Numerical study of the nocturnal urban boundary layer

A two-dimensional, time-dependent Earth-atmosphere model is developed which can be applied to the study of a class of atmospheric boundary-layer flows that owe their origin to horizontal inhomogeneities with respect to surface roughness and temperature. The authors' main application of the model is to explore the governing physical mechanisms of nocturnal urban atmospheric boundary layer flow. A case study is presented in which a stable temperature stratification is assumed to exist in the rural upwind area. It is shown through integration of the numerical model that, as this air passes over a city, the heat is redistributed because of increased surface friction (and, hence, increased turbulent mixing). This redistribution of heat results in the formation of an urban heat island. Additional numerical integrations of the model are conducted to examine the dependence of induced perturbations upon 1) the upwind temperature inversion; 2) the geostrophic wind speed; and 3) urbanization. The results show a linear relationship between heat island intensity and the rural temperature inversion, with the heat island increasing in intensity as the upwind inversion becomes stronger; that the heat island intensity close to the surface is inversely proportional to the geostrophic wind; and that the effects of anthropogenic heat cause an increase in the perturbation temperature with the perturbation extending to higher altitudes. From this study, it is concluded that, with an upwind temperature inversion, a city of any size should generate a heat island as a result of increased surface roughness. The heat island intensity should increase with city size because of two factors: larger cities are usually aerodynamically rougher; and larger cities have a larger anthropogenic heat output.