereitstellung von Daten. Svea List, Jakob Abermann

und Thomas Raab haben mir beim Korrekturlesen geholfen.

Besonders dankbar bin ich meinen “StudienkollegInnen”—vor allem Esther,

Gunther, Jakob, Nitsch, Philip, Svea und Tom—sowie meinen “Mitbewohnern” aus

dem Wetterherrenweg fur die schonen ersten 7 1/2 Jahre in Innsbruck

Weil ich weder Zeichnen noch Latein kann, ware ohne die liebevolle Arbeit

von Dr. Florian Schaffenrath (Ubersetzung) und Klemens Maya (Illustration—

http://www.kmaya.de) die Umsetzung des alternativen Deckblatts niemals moglich

gewesen.

Danke auch an Wolfgang Jentsch und den Arbeitskreis Klima/Wetter vom

Deutschherren-Gymnasium Aichach, die mein Interesse fur die Meteorologie geweckt

haben.

Schließlich mochte ich mich bei meinen Eltern fur die Unterstutzung wahrend

meiner ganzen Studienzeit und vor allem fur ihre Geduld bedanken!

75

Curriculum vitae

Martin Großhauser

Born: 22 April 1980 in Augsburg, Germany

Education

1990–1999 High school Deutschherren-Gymnasium Aichach, Germany.

Abitur.

2001–2009 Diploma study at the University of Innsbruck. Master of

Natural Science (Magister rerum naturalium) in Meteorology.

Scientific and academic experience

2003 Participation in CARBOMONT project at the University of

Innsbruck: Processing of eddy covariance measurements.

2004 Internship at DLR Oberpfaffenhofen: Processing of Doppler

lidar data from THORPEX project.

2007–2008 Field work at COPS campaign and glacier mass balance mea-

surements at the University of Innsbruck.

2006–2009 Tutor at the University of Innsbruck in FORTRAN and nu-

merical methods lectures.

77