The evaluation of models designed to represent the surface energy balance of urban environments

• Main Author: Best, Martin John
• Other Authors: Grimmond, Christine Susan Betham ; Baas, Andreas Cornelis Wilhelmus
• Published: King's College London (University of London) 2015
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.694044

Although urban models have been individually evaluated in numerous studies, it is not possible to objectively compare the performance between models in general as the level of optimisation during each model study is often not published. Also observational datasets are often of short duration and hence the schemes have not necessarily been tested over a seasonal cycle. The first urban model comparison project was designed to address both of these issues. Results from the comparison show that multiple reflections of shortwave radiation within street canyons, the reduction in the amount of visible sky from within the canyon (which impacts on the net long-wave radiation), the contrast in surface temperatures between building roofs and street canyons, and evaporation from vegetation are the dominant physical processes that govern the turbulent sensible and latent heat flux exchange between an urban surface and the atmospheric boundary layer. Hence models that use an appropriate bulk albedo, represent building roof surfaces separately from street canyons and include a representation of vegetation demonstrate the most skill. So models need to include these characteristics, but further complexity does not add to the skill of the model at the main study site used for the comparison, for minimizing errors in the turbulent sensible and latent heat fluxes. Furthermore, these models require parameter information on the bulk albedo, the height to width ratio of the building and the roof to impervious surface fraction, and plan area vegetation fraction. Urban models that contributed to the comparison are able to capture the seasonal cycle and the observed trends in the fluxes with respect to atmospheric forcing. However, initial conditions of soil moisture are critical because of the importance of evaporation from vegetation. Including the anthropogenic heat flux in the urban surface energy balance can give improved simulations, but the signal is small in the results for the comparison because of the small magnitude of the heat flux itself. Larger impacts are anticipated for urban environments with larger anthropogenic heat fluxes. Anthropogenic water sources from irrigation and street cleaning also have a critical impact on the surface energy balance, and it can be assumed that vegetation is never water stressed unless there is an irrigation ban enforced.