Parameterization of the Urban Energy Budget with the Submesoscale Soil Model

Abstract

The thermal component of the Soil Model for Submesoscales, Urbanized Version (SM2-U), is described. SM2-U is an extension on a physical basis of the rural Interactions between Soil, Biosphere, and Atmosphere (ISBA) soil model to urban areas. It evaluates the turbulent energy, moisture, and radiative fluxes at the urban canopy?atmosphere interface to provide lower boundary conditions of high-resolution mesoscale models. Unlike previous urban canopy schemes, SM2-U integrates in a simple way the physical processes inside the urban canopy: the building wall influence is integrated in the pavement temperature equation, allowing the model to compute directly the energy budget of street canyons. The SM2-U model is evaluated on the Marseille, France, city-center energy-budget components measured during the field experiments to constrain models of atmospheric pollution and transport of emissions [Expérience sur Site pour Contraindre les Modèles de Pollution Atmosphérique et de Transport d?Emissions (ESCOMPTE)] urban boundary layer (UBL) campaign (June?July 2001). The observed behavior of net radiation and heat fluxes is reproduced by SM2-U with a high level of quality, demonstrating that the influence of building walls may be well modeled by modifying the pavement temperature equation. A sensitivity analysis shows that the accurate account of wall area and the parameterization of both the fast response of artificial materials to environmental forcing variations and their heat storage capacity are essential for mesoscale simulations of the urban boundary layer; they are probably more important than accurate but complex computation of radiative trapping (effective albedo and emissivity)