| Based on the ERA5 hourly reanalysis data and the merged hourly precipitation products from National Meteorological Centre from May to June of 2010—2016,low-level jets(LLJ),low-level jet events and low-level jet days over South China are selected and classified using an objective method.The characteristics and diurnal variation mechanisms of boundary layer jet(BLJ)and synoptic-system-related low-level jet(SLLJ)are investigated during earlysummer,as well as their influence on the temporal and spatial distributions of the diurnal cycle of precipitation over South China.On this basis,the spatial distribution of heavy rainfall and the difference of weather background between heavy rainfall days and non heavy rainfall days are compared.Finally,the influence of the diurnal cycles of low-level jets on the diurnal cycle of precipitation over South China is studied.The main conclusions are as follows:(1)The SLLJ features a southwesterly wind maximum at 850—700 hPa located on land,whereas the BLJ usually features a southerly wind maximum at 975—925 hPa over Beibu Gulf and the northern area of South China Sea.BLJ and SLLJ weaken during daytime while intensify during nighttime,reaching their peaks in the early morning.The diurnal cycles are found to be associated with the clockwise rotation of ageostrophic winds caused by boundary layer inertial oscillations.(2)The differences of the three types of LLJ days in the low-level jets,the large-scale circulation background,the water vapor transport and the thermal dynamic conditions lead to the differences of the precipitation.The spatial distributions of the occurrence frequency of heavy rainfall and short-duration heavy rainfall are similar,and the short-duration heavy rainfall often occur in heavy rainfall days.The differences of weather background between heavy rainfall days and non heavy rainfall days are mainly manifested in the morphological distribution and spatial configuration of South Asia High,Western Pacific Subtropical High,BLJ and SLLJ.(3)The diurnal cycle of meso-?-scale rain events is a response to the diurnal cycle of lowlevel jets.The low-level jets develop at night,and the meso-?-scale rain events gradually strengthen and propagate from west to east.There is only a peak precipitation in the morning on the double low-level jet days and the pure SLLJ days,while there is a sub peak precipitation in the afternoon on the pure BLJ days.(4)The precipitation over South China affected by the double low-level jets shows obvious spatial differences with respect to the diurnal cycle due to the evolution and configuration of double low-level jets.The precipitation in central and northern Guangxi is mainly concentrated over the mountainous area at the left anterior to the SLLJ at nighttime and is characterized by only one single peak in the early morning,whereas in Guangdong and the coastal areas of Guangxi,there are two peaks in the morning and afternoon,respectively.The maintenance of convergence at the BLJ' exit and mid-level divergence near the SLLJ's entrance are conducive to the increasing frequency of precipitation and hence emerge the peak in the afternoon.In addition to the favorable configuration of the double low-level jets,the morning peak of precipitation is also attributed to the peak of precipitation intensity caused by the enhanced double low-level jets at nighttime in the diurnal cycle process.(5)Moreover,further diagnosis shows that,during the double low-level jet days,the moisture flux at the low-level troposphere at nighttime is higher than that at daytime by roughly30%.The net local moisture flux is closely associated with the moisture transport by ageostrophic winds.Correspondingly,the diurnal cycle of net moisture flux is similar to that of low-level jets,featuring a phase of 5—6 hours ahead of precipitation.The double low-level jets affect the temporal and spatial distributions of precipitation over South China via regulations on moisture transport and convergence by both dynamic and thermal effects. |
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