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 land surface, perturb the land surface- atmosphere balances of energy, mass and momentum. These changes lead to modifications of the atmospheric boundary layer which affect weather processes. Specifically, urban areas have been documented to change temperature distributions, wind patterns and air quality. They can also impact the development of clouds and precipitation in and around cities. Among the causes ascribed to the modification of convective precipitation induced by urbanisation, most studies suggest that the atmospheric destabilisation associated with the heat island and surface roughness is the most significant, more so than microphysical or moisture enhancement. However the relative importance of these mechanisms remains unclear. The present work reports an investigation of the effects of urban surface heterogeneity on the distribution of sensible heat flux and its impact on convective precipitation, in Greater Manchester. A simple numerical scheme is formulated to derive fields of surface sensible heat flux for a range of wind and temperature values over the urban area. This involves the derivation and mapping of urban surface morphologic characteristics such as the height of buildings and the frontal area index. Comparisons are made with previously published morphologies derived for other urban areas. The sensible heat flux values from the numerical procedure are compared to direct independent measurements for a number of days. The sensible heat flux field and the rainfall field measured using a C-band radar are compared. The possible influences of the urban morphology on the rainfall distribution, and the eventual initiation of convective cells by the sensible heat flux input generated over buildings in Manchester city centre, are discussed