Urban surface modeling and the meso-scale impact of cities

Summary New developments of the international community in modeling the urban canopy surface energy balance are presented and classified into five main categories: (i) models statistically fit to observations, (ii) and (iii) modified vegetation schemes with or without drag terms in the canopy, and (iv) and (v), new urban canopy schemes, that present both horizontal and vertical surfaces, again with or without a drag approach. The advantages and disadvantages of each type of model are explained. In general, the more the physics are correctly simulated, the more complex are the urban phenomenon that can be addressed, on the other hand however, the more consuming of computer-time and difficult to couple with atmospheric models the scheme becomes. Present use of these new models in meso-scale atmospheric models show their ability to reproduce the phenomenon of the urban heat island (UHI) and some of its consequences – urban breezes, storm initiation, interaction with sea-breeze. Their use opens up new perspectives, for example in the mitigation of the UHI, or assessment of the role of air-conditioning systems or the impact of urban dynamics on air pollution. However, there is need to validate further the different urban models available. In particular it is necessary to compare model output with urban surface energy balance measurements. An intercomparison exercise involving these urban schemes is suggested as an efficient way to assess and improve these models.