Abstract/Summary

A recently developed, height-distributed urban drag parameterization is tested with the London Model, a sub-kilometre resolution version of the Met Office Unified Model over Greater London. The distributed drag parameterization requires vertical morphology profiles in form of height-distributed frontal area functions, which capture the full extent and variability of building heights. London’s morphology profiles are calculated and parameterised by an exponential distribution with the ratio of maximum to mean building height as parameter. A case study evaluates the differences between the new distributed drag scheme and the current London Model set-up using the MORUSES urban land-surface model. The new drag parameterization shows increased horizontal spatial variability in total surface stress, identifying densely built-up areas, high-rise building clusters, parks and the river. Effects on the wind speed in the lower levels include a lesser gradient and more heterogeneous wind profiles, extended wakes downwind of the city centre, and vertically growing perturbations that suggest the formation of internal boundary layers. The surface sensible heat fluxes are under-predicted, which is attributed to difficulties coupling the distributed momentum exchange with the surface-based heat exchange.