Impact of Urban Expansion on Summer Heat Wave in Beijing

There is increasing evidence that rapid urbanization in China has intensified summer heat wave in recent decades in the background of global warming. This study implements two land-cover classification from the U.S. Geological Survey (USGS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) in the Weather Research and Forecast model (WRF), version 2.2, and investigates the regional effect of urban expansion during a typical heat wave event (July 11-15, 2002) in Beijing. The urban area from the MODIS classes (2002) apparently expanded compared with the USGS (1993) urban area. Results from the MODIS land-cover have shown that modeled near-surface air temperature field agrees closely with observations at urban enclosed area, and wind field convergence is well simulated in the urban area. Examining the differences in results from urban-cover changes, it is found that urban expansion can strengthen urban heating intensity, especially during the nighttime. This is due to stronger turbulent diffusion in the daytime than in the nighttime to transport heat to higher levels. In the meantime, the simulated wind convergence and planet boundary layer (PBL) height over the expanded urban area is enhanced, which can potentially add severity of pollution to the already heat stressed central city.