The influence of urban morphology on sensible heat flux and convective rainfall distributions over Greater Manchester

Human activities, and alterations of the nature and morphology of the landsurface, perturb the land surface- atmosphere balances of energy, mass andmomentum. These changes lead to modifications of the atmospheric boundary layerwhich affect weather processes. Specifically, urban areas have been documented tochange temperature distributions, wind patterns and air quality. They can also impactthe development of clouds and precipitation in and around cities.Among the causes ascribed to the modification of convective precipitationinduced by urbanisation, most studies suggest that the atmospheric destabilisationassociated with the heat island and surface roughness is the most significant, more sothan microphysical or moisture enhancement. However the relative importance ofthese mechanisms remains unclear.The present work reports an investigation of the effects of urban surfaceheterogeneity on the distribution of sensible heat flux and its impact on convectiveprecipitation, in Greater Manchester. A simple numerical scheme is formulated toderive fields of surface sensible heat flux for a range of wind and temperature valuesover the urban area. This involves the derivation and mapping of urban surfacemorphologic characteristics such as the height of buildings and the frontal area index.Comparisons are made with previously published morphologies derived for otherurban areas. The sensible heat flux values from the numerical procedure are comparedto direct independent measurements for a number of days. The sensible heat flux fieldand the rainfall field measured using a C-band radar are compared. The possibleinfluences of the urban morphology on the rainfall distribution, and the eventualinitiation of convective cells by the sensible heat flux input generated over buildings inManchester city centre, are discussed