Analyzing Radiation, Air Temperature and Soil Moisture Controls on Urban Surface Temperatures Using an Ecohydrologic Model

Abstract:

The western U.S and other regions of the World are experiencing drier summers and longer periods of consecutive days without wetting rain. Less frequent summer storms reduce the moisture available to dissipate heat, increasing the amount of energy available to heat the ground and the air. Increasing temperatures in urban environments have widespread societal and economic consequences and have been shown to be induced by surface moisture deficits in some regions and large-scale synoptic processes such as warm air advection in others. Warm air advection is attributed to high pressure areas caused by reduced cloudiness and enhanced by atmospheric blocking. Under these persistent synoptic conditions, soil moisture can be depleted further leading to evaporative losses resulting in positive feedbacks that enhance air temperatures. In this regard, summer temperatures and the occurrence of preceding summertime precipitation have become important indicators of the ability of urban temperatures to reach hazardous levels. To investigate the impact of summer storms and urban irrigation on the energy balance and thermal comfort across varying land cover uses we use a fully distributed ecohydrological model to simulate the urban microclimate of Missoula, MT, a typical temperate, mid-latitude town in the inter-mountain western US. We demonstrate that less frequent summer storms and less urban irrigation reduce the dissipation of available energy as latent heat, effectively increasing surface temperatures and decreasing thermal comfort in urban environments more significantly than the expected regional increase in air temperature associated with climate change. A comparison of surface temperature sensitivities to soil moisture and air temperatures during preliminary analyses show considerable differences between the two, modulated strongly by the presence of water.