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Urban Air Quality : Meteorological Processes

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Definition of the Subject

Meteorological processes in urban areas that are relevant to urban air quality. Of most significance are the impacts of the urban morphology on the mean flow and turbulence, which determines the transport and dispersion of pollutants and therefore their concentration.


Concentrations of pollutants within an urban area depend on a number of different factors. These include the emissions of pollutants within the urban area, pollutant concentrations transported into the area, and the meteorology within the urban area, in particular, the mean airflow and turbulence (which determines the movement and mixing of the emitted pollutants) and the temperature and solar insolation (which impacts on chemical transformation taking place). This entry discusses meteorology within the urban area with the focus being on the mean flow and turbulence, as these may be substantially different from the upstream flow. Discussed here are the current understanding,...

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Scale of weather systems smaller than synoptic scale but larger than microscale or urban scale; tens to hundreds of kilometers.

Neighborhood scale:

Scale typical of groups of buildings or streets; hundreds of meters to a few kilometers.

Building and street scales:

Scales of buildings or streets; tens to hundreds of meters.

Fully computational model (FCM):

A model which explicitly represents flow and turbulence around buildings.

Fast approximate model (FAM):

A model which uses approximations and parameterizations of the fine scale flow to speed up model run times and reduce complexity.


The volume fraction of air between buildings and therefore a measure of building density.

Urban air quality:

A general term representing concentrations of pollutants in an urban area. Good air quality corresponds to low concentrations of pollutants.

Urban meteorology:

Meteorology within an urban area; the urban environment significantly affects mean flow turbulence and temperature.


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Correspondence to David Carruthers .

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Editors and Affiliations



Building breadth


Measured/calculated concentration


Drag coefficient

c +

Normalized mean concentration for a line source


Gap or separation distance between buildings


Coriolis frequency


Froude number

F v

Surface water vapor flux

F θ



Gravitational acceleration


Boundary layer height


Building height

H c

Canopy height

H c

Standard deviation of canopy height, Hc


Mountain height


Normalized mean concentration for a point source


Kinetic energy and energy-dissipation model of turbulence


Vertical length scale of internal layer

l O , l N

Vertical length scales of internal layers over the urban area, neighborhood scale


Adjustment length for mean flow to adjust as it enters the porous canopy


Building length

L f

Coriolis advection length


Inner city length scale

L M , L N , L BS

Length scales of the (sub-)regions M, N, BS


Overall city length scale

L Ro

Rossby length scale

L Se

Effective source size


Buoyancy frequency


Hit rate test score


Emission rate


Fractional deviation


Ratio of the length of the sub-regionL M to the smallest scales resolved in that region


Distance to the nearest building

\( {u_*} \)

Friction velocity of the turbulent velocity profile of the atmosphere


Mean velocity, with subscripts denoting location/physical process


Typical wind speed associated with local buoyancy effects


Wind speed within the canopy


Geostrophic wind


Mean wind above the buildings (at height H)

U ref

Reference velocity

U c

Mean wind along the street canyon


Mean wind along street


Building length or width


Absolute deviation, Building width

x = (x, y, z)

Coordinates of a point

x B (i), y B (i)

Coordinates of staggered building i

x s , y s , z s

Coordinates of source

y c (x)

Streamline through source at xs, ys, zs

z 0 (x, y)

Roughness length for wind profile

z d

Displacement height for logarithmic wind profile

z S (x, y)

Surface elevation of the ground

Z s

Source height

Z *

Height of top of shear layer above buildings


“Porosity” of an urban canopy [β ∼ bw/d2]


Mean temperature

θ s

Surface temperature


Von Karman’s constant

λ p

Planar area index

λ f

Frontal area index

σ u , σ v , σ w

R.m.s velocity components (of the order of \( {u_*} \))


Angle between wind direction and normal direction to a street (Figs. 3 and 6), i.e., φ = 90° if wind is along the street.




Building/Street scale




Cloud concentration






At top of buildings/canyon




Neighborhood scale


Overall urban area




Surface, street




Effective source


Turbulence-related level for log profile, or turbulent source


Building/street sub-region


Computational fluid dynamics


Fast approximate model


Fully computational model


Large eddy simulation


Mesoscale region


Neighborhood sub-region


Reynolds averaged Navier–Stokes


Reynolds stress model


Root mean square


Sky view factor

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Carruthers, D., Sabatino, S.D., Hunt, J. (2012). Urban Air Quality : Meteorological Processes . In: Meyers, R.A. (eds) Encyclopedia of Sustainability Science and Technology. Springer, New York, NY.

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