A heat island model for large urban areas and its application to Milan

The study of the urban heat island has been carried out through two new enh anced versions of the UCLM (Urban Canopy Layer Model) model, Landsat/Themat ic Mapper data sets and meteorological data collected over a square area 30 km of side including Milan and its hinterland. The urban climate can be de scribed in different summer and winter radiative settings. The input data a re divided into two classes: 1) parameters related to urban and rural local properties (albedo and emissivity, vegetation index NDVI, surface roughnes s length, land cover...); 2) meteorological data related to the general syn optic conditions. The bulk system of the model is made up of four independe nt equations expressed in terms of four unknowns, ic., the temperature valu es at ground level, canopy level and reference level (100 m) and relative h umidity within the urban structure. The study area is divided by a regular square mesh of variable dimension (from 30 m to 1500 m); both the input and output data are average cell values. UCLM30 and UCLM60 calculate the tempe rature excess as well as the turbulent heat exchanges and the heat storage in the urban canopy as a function of the radiative and dynamic forcing. As can be observed in reality, the model shows that in summer the highest urba n heating occurs in early morning and after sunset and that, in extreme con ditions, the temperature can be up to 8 degreesC warmer in town than in the nearby rural lands.