Energy balance and surface temperatures of urban quarters

This work concerns modelling and simulating the energy budget and surface temperatures of the various quarters in urban areas. The complexity of the urban climatology is due to the heterogeneity of the surface such as roughness, surface types etc. Thus the urban climatology is composed of many microclimates corresponding to the various urban quarters. A novel urban soil model, SM2-U, has been developed for the refined simulations over urban cities at the scale of city quarters. It is a further development of the Sub-Meso Soil Model SM2 previously derived from the original two-layer model of Noilhan & Planton (1989) for natural and vegetated soils. This new model considers 5 types of surface in each computational grid cell; namely, natural surface (bare soil with vegetation), bare soil (without vegetation), artificial surface (with or without vegetation), built surface, and water surface. The urban canopy layer thickness is not considered here, i.e. the town is seen as a flat surface, and the roughness elements are modelled by roughness lengths and displacement heights. Two soil layers are considered: a superficial layer which is different for each type of surface, and a deep soil layer which is common for all surfaces in a grid cell. The model solves an equation for the time variation of the surface temperature and one for the surface specific humidity, for each surface type separately in each grid cell. These equations are based on the Force-Restore model of Deardorff (1978). Here, SM2-U is tested for 5 standard urban quarters: city centre, residential quarter, green quarter, industrial-commercial quarter, and high buildings quarter. The numerical simulations run over a complete year, using the meteorological data of the Hapex Mobilhy campaign, extrapolated above a virtual urban area. A comparison is presented between the energy budget and the surface temperature of each urban quarter, for an average diurnal cycle of each month in the year. The results are in good agreement with the previous experimental observations. It was found that the most urbanised quarter are the warmest, with a large sensible heat flux, a large heat storage and a weak latent heat flux. The green quarter is the coolest with the latent heat flux being larger than the sensible heat flux. In addition, the model shows that during summer, the deep soil humidity is larger in the urbanised quarters than in the green quarter, therefore the vegetation evapo-transpiration is larger in the urbanised quarters.