Numerical study of the effects of local terrain on "7.21" extreme torrential rain in Beijing

Based on NCEP 1°×1° reanalysis data and WRF model, numerical simulation for the extreme torrential rain in Beijing on 21 July 2012 was carried out. The results show that the WRF model can successfully simulate this storm event. In particular, it even has the ability to simulate the heavy rain with 24-hour accumulated precipitation more than 100 mm. Using control run as baseline, four terrain sensitivity tests were designed. The results of terrain sensitivity tests indicate that, the changes of the local terrain of Beijing have little influence on the simulationfor the location of the heavy rain, but have influence on the distribution and intensity of precipitation center. Especially, the local terrain
in southwest side of Beijing has a significant impact on the location of precipitation center; the effect of the local terrain in north and northeast side of Beijing is not evident. Further study shows that the local terrain in southwest side of Beijing changed the intensity and distribution of strong convergence area and vertical movement mainly through influencing the structure of southerly jet in the low troposphere, which in turn caused the variation of location and intensity of precipitation center.