Mesoscale meteorological modelling is an important tool to help understand air pollution and heat island effects in urban areas. Accurate wind simulations are difficult to obtain in areas of weak synoptic forcing. Local factors have a dominant role in the circulation and include land surface parameters and their interaction with the atmo-5 sphere. This paper examines an episode during the MCMA-2003 field campaign held in the Mexico City Metropolitan Area (MCMA) in April of 2003. High resolution

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... satellite observations are used to specify the land use, vegetation fraction, albedo and surface temperature in the MM5 model. Making use of these readily available data leads to improved meteorological simulations in the MCMA, both for the wind circulation pat-10 terns and the urban heat island. Replacing values previously obtained from land-use tables with actual measurements removes the number of unknowns in the model and increases the accuracy of the energy budget. In addition to improving the understanding of local meteorology, this sets the stage for the use of advanced urban modules. 20 the dispersion of gases and aerosols in the atmosphere. Mesoscale modelling is an important tool in analysing and understanding these changes, provided that accurate simulations can be obtained. Data assimilation (Stauffer and Seaman, 1994) has proved instrumental in improving certain model results. There are however situations where it is difficult to apply it successfully (Daley, 1991) . 25 This is especially true for cases with weak synoptic forcing and a strong dependence 9862 ACPD 5, 2005