Physical Modeling of Flow and Diffusion Over an Urban Heat Island

Publisher Summary This chapter discusses the physical modeling of flow and diffusion over an urban heat island. An urban complex acting as a heat source produces a positive temperature perturbation on the thermal field of the airflow in the lower layers of the atmosphere. In the study described in the chapter, the physical modeling of an idealized three-dimensional heat island in a wind-tunnel facility capable of simulating the stratified atmospheric boundary layer was undertaken in an attempt to reach a better understanding of the basic mechanisms involved. Similarity of two flow systems requires geometric similarity, kinematic similarity, dynamic similarity, thermic similarity, and similarity of boundary conditions. Partially similar flows representing urban heat islands that reproduce the gross full-scale flow features can be achieved in a meteorological wind tunnel. Partial similarity results from relaxing the requirements for exact similarity by not requiring equal Reynolds numbers and equal Rossby numbers for the laboratory and atmospheric flows but retaining the requirements for the equality of approach flow Richardson number and equality of height above heat island/Monin–Obukhov length over the heat island. The flow observations revealed that the buoyancy forces produced by heating cause larger perturbations in the oncoming two-dimensional boundary layer flow than are produced by the roughness of the heat island.