Characteristics of summer QPE error and a climatological correction method over Beijing-Tianjin-Hebei region

Radar Quantitative Precipitation Estimation (QPE) is a key component of short-term and nowcasting. The radar QPE plays an important role in many fields such as Quantitative Precipitation Forecast (QPF), heavy precipitation warning, urban waterlog, geological mountain torrent disaster, and fine weather service. In this paper, the spatial distribution characteristics of radar QPE over Beijing-Tianjin-Hebei region are analyzed using the radar QPE data and automatic weather station hourly precipitation data in the summers of 2011-2016. Specifically, a new type of climatological correction algorithm is proposed. Results show that the radar QPE can well reflect the northeast to southwest distribution of observed total rainfall. However, the radar QPE is underestimated over mountain areas in the northwest, northeast and southwest, and overestimated over the piedmont zone of the northeastern mountain area. Besides, false precipitation estimation is found over the northwest of Beijing-Tianjin-Hebei region. Precipitation over the urban area of Beijing estimated by radar is the closest to observations. The newly proposed climatological correction algorithm is then applied for hourly radar QPE to calibrate previous radar estimates. Results of tests show that the BIAS, the mean absolute error (MAE), the root mean square error (RMSE) and the relative RMSE (RRMSE) all are reduced after the correction. In particular, the reduction percentage of BIAS reaches more than 50% at most stations. The reduction percentages of MAE, RMSE and RRMSE are about 20% in the eastern and southern plain, but they are relatively small in the northern and southwestern parts. The test of precipitation cases shows that the strength of radar QPE is closer to station observations and the high-resolution structure of precipitation is well captured by radar after the correction. Furthermore, the BIAS, MAE and RMSE of precipitation cases all are reduced, and the correlation coefficient between the radar QPE and observations is increased. Hence, the climatological correction algorithm can improve the accuracy of the radar QPE. The correction algorithm is useful for operational weather forecasting and can be widely used in other areas.