On May 11, 2022, despite the favorable upper and lower-level circulation patterns of the high-altitude trough, shear line, and southwest jet stream, the urban cluster of the Guangdong-Hong Kong-Macao Greater Bay Area experienced light to moderate rainfall, deviating significantly from the forecasted heavy rain and local heavy rainstorm. This study explores the reasons for false alarms and predictability using ground observation data, radar data, ECMWF-ERA5 reanalysis field data, and ECMWF and CMA-TRAMS forecast data. The results indicate that the warm and moist airflow transported by the low-level jet stream was intercepted by the upstream MCS (mesoscale convective system) along the coastal area of western Guangdong, and inadequate conditions of negative vorticity dynamics led to insufficient moisture, thermodynamic, and dynamic conditions over the urban cluster, preventing the triggering of heavy precipitation. In addition, the 700 hPa westerly flow guiding the airflow and the stable low-level shear line, coupled with surface convergence lines, influenced the northward or southward movement of MCSs along the coastal and inland regions of western Guangdong. The weak and discontinuous intensity of echoes in the upstream Zhaoqing region further hindered the influence of surrounding echoes on the urban cluster. Numerical forecast models ECMWF and CMA-TRAMS overestimated the 850 hPa windspeed and 925 hPa meridional windspeed, resulting in the forecasted urban cluster experiencing heavy rain. Sensitivity tests of wind fields indicate that the 850 hPa wind field information is more sensitive to precipitation in the urban cluster. In this process, weak signal correction can be achieved in strong precipitation forecasts using the distinct signal of lower 850 hPa water vapor flux divergence compared to 925 hPa. Therefore, in the future, when the Guangdong-Hong Kong-Macao Greater Bay Area encounters similar warm-sector heavy rainfall events, adjustments to model forecasts can be made using specific 850 hPa elements such as wind speed, water vapor flux divergence, or specific humidity to enhance predictive accuracy.

中文翻译：

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粤港澳大湾区城市群一次强降雨过程误报原因及风场敏感性分析

2022年5月11日，尽管高空槽、切变线、西南急流等上下层环流格局有利，粤港澳大湾区城市群仍出现小到中雨，与预报的大雨和局地大暴雨偏离较大。本研究利用地面观测数据、雷达数据、ECMWF-ERA5再分析现场数据以及ECMWF和CMA-TRAMS预报数据探讨了误报和可预测性的原因。结果表明:粤西沿海低空急流输送的暖湿气流被上游MCS(中尺度对流系统)拦截,负涡动力条件不充分导致水汽、热力、和城市群上空的动态条件，防止引发强降水。此外，引导气流的700 hPa西风气流和稳定的低空切变线，加上地面辐合线，影响了粤西沿海和内陆地区MCS的北移或南移。肇庆上游地区回波强度较弱且不连续，进一步阻碍了周边回波对城市群的影响。ECMWF和CMA-TRAMS数值预报模型高估了850 hPa风速和925 hPa经向风速，导致预报城市群出现大雨。风场敏感性测试表明，850 hPa风场信息对城市群降水较为敏感。在此过程中，利用 850 hPa 水汽通量散度低于 925 hPa 的独特信号，可以在强降水预报中实现弱信号校正。因此，未来粤港澳大湾区遭遇类似暖区强降雨事件时，可利用特定的850 hPa要素如风速、水汽通量散度或特定的850 hPa要素对模式预报进行调整。湿度以提高预测准确性。