Vertical structure of the atmosphere over cities:
This contribution describes the fundamentals and peculiarities of the vertical structure of the atmosphere over cities When atmospheric air masses encounter a city, an urban boundary layer (UBL) is formed near the surface, extending approximately up to a height of 1-2 km above ground and persisting downwind of the city as an urban plume over tens to hundreds of kilometres.
In contrast to the atmospheric boundary layer in rural areas, the UBL reflects the increased roughness of the urban surface, the increased heat injections into the atmosphere, and the emissions of air pollutants from urban sources. The UBL can be divided in two 1 regions: The top 90%, characterised by thermals during the day (mixed layer) and the lowest 10% near the surface where mechanical turbulence dominates (surface layer).
The mixed layer (ML) undergoes a distinct diurnal cycle. The intensified daytime turbulence in the ML over a city can lift the capping inversion over the city by up to 20% relative to the rural surrounding. At night, turbulence breaks down to a large extent in the remains of the ML (residual layer). However, instead of the usual nocturnal surface inversion, inner cities can develop an elevated inversion at a height of a few hundred meters, while the surface layer below may remain slightly unstable due to the urban heat island.
The surface layer is further subdivided into an inertial sublayer (ISL) and a roughness sublayer (RSL). In the ISL, properties of the urban surface are mixed intensely so that air flow in the ISL is position-independent. In contrast, air flow in the RSL is horizontally dependent on the location relative to individual buildings and trees. As a lowest layer, we can define the urban canopy layer (UCL) within the RSL.
The UCL consists of the outdoor air and the atmosphere in the interior of buildings, extending from street level to the height of the tallest buildings and trees. In the UCL, air flow and temperatures are very location specific with regard to the three-dimensional, urban geometry in street canyons and courtyards. The radiation field shows small-scale differentiation (suns and shadows) and mean air quality differs on the scale of a few meters by the location of local emission sources relative to the approaching flow.