A New Approach for the Urbanization in the Unified Model

Higher horizontal resolution in NWP allows for representation of greater detail in urban areas. However, the requirement of limiting CPU cost in NWP forces a compromise between representing processes and simplicity. In this context, we have introduced a new urban parameterization within the Unified Model (UM), which 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 simplified parameterization consists of two facets, one representing the roof and the other representing the combined street canyon. The albedo, emissivity, transfer coefficients and ground heat flux of the street canyon are calculated from the four-facet model, and all then depend on morphology. The details of the new parameterization and its performance against the full four-facet model will be briefly presented. The parameterization will also be compared to the thermal canopy model, currently implemented in the operational NWP version of the UM.