Abstract:Based on the 24 h surface precipitation observation data from 08:00 BT to 08:00 BT of the next day and upper-air sounding data at 08:00 BT and 20:00 BT during 2016-2023 as well as ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF), this paper identifies a total of 83 torrential rain days in Beijing during the study period. On this basis, Beijing torrential rain events are classified according to the synoptic situations at 500 hPa, and a statistical analysis is conducted on the specific humidity characteristics at 925 hPa, 850 hPa, 700 hPa, 500 hPa and 200 hPa during the torrential rain days. The results are as follows. The torrential rain events in Beijing are divided into six types: torrential rain induced by Mongolian low vortex and trough type, subtropical high and westerly trough type, Hetao low-vortex type, northeast cold vortex type, tropical cyclone (typhoon) type, and westerly trough and typhoon type. The Hetao low-vortex type and tropical cyclone (typhoon) type tend to produce more extensive extreme torrential rains, while the northeast cold vortex type is dominated by localized torrential rain. In terms of specific humidity, it is markedly higher at all atmospheric levels during torrential rain periods than that during non-torrential rain periods. Among the six types of torrential rain events, tropical cyclone (typhoon) type and Hetao low-vortex type feature the highest specific humidity, which corresponds to the strongest precipitation intensity and the largest rainfall coverage of these two categories. Furthermore, based on hourly precipitation data, the precipitation conditions within one hour before and after the upper-air observation time (08:00 BT and 20:00 BT) are categorized into three grades: torrential rain (≥10 mm), light rain (0.1-9.9 mm) and no precipitation. The specific humidity is the highest at all levels during torrential rain period, followed by that in light rain period, and the lowest specific humidity is found in no precipitation period. In addition, the specific humidity of torrential rain events corresponding to winds from eight directions at different levels is statistically analyzed, and the results indicate that the specific humidity reaches its maximum when the wind direction ranges from 90° to 225° with jet streams, and drops to its minimum when the wind direction is between 270° and 315° accompanied by jet streams. Overall, this study has not only analyzed the specific humidity characteristics below 700 hPa during torrential rain events, but more specially found that the increase in specific humidity at 500 hPa can serve as a significant predictor for torrential rain forecasting.