Satellite-Supported Modeling of the Relationships between Urban Heat Island and Land Use/Cover Changes

Abstract:

Reliable assessment of the primary causes of urban heat island (UHI) and the efficiency of various heat mitigation strategies requires accurate prediction of urban temperatures and realistic representation of land surface physical characteristics in models. In this study, we expand the capabilities of the Weather Research and Forecasting (WRF) model and the Urban Canopy Model (UCM) by implementing high-resolution real-time satellite observations of green vegetation fraction (GVF), leaf area index (LAI), and albedo. We use MODIS-based GVF, LAI, and albedo to replace constant values that are assumed for urban pixels and climatological values that are used for non-urban pixels in the default WRF-UCM. Utilizing the improved model, summertime climate of Los Angeles is simulated over the span of three years (2010-2012). Next, thermal sensitivity of urban climate to anthropogenic land use/cover is assessed via replacing current urban cover with pre-development vegetation cover, consisting of shrubland and grassland. Surrounding undeveloped areas and inverse distance weighting method are utilized to estimate GVF and LAI of pre-development vegetation cover. Our analysis of diurnal and nocturnal surface and air temperatures shows cooling effects of urbanization in neighborhoods with high fractions of irrigated vegetation. However, urban warming is consistently detected over industrial/commercial and high-intensity residential areas. In addition to well-known mechanisms such as a shift in surface energy partitioning, high heat storage in urban material, and inefficiency of urban surfaces in transferring convective heat from the surface to the boundary layer, our results show decreased wind speed and sea breeze also contribute to the UHI intensity. We further evaluate the interactions between UHI and replacing irrigated and imported vegetation with non-irrigated native vegetation as a water conservation strategy in water-stressed Los Angeles metropolitan area.