The Energy Balance of an Urban Rooftop: A Case Study Addressing Cloudiness and Evaporative Cooling

By John E Frederick and Radhika Khosla
Advances in Building Energy Research, 1 May 2014

This work examines the heating and cooling of an urban rooftop using radiative and meteorological measurements plus numerical simulations for the months June through September. Longwave atmospheric radiation accounts for 63.6% of the radiant energy absorbed by the roof, compared to a solar contribution of 36.4%. Changes in incident solar and longwave irradiance in response to varying cloudiness have opposite signs. As a result, the mean radiative heating on clear days differs by only 3.35% from that on cloudy days with precipitation. In response to radiative forcing, the roof reaches a temperature that provides the thermal emission, sensible heat transport to the atmosphere and heat conduction into the interior required to balance the heating. These three processes offset 79.4%, 18.4% and 2.2%, respectively, of the average heating for dry days. After precipitation, evaporation provides a 24 h average cooling of 36.4 W m−2. Evaporative cooling comes primarily at the expense of sensible heat transport and is accompanied by smaller surface-to-air temperature contrasts than exist during dry periods. Comparison of measurements and simulations for wet days shows that evaporative cooling leads to a 24 h average drop in surface temperature of 1.2–1.3 K, with larger instantaneous reductions during daylight.