Experimental study on mean flow and turbulence characteristics in an urban roughness sublayer (Formerly Paper P6B.2)

The structure of the atmospheric boundary layer in urban areas is of particular interest for air pollution modeling. In urban-scale dispersion models, the lowest portion of the boundary layer is often represented using surface layer similarity parameterizations. The urban effects are taken into account by changes of surface roughness and heat flux. Strictly speaking, boundary layer formulations of this type are only applicable in the inertial sublayer well above the building tops, but not in the so-called roughness sublayer - the flow region in the immediate vicinity of the urban canopy elements where the flow locally depends on the particular building arrangements and thus has a rather complex structure. In the vertical, the roughness sublayer extends from the surface up to a level, at which horizontal homogeneity of the flow is achieved, that is at 2 to 5 times the average canopy-element height. In areas with high buildings, it can occupy a significant part of the boundary layer where most of the pollution problems occur.

New information on the mean flow and turbulence characteristics in the roughness sublayer was obtained in a wind-tunnel study of the flow field in a central part of the city Nantes, France. A detailed model of the building structure in a region of about 400 m in diameter was constructed in the scale 1:200 and investigated in a neutral boundary layer wind tunnel. Vertical profiles of mean and turbulent velocity components were measured with a Laser Doppler velocimeter at thirteen positions. The profile locations were chosen in the way to get information on the internal boundary layer development over the urban area and to trace horizontal variability of the flow inside and above a street canyon oriented perpendicular to the wind direction.

The influence of building pattern irregularities on the mean flow, turbulence kinetic energy, and shear stress distributions has been identified up to a level of about 2.5 times the average building height H_a. The observed wind profiles can be classified in two types. The first type includes characteristic canyon-flow profiles with almost zero or even negative mean wind velocities below the building roof level. Profiles referring to the second type are representative of the wind regime around street crossings or in open squares and characterized by higher mean and turbulent flow velocities in the canopy layer. Turbulence kinetic energy and shear stress profiles corresponding to both flow types in many cases show pronounced maxima in the flow region about 0.5 H_a deep just above the building roof level. Similar maxima are commonly observed in the flow transformation zone behind a step change of surface roughness.

It will be considered in detail whether the observed features of the mean flow and turbulence fields can be attributed to the suburban-urban roughness change or they reflect local flow disturbances by individual canopy elements. Possible parameterizations for horizontally averaged profiles of mean flow and turbulence statistics in the roughness sublayer will be discussed.