Modelling of Pollutant Transport in the Atmosphere - MANHAZ

Due to the rotation of the earth, the geostrophic balanced winds tend to
flow parallel (and not across - as one would intuitively think) the isobars (i.e.,
the lines of equal pressure). This happens because on the regional or "synoptic
scale" (i.e., on horizontal scales of the order of ~1000 km or more) there is a
balance on air movement between the pressure gradient force, and the Coriolis
force, the latter being associated with the rotation-rate of the Earth. Other
forces, such as the sheering stress caused by the drag of the ground, and air
channeling through valleys and over passes, will result in strong local wind
deviations from the regional geostrophic wind fields. On the local scale, say out
to distances within 10-20 km from the release point, effects of the underlying
surface with its topographical features also tends to be important for the wind
speed and direction, each tree, house and hill exerts drag on the air, and heat
and moisture profiles of the air becomes important.
The vertical wind profile. The "free" geostrophic wind mentioned above
prevails above the ABL, i.e. above say above ~1 km up in the atmosphere. It
flows parallel to the isobars. Closer to the Earth's surface, that is, within the
strong turbulent ABL, the wind speed is gradually decreasing, due to the
friction with the rough ground surface. Near the surface, houses, trees, hills
and forest perturb the velocity profile.
The local turbulence level depends on the local "atmospheric stability" which in
turn is characterized by 1) the magnitude of the wind speed 2) the surface
roughness, and 3) the vertical temperature profile.
Finally, because of the drag forces that exercises on air that flows near the
Earth's' surface, the wind direction is often turned some 20 degrees counterclockwise
(towards the centre of the low pressure) relative to the direction of
the geostrophic "free" winds that blows parallel to the isobars near the top of
the ABL at ~1 km height. This "veering of the wind direction with height" can
easily be observed on a "cumulus cloudy" day, where the wind direction near the
top of the ABL is marked by the motion of the cumulus clouds, while the near
ground wind direction can be read from a nearby flag, a wind turbine or from a
smoke plume.
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