双低空急流影响下华南初夏降水日变化的时空分布特征

利用2010—2016年5—6月ERA5逐小时再分析数据集和国家气象信息中心逐小时降水量融合产品，对影响华南地区的低空急流事件进行筛选和分类，并分析天气系统相关的低空急流(Synoptic-system-related Low-Level Jet, SLLJ)和边界层急流(Boundary Layer Jet, BLJ)的日变化及其影响下的华南降水日变化的时空分布特征。结果表明，BLJ和SLLJ在白天减弱、夜间增强，并在凌晨达到峰值，其日变化主要与边界层惯性振荡引起的非地转风的顺时针旋转有关。双急流日华南地区降水量显著增加，且降水日变化有明显的区域差异，这与双急流的演变和配置密切相关。广西中北部主要为SLLJ左前方发生的夜间山区降水，且降水量仅有凌晨的单峰。广西沿海和广东地区存在早晨和午后两个峰值，BLJ出口区辐合和SLLJ入口区辐散的维持有利于降水频率的增大，从而导致午后峰值的出现，而早晨的峰值除了受双急流有利配置的影响外，主要归因于早晨降水强度的增加。

Temporal and spatial distributions of diurnal cycle of early-summer precipitation in South China influenced by double low-level jets

Based on the ERA5 hourly reanalysis dataset and the merged hourly precipitation products from National Meteorological Centre from May to June from 2010 to 2016, the low-level jet events affecting South China were selected and classified using an objective method.This paper also investigated the diurnal cycles of boundary layer jet (BLJ) and synoptic-system-related low-level jet (SLLJ) during early-summer, and analyzed their influences on temporal and spatial distributions of diurnal cycle of precipitation in South China.Results show that BLJ and SLLJ weaken during the day and increase at night, and reach the peak in the early morning.Their diurnal cycles are mainly related to the clockwise rotation of ageostrophic winds caused by inertial oscillations of boundary layer.The precipitation in South China increases significantly during the double low-level jet days, and the diurnal cycle of precipitation has obvious regional differences, which is closely related to the evolution and configuration of the 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 night and is characterized by only one single peak in the early morning.There are two peaks in the morning and afternoon in coastal Guangxi and Guangdong.The maintenance of convergence in BLJ exit area and divergence in SLLJ inlet area are conducive to the increase of precipitation frequency, resulting in the emergence of afternoon peak.In addition to the influence of the favorable configuration of the double low-level jets, the morning peak of precipitation is mainly due to the increase of the morning precipitation intensity.