Urban climatology in arid regions: Current research in the Negev desert

Desert cities provide climatologists with circumstances that can be qualitatively different from those encountered in more temperate environs. These distinctions are expressed in the balance of heat and moisture between a dry atmosphere and an urbanized terrain, and they are experienced by city dwellers in urban outdoor spaces. But, while such places are characterized by harsh thermal extremes, they also present unique opportunities for microclimatic enhancement. This potential has motivated a series of studies in the and Negev region of Israel, beginning with a measurement campaign showing that a dense urban fabric can provide significant thermal benefits due to protection from solar radiation, which is dominant within the overall energy exchange. To generalize these findings, an innovative modelling approach - using an open-air, scaled urban array - was used to study variations in urban geometry under realistic radiative loading and boundary-layer turbulence. The surface energy budget above this small-scale urban canopy was found to be similar to observed energy exchange patterns of actual cities. Measurements within the scaled urban canyons served as input to a pedestrian-centred energy exchange model, and a semi-empirical model was also developed for predicting pedestrian energy exchange and thermal discomfort as a function of urban attributes and meteorological data for a given land-use area. This integrated modelling approach revealed that increased urban density, while serving to increase radiative trapping and storage of heat within the urban fabric, also reduces pedestrian thermal stress during the critical daytime hours. This 'cool island' effect is enabled by the high thermal inertia of the built-up area, and the sharp diurnal fluctuations that are peculiar to a hot-arid climate. Its impact is contingent, however, upon the orientation of the street canyon in question - increasing as street-axis orientation approaches north-south and becoming negligible in the east-west direction.