Understanding the sensitivity of WRF hindcast of Beijing extreme rainfall of 21 July 2012 to microphysics and model initial time

The extreme rainfall during 21-22 July 2012 brought the heaviest rain to Beijing (190 mm) and Fangshan (460 mm) in six decades, which caused widespread damage in the city and its surroundings. To understand the evolution of spatio-temporal variability of precipitation and to quantify the inherent uncertainties in short-range weather forecasts of such an urban extreme precipitation event, we use an ensemble of high-resolution urbanaware simulations. In particular, we focus on the sensitivity of WRF hindcasts to microphysics parameterizations since reliable forecasting of extreme precipitation depends heavily on accurate descriptions of the processes of cloud microphysics. Three widely used microphysics schemes (Thompson, WSM6, and WDM6) were examined. Apart from the microphysics schemes, we also evaluated the performance of the simulations to four different model initial times (starting from 1200 UTC 19 July with 12 h-intervals) and two different boundary conditions (ERA-Interim and NCEP FNL). The results indicate that all three microphysics schemes underestimated total rainfall. The Thompson microphysics scheme, when initialized at the beginning of convection intensification (0000 UTC 20 July 2012), captures the intense rain areas. The WSM6 scheme captures the total amount of intense rainfall better than other schemes but with a spatial bias, and the WDM6 captures the location of the heavy precipitation but underpredicts the amount of precipitation compared to other schemes. The simulations starting at 0000 UTC 20 July 2012 perform better compared with other three other initialization times. The simulation using NCEP-FNL captures the location of maximum precipitation better compared with ERA-Interim. Overall, it is found that the results of these hindcasts are most sensitive to the time of initialization.

Automated summary

This article investigates the spatio-temporal variability of precipitation during the extreme rainfall event in Beijing in 2012, as well as the uncertainties in short-range weather forecasts. By using high-resolution urban-aware simulations, the authors find that the microphysics parameterizations and model initialization time greatly affect the forecast accuracy. Recommendations for improvement are also proposed.