Evaluation of Urban Surface Energy Fluxes Using an Open-Air Scale Model

Abstract

The thermal behavior of an urban surface is crucial to understand, but it is difficult to predict using conventional measurement or modeling approaches. In this study, an integrated method is proposed for evaluating urban energy exchanges with an open-air scale model of a building?street canyon surface array. The technique, which potentially combines the flexibility of modeling with the reliability of empirical observation under natural turbulence and radiative loading, is tested in hot, arid summer conditions to gauge its ability for reproducing surface?atmosphere energy fluxes that are representative of diurnal patterns in actual urban settings. After identifying the inertial sublayer, which is created above the scaled roughness array at a point near its downwind edge, roughness parameters utilized in the calculation of turbulent sensible heat flux are determined for two different array configurations of varying frontal area density and compared with existing data from field studies and morphometric models. For each geometric configuration, the relative sharing of radiant energy between storage and turbulent fluxes is compared with published findings obtained by conventional methods, as is the diurnal pattern of each component flux. Roughness parameters that are obtained conform to the expected ranges, as do daytime and overall daily fluxes and flux ratios. Overall, radiation absorption and heat storage are higher in the array with deeper canyons, and in both arrays the share of sensible heat channeled into the atmosphere is both higher in magnitude and later in reaching its peak intensity than that which is stored within the scaled urban fabric. This thermal time lag, when evaluated by fitting data to a published model for parameterizing heat storage from net radiation, shows a high correlation with hysteresis behavior in actual cities.