Sensitivity of Extreme Temperature Events to Urbanization in the Pearl River Delta Region

The urbanization effect on extreme temperature related to heat wave events in the Pearl River Delta (PRD) of China is investigated using a regional model. Several experiments with different urbanization levels are carried out. Results indicate that the air temperature of heat wave events is sensitive to urban land cover and the urbanized warming intensifies along with urban expansion. The regional model performs better on the simulations of maximum and minimum 2-m air temperature when urban land cover is close to the observed value. Because of the urbanization effect, the surface skin temperature shows a maximum warming (~2.5 °C) at daytime, while the maximum warming of 2-m air temperature (~1.5 °C) occurs at nighttime. Consequently, an increased (a decreased) diurnal range in surface skin temperature (2-m air temperature) is induced. According to the diagnostic equations, the 2-m air temperature is not only related to surface skin temperature, but also regulated by surface sensible heat flux. At daytime, although the increased solar absorption and the decreased upward latent heat flux (due to the deficit of surface evaporation) sharply enhance the surface skin temperature in the higher urbanization experiment, the simultaneously increased sensible heat flux suppresses the 2-m air temperature. At nighttime, however, the difference in surface sensible heat flux is relatively small, thus 2-m air temperature largely depends on surface skin temperature influenced by ground heat release. This study emphasizes the different variations and physical mechanisms of surface skin temperature and 2-m air temperature under the urbanization in the PRD region.