Idealized studies with the new urban scheme of the Met Office Unified Model

With increasing resolution of numerical weather prediction models, highly heterogeneous urban areas have to be treated more accurately in numerical models, since they are beginning to be resolved within the models grid cells. Since urban areas differ from rural areas in many ways, e.g. the surface energy balance, surface drag, limited application of Monin-Obukhov-theory, the relevant physical processes leading to the typical urban features like an urban heat island and a weaker nocturnal stable boundary layer or even unstable layer have to be parameterized sufficiently and analysed thoroughly.

Within urban areas horizontal changes in building morphology can result in strong horizontal gradients of the surface fluxes, especially, during the night, when the boundary layer is much shallower than during daytime. To account for the effects of strong heterogeneity in urban surface morphology a new urban scheme is implemented in the Met Office Unified model representing the urban land use by a 2 tile approach (roof, street and walls). The new urban scheme is a rational simplification of a more complete parameterization that treats the roof, street and the two wall facets separately (i.e., a four-facet model). The new parameterization was validated against observations in Mexico City. Additionally, idealized 2D simulations are carried out with the new Unified Model version investigating the adjustment of the surface fluxes and the internal boundary layer.