ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
Analysis of Statistical Characteristics of Upper-Air Specific Humidity during Heavy Rainfall Events in Beijing
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Abstract:
This study utilizes 24-hour surface precipitation data from 08:00 to 08:00 (local time) and focuses on 83 heavy rainfall days in Beijing from 2016 to 2023. Based on the 500 hPa geopotential height field, the circulation patterns are classified into six categories: heavy rainfall associated with the Mongolian vortex and trough, heavy rainfall from the interaction between the subtropical high and westerly trough, heavy rainfall of the Hetao vortex type, heavy rainfall of the Northeast cold vortex type, heavy rainfall from tropical cyclones, and heavy rainfall resulting from the combination of westerly trough and typhoon. It was found that the Hetao vortex type and tropical cyclone type are more likely to produce widespread extreme heavy rainfall, while the Northeast cold vortex type mainly causes localized heavy rainfall and rarely results in widespread events.Statistical analysis of observational data from the Beijing station (54511) at 925, 850, 700, 500, and 200 hPa levels revealed that specific humidity at all levels during heavy rainfall periods is significantly higher than during non-heavy rainfall periods. Further analysis showed that, among the six types of heavy rainfall events, tropical cyclone and Hetao vortex events exhibit the highest specific humidity values, corresponding to the highest precipitation intensity and coverage.Using hourly precipitation data, precipitation occurring within one hour before and after the upper-air observation times (08:00 and 20:00) was categorized into three cases: heavy rainfall (≥10 mm), light rain (0.1–9.9 mm), and no precipitation. Statistical analysis of specific humidity at different levels showed that specific humidity is highest during heavy rainfall periods, followed by light rain periods, and lowest during no-rain periods.By combining jet stream analysis, the specific humidity characteristics of heavy rainfall processes corresponding to winds from eight directions at different levels were examined. It was found that specific humidity is highest when the wind direction is between 90° and 225° with a jet stream present, and lowest when the wind direction is between 270° and 300° with a jet stream. This study addresses the previous research focus on specific humidity characteristics below 700 hPa, particularly at 850 hPa, during heavy rainfall processes, while lacking analysis of higher levels. Innovatively, it reveals that an increase in specific humidity at the 500 hPa level has significant indicative value for heavy rainfall forecasting..
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