Interaction between slope flows and an urban heat Island

The local atmospheric circulation due to a city located at the bottom of a valley is reproduced by laboratory experiments analyzing the interaction between an urban heat island (UHI) and anabatic or katabatic slope flows. Slope flows are generated by the horizontal temperature difference between air adjacent to a mountain slope and the ambient air at the same altitude over the neighboring plane (or over the valley center). The thermal disomogeneity is a consequence of the daily heating due to the solar radiation and to the nightly cooling related to the infrared radiation emitted by the ground. Assuming clear sky and weak synoptic wind conditions, the slope flow is upslope (anabatic) during the daytime and downslope (katabatic) during the nighttime. The cool air settles down in the valley, starting the cold pool formation, a still and steady stratified environment. The slope flows present counter current compensating flows of lower velocity and larger thickness. The circulation associated to slope flows was studied in the past via field observations (Manins and Sawford, 1979; Hunt et al., 2003), analytical (Prandtl, 1952; Horst and Doran, 1983), numerical (Tripoli and Cotton, 1989) and experimental investigations (Fernando et al., 2000; Cenedese and Monti, 2004). Buoyancy-driven UHI circulation has been investigated in experimental and analytical study by Lu et al. (1997). The experiments described here are performed in a temperature controlled water-tank, the same employed by Cenedese and Monti (2003) and (2004) to investigate urban heat islands and sea-breeze flows, respectively.