Urbanization impacts on Pearl River Delta extreme rainfall - sensitivity
to land cover change vs anthropogenic heat
Abstract
Impacts of urban land cover and anthropogenic heat (AH) on extreme local
rainfall over the coastal PRD megacity region during boreal summer are
investigated by conducting numerical experiments using the Weather
Research and Forecasting (WRF) model coupled with a single-layer urban
canopy model (SLUCM). To examine the relative importance of land cover
change vs the presence of AH, three numerical experiments corresponding
to different levels of urbanization in the PRD area were designed: one
with cropland covering the whole region, one with urban land cover but
zero AH, and one with urban land cover and a strong diurnal maximum of
300Wm-2 of AH in the model
simulations. Results show that the increase of accumulated rainfall in
the urban area is much more sensitive to the intensity of AH than the
mere change of surface properties. Urbanization with intense AH can
enhance both the intensity and frequency of extreme rainfall, which can
be attributed to higher surface temperature (of about 3.5 to
4oC), higher convective available potential energy
(CAPE), and lower convective inhibition (CIN), thus creating an
environment more conducive to strong convection over the urban areas.
Moreover, enhanced rainfall is supported by moisture supply from the
South China Sea and increased water vapor flux convergence over the PRD
city area. The amount of moisture flux converging over the coastal
megacity area was found to depend on the direction of prevailing
background wind.