Evaluation of the urban soil model SM2-U on the city center of Marseille (France)

The single canopy layer SM2-U (Soil Model for Sub-Meso scales Urbanized version) model is built on a physical basis from the ISBA (Interactions between Soil, Biosphere, and Atmosphere) scheme by including urban surface cover modes to evaluate the heat and humidity fluxes at the urban canopy-atmosphere interface. SM2-U has the advantage to be a unique model for rural and urban soils, allowing simulation of all intermediate areas. Unlike previous urban canopy models, SM2-U solves directly the street heat fluxes by integrating building walls to paved surfaces: it does not solve separate energy budgets for walls and roads, nor use any canopy wind speed parameterization, but computes the energy budget of the whole street canyon instead of the paved horizontal surfaces alone. This has the advantage of simplicity and computer efficiency, which is essential for coupling with atmospheric, weather prediction models. The building wall influence appears in three terms of the modified energy budget of paved surfaces: an additional heat flux stored or released by walls; a modified pavement inverse heat capacity including wall storage capacity; and parameterized effective albedos and emissivities accounting for radiative trapping.

The SM2-U model is evaluated without soil-atmosphere feedback on the city center of Marseille (France) against heat flux measurements collected during the UBL-ESCOMPTE campaign (June-July 2001) by groups from the Universities of Indiana, British Columbia and Western Ontario, and Météo-France. It is also compared to the Town Energy Budget (TEB) scheme. The average pattern of the observed heat fluxes is accurately simulated by SM2-U, with the same quality as TEB, thus demonstrating that building wall influence can be easily modeled by only modifying the pavement temperature equation. The communication will also show that an accurate account of wall areas and an accurate parameterization of the physical processes, such as the heat storage and the fast response to environmental forcing variations of artificial surfaces, are essential factors for mesoscale urban boundary layer simulations, and probably more important than complex parameterizations of effective albedo and emissivities of streets.