Urban Heat Island: Modeling, Sensing and Mitigation Strategies

Urbanization is one of the important drivers of micro and regional climate change. For example, the observation that cities are hotter than their surrounding rural areas, also known as urban heat island effect, has been shown to play a role in modifying urban rainfall climatology and altering the stability of the lower atmosphere over cities. However, urban modeling still faces significant challenges mainly due to difficulties in representing small-scale physical processes occurring in urban canopies and in parameterizing the highly heterogeneous urban surfaces at regional scales. The Weather Research and Forecasting (WRF) model can be a powerful tool in overcoming these challenges due to its nesting and large-eddy simulation capabilities. In this study, we assess the ability of WRF to capture the complex atmospheric boundary layer dynamics over urban terrain, using WRF-LES. The findings indicate that, over urban and rural areas, the land surface temperatures are highly sensitive to the parameterization of the thermal roughness length for the different surface types; different approaches are required for this parameterization over porous forests and over bluff urban canopies. We also observe that the default single-layer urban canopy model (UCM) in WRF overestimates the surface temperatures along Washington-Baltimore Corridor when compared to the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations. To improve the model performance, a new urban canopy model, calibrated using field observations, with two surface types for the roofs (conventional roof and green roof) and three for the ground (asphalt, concrete and grass) is implemented into WRF. The new urban canopy model significantly reduces the errors in land surface temperature over urban areas. WRF with the new model is also shown to be able to reproduce the vertical structure of the ABL fairly accurately when compared to aircraft observations, and is then used to evaluate urban heat island mitigation strategies at city and regional scales.