Analysis of the Characteristics of the Low-level Jets in the Middle Reaches of the Yangtze River during the Mei-yu Season

Here,we analyze the characteristics and the formation mechanisms of low-level jets(LLJs)in the middle reaches of the Yangtze River during the 2010 mei-yu season using Wuhan station radiosonde data and the fifth generation of the European Centre for Medium-Range Weather Forecasts(ERA5)reanalysis dataset.Our results show that the vertical structure of LLJs is characterized by a predominance of boundary layer jets(BLJs)concentrated at heights of 900-1200 m.The BLJs occur most frequently at 2300 LST(LST=UTC+8 hours)but are strongest at 0200 LST,with composite wind velocities>14 m s^(-1).Synoptic-system-related LLJs(SLLJs)occur most frequently at 0800 LST but are strongest at 1100LST,with composite wind velocities>12 m s^(-1).Both BLJs and SLLJs are characterized by a southwesterly wind direction,although the wind direction of SLLJs is more westerly,and northeasterly SLLJs occur more frequently than northeasterly BLJs.When Wuhan is south of the mei-yu front,the westward extension of the northwest Pacific subtropical high intensifies,and the low-pressure system in the eastern Tibetan Plateau strengthens,favoring the formation of LLJs,which are closely related to precipitation.The wind speeds on rainstorm days are greater than those on LLJ days.Our analysis of four typical heavy precipitation events shows the presence of LLJs at the center of the precipitation and on its southern side before the onset of heavy precipitation.BLJs were shown to develop earlier than SLLJs.