Impacts of land use changes and synoptic forcing on the seasonal climate over the Pearl River Delta of China

In this study, the impact of the rapid rate of urbanization over the Pearl River Delta (PRD) region of China since the 1980s on its seasonal climate is investigated using the Weather Research and Forecasting Model. Two land-cover data are employed to simulate the urbanization effects on the regional climate in both summer (June and August) and winter (December and February) for 26 years from 1984 to 2009. Spatial and temporal features of the urbanization effect on temperature and precipitation are found to be different between summer and winter. Urban warming and modifications of surface variables such as surface latent heat flux and 2-m relatively humidity are more pronounced in summer. Moreover, summer rainfall is enhanced but winter precipitation is reduced. The increase in summer rainfall is mainly contributed by the strong urban heat island effect, which enhances the convective circulation over the urban area. On the other hand, a reduction in winter precipitation is observed. The different behavior during winter time is due to drier northerly winds and increased atmospheric stability. Furthermore, the changes in winter precipitation are associated with the synoptic forcing such as El Niño – Southern Oscillation and monsoon, but no significant correlation can be found between changes in summer rainfall and the synoptic forcing. In other words, the local surface forcing (including radiative fluxes, heat, momentum and water transfer at the air–land interface) dominates over the synoptic forcing in summer but the reverse is true in winter. Under conditions of weak synoptic flow, local thermodynamic effects of urbanization dominates, setting up convergence zone over the urban area and leading to increased precipitation. However, during winter time, under the influence of strong northeasterly synoptic flow, dynamic effects of increased surface roughness urban areas dominate and lead to convergence upwind of the urban region. ► Urban warming and modifications of surface variables are more pronounced in summer. ► Summer rainfall is enhanced but winter precipitation is reduced. ► The local forcing dominates over the large-scale forcing in summer but the reverse is true in winter. ► A strong synoptic flow can reduce the urbanization effects over an urban area.