|Ph.D Student||Pearlmutter David|
|Subject||The Microclimatic Influences of Urban Surface Geometry:|
A Physical Modeling Study in Hot-Arid Conditions
|Department||Department of Architecture and Town Planning||Supervisors||Dr. Pedro Berliner|
|Professor Emeritus Edna Shaviv|
While deserts are characterized by harsh thermal extremes, they also present unique opportunities for microclimatic enhancement. This potential motivated the present urban climate study, which analyzes the influences of urban surface geometry on local climate and pedestrian thermal comfort under the arid conditions of the Negev. In a previous study, measurements in an existing residential area indicated that a densely-packed urban fabric can provide a significant thermal benefit for pedestrians under hot-arid conditions. However, the study also highlighted the limitations of existing research approaches: on-site measurements describe a particular situation and do not necessarily show generalized trends, and mathematical or laboratory models provide only simplified representations of the complex interactions between urban geometry and climatic variables. Thus in an effort to generalize the findings of the previous study, an innovative modeling approach - using an open-air, scaled urban array - was developed to systematically compare a wide range of physical urban configurations under realistic climatic conditions. Through a series of preliminary tests, it was demonstrated that this small-scale model generates patterns of air flow and energy exchange which are highly similar to those found in an actual urban area.
Microclimatic measurements made within a variety of scaled urban “street canyons” were used in a pedestrian energy exchange model describing the connection between geometric urban parameters and physiological thermal stress. By linking these street-level results with simultaneously measured data from above the scaled “city,” a semi-empirical model was developed for predicting pedestrian energy exchange and thermal discomfort as a function of physical urban attributes and meteorological data for a given land-use area. It was found that this integrated modeling approach is capable of revealing relatively detailed distinctions in the relationship between urban geometry and microclimate. The effect of increasing 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. Clearly this further demonstrates the dominance of solar effects, though differences in long-wave radiation and convective heat removal by varying degrees of wind attenuation were shown to have a significant secondary impact.