关键词: 极端降水, 评估检验, 地形, 风场特征

Abstract:

From July 27 to August 2, 2023, an extreme heavy rainfall event occurred in North China (referred to as the “23·7” event), and significant discrepancies are found among different numerical weather prediction models. In this study, three global models and seven regional models are selected to systematically evaluate the forecast performance for this heavy rainfall event under complex terrain conditions, based on a categorical analysis of 3-hourly accumulated precipitation, combined with precipitation verification and surface wind field analysis. The results show that the global models perform relatively well in predicting precipitation of ≥0.1 mm and ≥1 mm, but with significant overforecasting, and exhibit obvious underprediction for precipitation of ≥10 mm. The regional models exhibit superior overall forecasting performance to global models, especially for precipitation of ≥10 mm, for which their forecasting skill is significantly higher. Model resolution is found to exert a considerable influence on the prediction of rainfall at different intensities. Better performance is achieved by high-resolution models compared with low-resolution models, particularly for precipitation of ≥5 mm, though exceptions do exist. With regard to the fitting of precipitation frequency distribution against precipitation intensity, regional models are demonstrated to be overall distinctly superior to global models. Analysis based on the Froude number (Fr) reveals the synergistic mechanism between terrain and wind fields. Overestimation of wind speed or underestimation of terrain height in models leads to the misrepresentation of flow-around as orographic updraft, inducing biases in the predicted precipitation location. In contrast, the characteristics of orographic lifting or flow around can be better preserved in regions with reasonable local Fr values.

Key words: extreme precipitation, forecast evaluation, topography, wind field characteristics