the planetary boundary layer in complex terrain john horel noaa cooperative institute for regional...
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The Planetary Boundary Layer in Complex Terrain
John HorelNOAA Cooperative Institute for
Regional PredictionDepartment of Meteorology
University of [email protected]
Photo: J. Horel
What is CIRP? CIRP: NOAA Cooperative Institute for Regional Prediction
at the University of Utah Mission: Improve weather and climate prediction in regions
of complex terrain People:
Staff: John Horel, Jim Steenburgh, Mike Splitt, Judy Pechmann, Will Cheng, Bryan White, Brian Olsen
Students: Justin Cox, Jay Shafer, Ken Hart, Dave Myrick, Dan Zumpfe, Erik Crossman, Greg West
References •Barry, R., 1992: Mountain Weather and Climate. Rutledge•Blumen, W., 1990: Atmospheric Processes Over Complex Terrain. American Meteorological Society, Boston, MA.•Clements, C., D. Whiteman, J. Horel, 2003: Cold pool evolution and dynamics in a mountain basin. J. Appl. Meteor., 42, 752-768.•Garratt, J., 1992: The Atmospheric Boundary Layer. Cambridge•Horel, J., M. Splitt, L. Dunn, J. Pechmann, B. White, C. Ciliberti, S. Lazarus, J. Slemmer, D. Zaff, J. Burks, 2002: MesoWest: Cooperative Mesonets in the Western United States. Bull. Amer. Meteor. Soc., 83, 211-226. •Kalnay, E., 2003: Atmospheric Modeling, Data Assimilation and Predictability. Cambridge•Kossmann, M., and A. Sturman, 2003: Pressure-driven channeling effects in bent valleys. J. Appl. Meteor., 42, 151-1158.•Lazarus, S., C. Ciliberti, J. Horel, K. Brewster, 2002: Near-real-time Applications of a Mesoscale Analysis System to Complex Terrain. Wea. Forecasting. 17, 971-1000.•Stull, R. B., 1999: An Introduction to Boundary Layer Meteorology. Kluwer•Whiteman, C. D., 2000: Mountain Meteorology. Oxford•Zhong, S. and J. Fast, 2003: An evaluation of the MM5, RAWMS, and Meso-Eta Models at Subkilometer resolution using VTMX field campaign data in the Salt Lake Valley. Mon. Wea. Rev., 131, 1301-1322.•Notes: Summer School on Mountain Meteorology 2003. http://www.unitn.it/convegni/ssmm.htm
Outline
Part I- Characteristics/impacts of complex terrain Part II- Resources for observing surface weather Part III- Basin boundary layer Part IV- Mountain-valley and lake breezes
Field Programs CASES-99 Cooperative Atmosphere-Surface
Exchange Study. Kansas. Poulos et al., 2002: BAMS, 83, 555-581.
MAP Mesocale Alpine Program. Alps. Bougeault et al., 2002, BAMS, 82, 433-462.
VTMX Vertical Transport and Mixing Experiment. Salt Lake Valley. Doran et al. 2002, BAMS, 83, 537-551.
PBL IssuesVTMX Science Plan: Measurement and modeling of vertical transport and mixing processes in the
lowest few kilometers of the atmosphere are problems of fundamental importance for which a fully satisfactory treatment has yet to be achieved
Although a general theoretical understanding of many of the physical phenomena relevant to vertical transport and mixing processes exists, that understanding is incomplete, the representation of various phenomena in models is often poor, and the data needed to test those models are lacking.
The upward and downward movements of air parcels in stable and residual layers of the atmosphere and the interactions between adjacent layers are particularly difficult processes to characterize, and significant difficulties also exist in describing the behavior of the atmosphere during morning and evening transition periods.
Complications due to heterogeneous land surfaces and complex terrain further compromise our ability to treat vertical transport and mixing processes properly.
www.pnnl.gov/vtmx
VTMX Science Questions
What are the fundamental processes that control vertical transport for stable and transition boundary layers?
How can momentum, heat, and moisture fluxes be modeled and predicted in a stratified atmosphere with multiple layers?
What improvements in numerical simulations and forecasts of vertical transport and mixing during stable and transition periods are feasible and how can they be implemented?
What formulations are most appropriate for the description of vertical diffusion in stable air? For example, how rapidly will an elevated layer of pollutants mix towards the ground in a stable pool trapped within a basin, and how can that mixing be modeled?
What is the sensitivity of current local weather forecast and dispersion model predictions to variations in the treatment of vertical diffusivity and turbulence?
What limits our ability to forecast vertical transport in current numerical prediction models?
How do traveling weather systems remove stable stagnant air out of a basin, and under what conditions do these removal mechanisms fail?
What is the nature of the interaction of terrain-induced flows (e.g., drainage winds at night, upslope winds during the day, and waves) with cold air pools in basins, and how do such flows affect the formation and erosion of those pools and the dispersion of pollutants in them?
What are the effects of complex terrain?
Substantial modification of synoptic or meso scale weather systems by dynamical and thermodynamical processes through a considerable depth of the atmosphere
Recurrent generation of distinctive weather conditions, involving dynamically and thermally induced wind systems, cloudiness, and precipitation regimes
Slope and aspect variations on scales of 10-100 m form mosaic of local climates
(Barry 1992)
Effects of Complex Terrain
Carruthers and Hunt 1990
Billiard ball analogy “If the earth were greatly reduced in size while maintaining its shape, it
would be smoother than a billiard ball”. (Earth radius = 6371 km; Everest = 8.850 km)
Nonetheless, mountains have a large effect on weather. Why is this, if they are so insignificant in size?
Answer: the atmosphere, like the mountains, is also shallow (scale height 8.5 km) so mountains are a significant fraction of atmos depth.
But, this answer underestimates mountain effect for two reasons: Stability gives the atmosphere a resistance to vertical displacements The lower atmosphere is rich in water vapor so that slight adiabatic ascent
brings the air to saturation.
Example: flow around a 500-m mountain (<< 8.5 km) could include 1) broad horizontal excursions, 2) downslope windstorm on lee side, and 3) torrential orographic rain on windward side.
Smith (1979)
Distribution of mountains on the globe (Barry 1992)
Elevation range Mountains (106 km2) Plateau (106 km2) Mountains/Land Surface (%)
>3000 m 6 --- 4.0
2000-3000 m 4 6 2.7
1000-2000 m 5 19 3.4
0-1000 m 15 94 10.1
Total 30 119 20.2
Total land surface is about 149 million km2. Oceanic islands covering 2 million km2 are not included in the listed areas. Plateau & mountains are both included in the table’s 1st line.
Louis (1975)
Energetic Considerations
Since the atmosphere is heated mainly from the ground, cooling effect upon earth’s surface of latent and sensible heat fluxes is nearly double that of radiative fluxes
Since much of the land surface is hilly, thermally driven circulations play important role in global energy balance
F. Fiedler. Summer School Trento
Chen, C.-C., D. Durran and G. Hakim(2003) ICAM
Surface Wind and Vorticity Around Isolated Mountain:Interaction with Large-scale flow
Potential Temp, Vertical Velocity, and Turbulent Mixing
Chen, C.-C., D. Durran and G. Hakim (2003) ICAM
Energy and mass exchanges near ground---interactions among soil science, hydrological cycles (ground and air), ecosystems, and atmosphere.•Canopy •Terrain•Heterogeneous surfaces•Clouds/fog•Urban environment, air pollution
Heig
ht
(m)
Planetary boundary layer
1 km
D. Lenschow
Shallow Drainage Flows – Mahrt, Vickers, Nakamura, Soler, Sun,Burns, & Lenschow – BLM, 101, 2001.
Schematic cross-section of prevailing southerly synoptic flow, northerly surface flow downThe gully, and easterly flow likely drainage flow from Flint Hills. Numbers identify theSonic anemometers on the E-W transect. E is to the right and N into the paper.
Pollutant Transport in Valleys
Savov et al. (2002; JAM)
Nighttime Stable Layer in Valley
After Breakup of Nighttime Stable Layer in Valley
Daytime vertical mixing processes
Jerome Fast
Diurnal mountain wind systems
Whiteman (2000)Whiteman (2000)
Mountain-plain circulation, Rocky Mountains
2 m s-1
A A'
AZ
WS
MB
PV MC FBLT WC
a)
PV
MC
FBLT WC
A A'
b)
00 UTC/18 CST
00 UTC/18 CST
2 m s-1
0
1
2
3
4
5
6
US radar profiler network, 1991-1995, Jun-Aug, 500 m gate, max=3.5 m/s US radar profiler network, 1991-1995, Jun-Aug, 500 m gate, max=3.5 m/s
Whiteman and Bian (1998)Whiteman and Bian (1998)
Alpine pumping
Mountain-plain circulation in Alps (Vertikator)
Innsbruck
Boundary of Alpine pumpingsynoptic conditions modify shape 100 km
Munich
MilanTurin
Zürich
Graz
Emissions within the area of Alpine Pumping are transported
into the Alps and mixed convectively to higher levels
Lyon
Lugauer et al. (2003)
Mountain venting, anti-slope flow
Local Time (PDT)
0
100
200
300
400
500
600
700
800
900
1000
Heig
ht A
bove
Gro
und (
m)
0900
3 m/s
CBL Height from Lidar
Vertical cross-section of slope flow (upslope to the right)
25 July 200125 July 2001
ReutenReuten et al.( 2002) with Steyn et al.( 2002) with Steyn
Valley cross sections
Whiteman (2000)
Whiteman (1980)Whiteman (1980)
temperature temperature and wind and wind structure structure layers at a layers at a time midway time midway through the through the transitiontransition
Channeling of synoptic/mesoscale winds
Forced ChannelingForced Channeling
Pressure Driven ChannelingPressure Driven Channeling
Whiteman (2000)Whiteman (2000)
Bent valley with 45° changes in wind direction above valley
Kossmann & Sturman (2003)
Dynamic Channeling
Kossman and Sturman 2003
Western U.S. Terrain(high- dark;low-light)
High terrain (dark)
Flat (tan)Mtn. Valleys
(light)
A. Reinecke
Normalized surface-layer velocity standard deviations for near neutral conditions in the Adige Valley in the northern Italy alpine region. a is from Panofsky and Dutton, 1984; b the average valuesfrom MAP; e/u*
2 is the normalized turbulence kinetic energy (From de Franceschi, 2002).
σu /u* σv /u* σw /u* e /u*2
Flat uniform terrain
2.39 1.92 1.25 5.48
Rolling terrain
2.65±4.50 2.00±3.80 1.20±1.24 6.23±18.11
Along valley 2.19 2.13 1.55 5.88
D. Lenschow
West DEM Grid Points vs. MesoWest Stations
0
10
20
30
40
50
60
-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12
Valley Flat Mountain
% o
f T
otal
Green-WestBlue-MesoWest
Adding Physiographic Information to MesoWest
Land Data Assimilation Systems
(LDAS)
UMD Vegetation Types
Exposure? Forested? Nearby Water? Mountain/Valley? Urban? Slope? Aspect?
MesoWest land characterization
* Sites located disproportionately in urban areas and near water resources.
-100
0
100
200
300
400
500%
diff
eren
ce fr
om W
est
Diurnal Temperature Range
A. Reinecke
Diurnal fair weather evolution of bl over a plain
Whiteman (2000)Whiteman (2000)
surface →
layer
mixed →
layer
free →
troposphere
D. Lenschow
D. Lenschow
D. Lenschow
D. Lenschow
Diurnal evolution of the convective and stable boundary layers in response to surface heating (sunlight) and cooling.
D. Lenschow
Atmospheric structure evolution in valley terrain
Whiteman (2000)Whiteman (2000)
Roughness Effects
For well-mixed conditions (near neutral lapse rate) U2 = u1 ln (z2/zo)/ln(z1/z0)
Roughness length zo=.5 h A/S where h height of obstacle, A- silhouette area, S surface area A/S< .1
Zo- height where wind approaches 0
Roughness lengths zo for different natural surfaces (from M. de Franceschi, 2002, derived from Wieringa, 1993).
zo (m) Landscape Description________________________________________________________________0.0002 Open sea or lake, tidal flat, snow-covered plain, featureless desert, tarmac, concrete with a fetch of several km.0.005 Featureless land surface without any noticeable obstacles; snow covered or fallow open country0.03 Level country with low vegetation and isolated obstacles with separations of at least 50 obstacle heights0.10 Cultivated area with regular cover of low crops; moderately open country with occasional obstacles with separations of at least 20 obstacle heights0.25 Recently developed “young” landscape with high crops or crops of varying height and scattered obstacles at relative distances of about 15 obstacle heights0.50 Old cultivated landscape with many rather large obstacle groups separated by open spaces of about 10 obstacle heights; low large vegetation with with small interstices1.0 Landscape totally and regularly covered with similar sized obstacles with interstices comparable to the obstacle heights; e.g., homogeneous cities
Effects of irregular terrain on PBL structure
Flow over hills (horizontal scale a few km; vertical scale a few 10’s of m up to a fraction of PBL depth)
Flow over heterogeneous surfaces (small-scale variability with discontinuous changes in surface properties)
Inner layer – region where turbulent stresses affect changes in mean flow
Outer layer – height at which shear in upwind profile ceases to be important
(Kaimal & Finnigan, 1994).
(Kaimal & Finnigan, 1994).
(Kaimal & Finnigan, 1994).
D. Lenschow
Effects of horizontal heterogeneity in surface properties
Changes in surface roughness Rough to smooth Smooth to rough
Changes in surface energy fluxes Sensible heat flux Latent heat flux
Changes in incoming solar radiation Cloudiness Slope
Summary- Impacts of Complex Terrain
Terrain affects atmospheric circulation on local to planetary scales
Terrain induced eddies modify and contribute to the vertical and horizontal exchange of mass, temperature, and moisture in a much stronger manner than turbulent eddies over flat terrain
Photo: J. Horel
Problems and possible future directions
Most theoretical, modeling and observational results are applicable to a horizontally homogeneous PBL and underlying surface.
Non-uniform surfaces predominate over land. New tools are needed and are becoming available to
address PBL structure over heterogeneous terrain.
D. Lenschow