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Urban model: SURFEX

The newly developed surface scheme of Météo-France-Surface Externaliseé (SURFEX; Le Moigne 2009) is an externalized surface scheme that can be run either in a coupled mode in which case the atmospheric forcing is provided by the host atmospheric model, or in a stand-alone mode where the atmospheric drivers are derived from observations and fed to the surface scheme such that it is decoupled from the atmospheric part of a global circulation model. The latter case is possible by relying on the algorithmic structure proposed by Best et al. (2004). SURFEX contains various modules allowing one to describe the exchanges of water, momentum, and energy over four surface area tiles: sea, lake, vegetation, and the city. A grid value is then simply an area averaged value of the different tiles present in the grid cell. Over vegetated areas, SURFEX includes the Noilhan and Planton (1989) Interactions between Soil, Biosphere, and Atmosphere (ISBA) scheme. ISBA solves simultaneously the energy and water budget of the soil and vegetation. Vegetation parameters come from the ‘‘Ecoclimap’’ database (Masson et al. 2003). Over urban surfaces, SURFEX includes the Town Energy Balance (TEB) (Masson 2000) single-layer urban canopy module. Urban canopy is assumed to be an isotropic array of street canyons.

TEB simulates heat and water exchanges of three generic surfaces (roof, wall, and road), where heat transfers are computed through several layers of materials, generally four. Anthropogenic heat and vapor releases from buildings, vehicles, and  chimneys can also be added. TEB is forced with literature-based surface thermal parameters and observed or simulated atmospheric and radiation data from above roof level. Despite the simplification hypotheses, offline simulations of TEB have been shown to accurately reproduce surface energy balance, canyon air temperature, and surface temperatures observed in dense urban areas - Vancouver and Mexico City (Masson et al. 2002), Marseille (Lemonsu et al. 2004), Basel (Hamdi and Masson 2008) - during dry and hot seasons. In the study by Pigeon et al. (2008), the evaluation of TEB is extended to two other seasons, fall and winter. In their study, a validation of the parameterization of anthropogenic heat sources against an inventory of energy consumption in the city of Toulouse, France, was also performed. SURFEX can be run in-line within the ALARO model, also for climate simulations (see Hamdi et al., 2014).