Abstract
The heaviest rainfall over 61 yr hit Beijing during 21–22 July 2012. Characterized by great rainfall amount and intensity, wide range, and high impact, this record-breaking heavy rainfall caused dozens of deaths and extensive damage. Despite favorable synoptic conditions, operational forecasts underestimated the precipitation amount and were late at predicting the rainfall start time. To gain a better understanding of the performance of mesoscale models, verification of high-resolution forecasts and analyses from the WRF-based BJ-RUCv2.0 model with a horizontal grid spacing of 3 km is carried out. The results show that water vapor is very rich and a quasi-linear precipitation system produces a rather concentrated rain area. Moreover, model forecasts are first verified statistically using equitable threat score and BIAS score. The BJ-RUCv2.0 forecasts under-predict the rainfall with southwestward displacement error and time delay of the extreme precipitation. Further quantitative analysis based on the contiguous rain area method indicates that major errors for total precipitation (> 5 mm h−1) are due to inaccurate precipitation location and pattern, while forecast errors for heavy rainfall (> 20 mm h−1) mainly come from precipitation intensity. Finally, the possible causes for the poor model performance are discussed through diagnosing large-scale circulation and physical parameters (water vapor flux and instability conditions) of the BJ-RUCv2.0 model output.
Highlights
Heavy rains are commonly responsible for weather disasters in China
This work uses the contiguous rain area (CRA) method (Ebert and McBride, 2000) to verify the quantitative precipitation forecasts (QPFs) of the BJ-RUCv2.0 system, with total error decomposed into the components due to horizontal displacement of the system, error in the mean rain intensity and pattern errors
The 24-h forecast released by the National Meteorological Center of China Meteorological Administration at 2100 UTC 20 July 2012 indicated heavy rainfall in Beijing area with a blue rainstorm warning
Summary
Heavy rains are commonly responsible for weather disasters in China. Associated with the summer monsoon, three primary rain zones of China in the warm season are located in southern China, Jiang-Huai area, and northern China, respectively (Tao, 1980). A number of papers investigated detailed features such as the sources of water vapor, frontogenesis, upper-level jet stream, and so on (Chen et al, 2013; Li et al, 2013; Liao et al, 2013; Quan et al, 2013; Sun et al, 2013) These studies focused on the precipitation dynamics and conditions of this extreme event using various observations, trying to gain a better understanding of its mechanisms. As an alternative, Ebert and McBride (2000) developed an objective-oriented verification method within the framework of contiguous rain areas (CRAs) Through this method, the total rainfall error of model forecasts can be decomposed into the components of location, rain volume and pattern, providing quantitative analysis of precipitation errors.
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