The energetics of urban microclimates

The goal of this study is to understand physical mechanisms influencing air and surface temperatures in different types of urban and rural sites. An urban canyon in downtown Chicago with an aspect ratio (building height to street width) approaching 8 is compared to another urban site 10 km distant with an aspect ratio of approximately 1.5. The latter is referred to as the urban control. Measurements are recorded at the bottom of the canyon, and at roof level of the urban control, which is not surrounded by tall structures. The effects of these structures on ground-level radiation and meteorology can therefore be isolated. Data from both sites are collected with a net radiometer and weather station. Dual stations collect contemporaneous data from each site. Data are incorporated into an energy balance model for urban and natural surfaces. Energy fluxes for the 2 locations are computed from recorded values and model results. The model accounts for greater than 95% of the variance at the urban control site and over 80% of the variance at the urban canyon. The energy balance for this model is defined as:

0 = Q_LW(up) - Q_LW(dn) - Q_SOL + Q_SENS + Q_EVAP - Q_CONDUC

Where Q_LW is the heat flux associated with up- and down-welling thermal infrared (longwave) radiation; Q_SOL with absorbed solar radiation; Q_SENS with sensible heat transfer into or out of the system; Q_EVAP with latent heat transfer due to vaporization of water; and Q_CONDUC with the conduction of energy into a surface. These terms are either heating or cooling effects based on their sign (positive is cooling). Measurements reveal that the urban control location receives almost twice the net solar radiation as the urban canyon; this produces cooler air and surface temperatures during daytime hours in the canyon. Yet incoming thermal radiation is increased in the urban canyon due to the vertical structures that surround it, which absorb and retain significant amounts of radiation emitted as heat throughout a 24-hour cycle. Sensible heat transport out of the canyon is almost one-third that of the urban control, due to decreased wind and a smaller surface to air temperature differential. These contribute to the development of an urban heat island between these two sites. The urban canyon is consistently warmer during evening and pre-dawn hours in early August, with a maximum intensity of 1.6 °C (2.9 °F) between urban canyon and control locations; 3.7 °C (6.7 °F) between the canyon and nearby suburban Midway airport; and 8.8 °C (15.8 °F) between the canyon and rural De Kalb airport. While surface temperatures at the urban control location can reach values up to 20 °C warmer than the canyon, 24-hour average surface temperatures of the two sites are almost identical.