Multi-model comparison of urban heat island modelling approaches

Cities are characterized by different physical properties of surface compared to their rural counterparts, resulting in a specific regime of the meteorological phenomenon. Our study aims to evaluate the impact of typical urban surfaces on the central European urban climate in several model simulations, performed with the Weather Research and Forecasting (WRF) model and Regional Climate Model (RegCM). The specific processes occurring in the typical urban environment are described in the models by various types of urban parameterizations, greatly differing in complexity. Our results show that all models and urban parameterizations are able to reproduce the most typical urban effect, the summer evening and nocturnal urban heat island, with the average magnitude of 2–3 °C. The impact of cities on the wind is clearly dependent on the urban parameterization employed, with more simple ones unable to fully capture the wind speed reduction induced by the city. In the summer, a significant difference in the boundary-layer height (about 25 %) between models is detected. The urban-induced changes of temperature and wind speed are propagated into higher altitudes up to 2 km, with a decreasing tendency of their magnitudes. With the exception of the daytime in the summer, the urban environment improves the weather conditions a little with regard to the pollutant dispersion, which could lead to the partly decreased concentration of the primary pollutants.

Sensitivity of WRF Model to Urban Parameterizations, With Applications to Chicago Metropolitan Urban Heat Island

Chicago is one of the most populated cites of US. It is located next to a freshwater source, Lake Michigan, and surrounded by productive agricultural land and diverse natural habitats. This study explores the sensitivity of mesoscale urban heat island (UHI) simulations to urban parameterizations, focusing on the Chicago metropolitan area (CMA) and its environs. For this purpose, a series of climate downscaling experiments using the Weather Research and Forecasting (WRF) model at 1 km horizontal resolution. A typical summer hot day in Chicago was considered, which is imitative of a summer day in the late 21st century. This study utilizes National Land Cover Database (NLCD) 2006 classifications to test UHI sensitivity for CMA. Among different urban parameterization schemes, BEP+BEM best reproduces the urban surface temperatures in comparison to other urban schemes. Results show that UHI is more pronounced with BEP and BEP+BEM schemes due to explicit accounting of anthropogenic heat (AH). The study also investigates the effects of urbanization on regional climate by replacing Chicago metropolitan area by agricultural landscape, which yielded increased surface wind speeds due to reduced mechanical and thermal resistance.