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OROGRAPHIC INFLUENCE ON URBAN CLIMATE

INTRODUCTION

The aim of this review paper is to evaluatehow processes, which are mainly induced byorographical forcing, may effect the urbanclimate. I t represents the results of discus-sions between urban climatologists and land-use planners, and concentrates therefore, onthe processes which have a strong effect onthe urban atmosphere.

Orography influences the atmosphere andatmospheric motions by providing sources (orsinks) of sensible and latent heat. by deflect-ing the airflow horizontally and vertically andby enhancing the frictional dissipation (ICSU-WMO, 1982).

An urban land surface is normally rougher,warmer and drier and produces much moreair pollutants than a rural one. Looking atheat f lux anomalies and a t the roughnesseffects, i t can be stated that mountains andbuildings cause similar modifications of theoverlying atmosphere. However, one charac-teristic difference consists in the scale whichis affected by mountains or urban areas. Fig.1 and Table I show that interactions between

Heinz Wanner and Paul RingerInstitute of Geography

Bern, Switzerland

ABSTRACT

Urban induced meteorological phenomena are normally restricted to time and lengthscales between a single roughness element or heat source (micro-scale ) and a largemetropolitan area (meso-scale a). For this reason, orographically induced phenomenapertaining to these scales exert the most important influence upon the four relevantbudgets (energy, water, air contaminants and momentum) of an urban air volume.

A comparison of both orographically induced meteorological phenomena and urbanbudgets leads to four processes. or process groups, being important for the urbanclimate: Air deflection and channelling in broad valleys. Strong downslope winds on theleeside foothills of high mountain chains. Orographically induced precipitation in highslope areas and on mountain tops, and finally, the most important phenomena, thethermotopographic wind systems. These winds interact in a complex manner with thegeostrophic wind and the various thermal and mechanical effects caused by the urbanland surface.

4 10'

2000

200

20

2

0.2

0,02

Weather and Climate (1989) 9: 22-28

0minutes ; h o u r s

Tenn scate

days /months

Fig. 1. Typical time and length scales o f atmosphericprocesses (after Orlansky 1975). The box denotes the areaof the scales which are relevant to urban climatology (seealso Table 1).

the mountains and the atmosphere occur onabout every scale, because the mountains areable — due to their mass and elevation — toinfluence the whole troposphere.

Urban Climate

On the other hand, the urban induced clima-tological modifications a re l imited t o therange between about a single roughness ele-ment or heat source (micro-scale f l ) and alarge metropolitan area (meso-scale a). Con-sequently, the orographic elements corre-sponding to these scale are most important,because they are uniquely able to generatemeteorological phenomena which are spatial-ly restricted to a single urban area o r anurban district.

TABLE 1. TYPICAL SCALES AND ASSOCIATED ORO-GRAPHICALLY INDUCED PHENOMENA

Macro-scale— Modification of planetary waves;— generation of anomalies in the angular momentum

flux;— causation of heat flux anomalies;— generation of baroclinic fields and waves.

Meso-scaleMeso-scale a:

— Upstream blocking and flow splitting;— cold air deflection and deformation of fronts;— lee cyclogenesis— mountain-plain circulation systems.

Meso-sca let3— Mountain drag effect, blocking and channeling of air

flow;— lee waves and strong downslope winds;— mountain generated disturbances;— modification o f the radiation budget due to the

elevated horizon;— (mountain and) valley winds;— generation of nocturnal low level jets.

Meso-scale a:— Channeling and air deflection;— mountain induced thunderstorms;— valley and slope winds (anabatic/katabatic flow).

Micro-scale— Thermal and mechanical forcing through small

valleys and hills (turbulence, convection, short gravitywaves, thermal wakes).

A number of papers, books and review arti-cles deal with general aspects of mountainmeteorology and climatology, or orographicalprocesses (e.g. Yoshino, 1975 and 1981; Smith,1979; Atkinson, 1981; Barry, 1981; ICSU-WMO,1986; Sturman, 1987). Only a few articles existwhich t ry to combine mountain and urbanmeteorology and climatology, because the in-teractions between the different scales andprocesses are very difficult to recognize andand t o explain (Goldreich, 1984; Wanner,1984).

23

This paper describes mechanical and ther-mal effects of the orography upon the urbanatmosphere. I t does not include the influenceof variable land use or surface cover, such asthe different thermal responses of water sur-faces, forests and buildings, o r small-scalefrictional effects. Af ter discussing how theorography influences the different budget pa-rameters of the urban climate, the most im-portant orographic effects are described.

FOUR IMPORTANT URBAN BUDGETS

In the urban soil-building-air volume, fourimportant climatological budgets are influ-enced by orographically induced meteorologi-cal phenomena:

The energy budget,Q* + QF Q n + Q s + ( ) A P/M-1is influenced by virtually al l orographic ef-fects including shadowing, local airstreamsand precipitation.

The water budget, f i rst of two importantmass budgets,

p F + I = E r S [ m m ]is mostly influenced by orographic precipita-tion (symbols according to Oke, 1978).

The second important mass budget, the bud-get of air contaminants or pollutants , esti-mates the mean concentration of these a i rcontaminants or pollutants (g m-') in a station-ary flow through a box of urban air:

S - Px — [ g m - 3 ]

b.h*.0S: Sources, emitting pollutants (g s-1)P: Sinks of pollutants (g s-1)b: Width of the observed air volume (m)h*: Mixing height (m)u: Mean horizontal wind speed (M s-1)

This budget is primarily influenced by thediurnal wind systems.

The fourth important budget, the momen-tum budget, i s very difficult to determine,especially in a small air parcel over a non-homogeneous urban area. Nevertheless it canbe stated that this budget is affecdted by allairflow phenomena, like strong gravity wavegenerated downslope winds or diurnal windsystems (Stull, 1988).

24 U r b a n Climate

Orographic U r b a n budgetsinduced phenomena

Energybudget

Budget ofair pollut.

Momentumbudget

Waterbudget

1. Predominantly mechanical

(1) i

Air deflectionand channeling

1.1 Channeling, airdeflection (micro- 2scale • N

1.2. L e e waves, strong tdown-slope winds

I ( l e e )1.3. D r a g , upstreamblocking, air de- 1Section (torso-scale) 9,

1.4. Deformation of fronts, r ilee cyclogenesis

(2)Strong downslope

winds

i(3)

Storms andgeneration oforographic in-duced preci-pitation maxi-2. Predominantly thermal

2.1. Mountainlalaincirculation 5

2,2.. Modification of theradiation budget

2.3. Va l l e y winds, noctur-nal low level jets 2

2.4. S lope winds I

(4)

Influenceof diurnalwindsystems

ma and minimami

OROGRAPHIC CLASSIFICATION OF URBAN SITES

In a next step it seems appropriate to clas-sify the urban sites in relation to the surround-ing orographic elements or features (Fig. 2).This classification has been sketched withoutconsidering the thermal or mechanical influ-ence of the different land-use categories likeforests, fields or water surfaces.

In addition, i t has also to be emphasized,that only a few smaller towns can be associat-ed with one single orographical configuration.Larger urban areas which are situated near orwithin mountains, normally show a combina-tion of different orographic elements like aplateau with a small h i l l o r a slope wi thseveral valleys.

OROGRAPHICALLY INDUCED EFFECTS ON URBANCLIMATE

After describing the orographically inducedmeteorological phenomena and the importanturban budgets, these two fields have to beconsidered together with the goal to define theprocesses and impacts which are importantfor the urban climate. This idea is realized inFig 3. The abscissa shows the four budgetsand along the ordinate, the orographic phe-nomena are represented. Looking at the corre-lation between these different budgets and

Fig. 3. Orographically induced phe-nomena, urban climatological bud-gets and related processes which are

important for the urban climate.

1. Orographic element ? urban area

1.1. Mountain top or ridge 1 .2 . Plateau

1.3. Slope

1.5. Valley

1.7. Slope base

N I i i2. Orographic element < urban area

21. Valley within 2 . 2 . Small mountain or hillurban area w i t h i n urban area

• -11-, tot-

\ • i l l1.4. Slope terrace

\ I lo I

1.6. Basin

1.8. Passage moan -ain — plain

1,0Fig. 2. Orographic classification of urban sites.

Urban Climate

phenomena, four key processes can be de-duced, which are mainly important for theurban climate. In the following sections, eachof t hese processes i s descr ibed a n ddocumented with a figure.

Air deflection and channelingMany metropolitan areas in the world are

situated on a plain surrounded by mountainsor in broad valleys. Fo r that reason i t i simportant to study how the air is being de-flected or channeled along ridges or withinthese valleys. Fig. 4 shows how a geostrophicwind over the northern hemisphere i s de-flected anticlockwise because of the decelera-tion of the flow within the valley, due to thelarger cross section through which the a i rhas to stream (Gross and Wippermann, 1987).Because the air is forced to flow parallel tothe side-walls, the large-scale pressure gradi-ent along the valley governs the wind withinthe surface layer. In the case of a geostrophic

25

wind from NW (C) or SE (F) a countercurrentforms, especially when the air within the val-ley is stably stratified.

This paper does not include other effects ofchanneled airflow like small-scale accelera-tions or the generation o f supergeostrophicwinds and low level jets in broad valleys orover the mountain foreland (Weisel, 1986).

It is evident that channeling effects influ-ence the energy, as well as the air pollutantand momentum budget, through the increasedvertical fluxes and the stronger advection.With regard to the air pollutant budget, it hasto be emphasized that a rotation of the geo-strophic wind may easily cause a 180' switchof the surface wind within the valley and leadto a backtransport of air masses which werepolluted by the same urban area some hoursbefore.Strong downslope winds

Strong downslope winds are highly related

F

Fig. 4. Channeling and countercurrent in a broad north-south orientated valley of the northern hemisphere (modifiedafter Gross and Wippermann 1987).

26

to the existence of lee waves (gravity waves).Smith (1986) points to the fact that differentcross-mountain flows show similar character-istics with a stagnant area aloft, which inter-acts with the very turbulent, descending flow

Fig. 5. Three cross-mountain flows with strong downslopelee winds (after Smith 1986).(a) shallow foehn with blocked cold a i r on the upwind

(lull) side and a flow aloft parallel to the range (0)(b) bora with reversed flow aloft(c) Boulder windstorm with uniform flow upstreamA. Stagnant area aloft.

in the lee of the mountain. The stagnant areahelps to decouple the upper air stream from

Urban Climate

this f l ow and favours f ina l ly the strongdownslope winds like the foehn, the bora andthe well known Boulder windstorms (Fig. 5).

Urban areas are often situated on the foot-hills o f high mountains where the strongdownslope f l o w fo rms a hydraulic j u m p(bora, Boulder windstorms) or glides up thecold air layer of the adjacent plain, basin orplateau (foehn over t h e northern Alp ineforeland).

It is evident that each of the four urbanbudgets is influenced by such strong windsappearing near the surface, though the mo-mentum budget is most affected.

Orographically induced precipitationBesides the precipitation forming processes

in connection with lee cyclogenesis, which is ameso- a —scale phenomenon and thereforeless important for a single town, the genera-tion of orographically induced precipitationhas to be considered. Fig. 6 shows three im-portant mechanisms of control for orographicprecipitation (Smith, 1979). I t demonstratesclearly that urban sites in higher slope areas,or on mountain tops, are most affected by

Fig. 6. Three mechanisms of orographic control of precip-itation (after Smith 1979)(a) larger scale upslope precipitation(b) enhancement o f rainfall over small hills ("seeder-

feeder cloud mechanism"; Bergeron 1965)(c) orographic control of cumulonimbus formation in a

conditionally unstable airmass

orographic rain. In comparison with the stud-ies dealing with urban temperature and windfields, only a few papers have been concernedwith the orographic dependence of rainfall inurban areas (e.g. Huff et al., 1975; Huff andVogel, 1978).

Urban Climate

Orographically induced diurnal wind systemsThere i s n o d o u b t t h a t t h e

thermotopographic induced diurnal wind sys-tems are the most important meteorologicalphenomena caused by the orography, whichmay influence urban climates. Winds of dif-ferent scales (slope and valley winds, moun-tain-plain circulation systems) interact notonly with the geostrophic wind but also withthe various thermal and mechanical effectscaused by the urban land surface.

The hypothetical model in Fig. 7 representsa geostrophic wind (Vg) flowing towards anurban area situated near a mountain slope.During the night, the urban plume is shallowand the air pollutants are concentrated withinthis stable boundary layer. The wind profile ismodified by the roughness of the urban area.Shallow katabatic winds from the adjacent

0 . 3 -0.5 km

1 km

slopes or valleys interact with the geostrophicwind and penetrate into the urban area. Dur-ing the day, the synoptic wind is modifiedthrough thermal and mechanical forcing fac-tors caused by the urban area and the orogra-phy. Wi th in t h e w e l l m ixed convectiveboundary layer, convection cells form andanabatic winds lead to a transport of pollutedair towards the elevated areas.

In general, anabatic and katabatic windscause a modification of the budgets of energy,air pollutants and momentum. The coolingand cleaning effect o f nighttime drainagewinds is especially important in regard t ourban planning.

CONCLUSIONS

Mountains affect the overlying atmospherethermally and mechanically. The influence of

27

Fig. 7: Hypothetical model o f the interactions between the geostrophic wind (Vg), urban induced meteorologicalphenomena and slope winds.

(a) During a midsummer night ( b ) During a midsummer day

28

orographically induced meteorological phe-nomena on the urban atmosphere i s mostpronounced in the scale which corresponds tothe range between large metropolitan areas(meso- ) and single land-use elements (mi-cro-p). Although four important mountain-in-duced processes have been descr ibedseparately i n t h i s paper, t h e rea l i t y i scharacterized by the combined interaction ofdifferent scales and phenomena.

There is no opportunity to discuss corre-sponding results or working methods in thispaper. Nevertheless, the most important andcomplex phenomenon has to be mentioned. I tconcerns the interactions between synopticeffects (gradient wind), orographic inducedphenomena, and surface cover (say "urban")induced meteorological processes. In consid-ering simultaneously, the influence of the cy-cle o f daytime radiational warming andnighttime cooling, very complicated struc-tures and processes and created. These aremanifested in the complex data that are rou-tinely collected and interpreted, a l l aroundthe world.

ACKNOWLEDGMENT

The authors are grateful to; the Swiss Na-tional Science Foundation (NRP 14) for thefinancial support; U. Neu and M. Leibundgutfor their help in typing and drawing, and tworeviewers for their valuable comments on thetext.

Urban Climate

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en, London, 313 pp.Bergeron, T. , 1965: On the low-level redistribution o f

atmospheric water caused by orography. Suppl Proc.Int. Con!. Cloud Phys., Tokyo, 96-100.

Goldreich, Y. 1984: Urban topoclimatology Progress inPhys. Geogr. 8, 336-364.

Gross, G. and F. Wippermann, 1987: Channeling andcountercurrent in the Upper Rhine Valley: Numericalsimulations. J. Climate and AppL Meteor. 26, 1293-1304.

Huff, F. A. and J. L. Vggel, 1978: Urban, topographic andPrecipitation increases in the low hills of southernIllinois: Part 1. Climatic and network studies. Mon.Wea. Rev. 103, 823-829.

Huff, F. A. and J. L. Vogel, 1978: Urban, topographic anddiurnal effects on rainfall in the St. Louis region.Appl. Meteor. 17, 565-577.

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Smith, R. B., 1979: The influence of mountains on theatmosphere. Adv. in Geophysics 21, 87-230.

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