Temporal variation of urban surface and air temperature

Abstract

The urbanization process alters the radiative, thermal, moisture and aerodynamic characteristics of a surface, which significantly affect the surface energy balance within the atmospheric boundary layer. Such modifications can lead to the so-called urban warming phenomenon, where the extent and rate of urban surface and air temperature is substantially higher than the corresponding rural region, which has caused serious concern in recent decades. However, the understanding of the causes of urban warming is incomplete, and the same applies to lack of effective mitigation strategies. Therefore, in order to have a complete understanding of the formation of urban warming, the present thesis focusses on the estimation of temporal variations of urban surface and air temperatures by using numerical simulations, analytical methods and field measurements.

To better understand the mechanism(s) of urban surface temperature variation, a three-dimensional model that incorporates the energy exchange processes is first developed for a realistically complex city. In order to reduce the computational effort for the radiation heat transfer calculation, the compressed row storage scheme is applied, which permits the rigorous consideration of multiple reflections in a realistic urban area with hundreds of buildings.

The developed surface energy balance model is then used to investigate the effects of the urban canopy geometry on urban albedo and surface temperature. The average urban albedo is less for a moderately compact city having high rise buildings with varying building heights than other cases. A cooler urban street surface temperature with smaller amplitude and earlier occurrence of the daily maximum temperature is observed in a high rise compact city than a low-rise sparse city.

In order to understand quantitatively the causes of urban air warming, a new analytical zero-dimensional urban air temperature model is also developed, which is able to capture the features of the urban temperature variation. Results show that solar heat gain, evapotranspiration and the anthropogenic heat affect the mean air temperature, while heat storage and thermal convection affect the amplitude and phase shift of the daily cycle. A high-rise, high-density city generates low surface temperature, resulting in low air temperature during the day.

The main conclusion of this study is that on the condition rural air temperature cycle is given, the mean temperature of the urban air and surface temperature is determined by the net heat gain and ventilation rate, and the amplitude and phase can be obtained from thermal storage and ventilation rate. Essentially, the net heat gain, thermal storage and ventilation are affected by urban morphology, and hence a city thermal environment can be designed. (419 words)