On the suppression of the urban heat island over mountainous terrain in winter

Most of the urban areas generate an Urban Heat Island (UHI) because of urban-rural differences such as in albedo or heat capacity. The UHI effect is often more noticeable either at night or in winter since it may balance the effects of stable stratification. Over complex topography, terrain-induced exchange processes interfere with the urban-scale circulations. Also, in a valley environment, temperature changes are larger than over flat terrain, so that the UHI may be either enhanced or suppressed depending upon the stratification of the atmosphere. Numerical simulations were conducted in the area of the Grenoble valley (France) for a selected PM10 pollution episode in February 2005 using, a set of numerical codes (e.g. WRF and METPHOMOD). The UHI index was usually found to be in the order of 5 K at night and 3 K during the day. Ground surface measurements of radon were used to investigate the role of the UHI in the dispersion of locally emitted primary pollutants.