云南区域夏季风雨季爆发前后大气热源和云量特征

Characteristics of Atmospheric Heat Source and Cloud Amount over Yunnan before and after Summer Monsoon Rainy Season Onset

本研究应用2001~2020年欧洲中期天气预报中心第五代再分析资料(ERA5)的气象场及卫星反演的降水和云量资料,研究了云南区域夏季风雨季爆发前后大气热源和云量的气候特征。研究结果表明:(1)云南区域的大气热源和云量会受夏季风环流的强烈影响,有着明显的逐月变化。6月受来自孟加拉湾东部的西南气流水汽输送影响,云南季风雨季爆发,大气总热源(特别是降水凝结释放的潜热)和云量较4~5月明显增强,而地表感热和大气辐射冷却作用减小。(2)基于Wang and LinHo(2002)方法计算的2001~2020年期间云南夏季风雨季的平均爆发时间约为第31候,区域大气总热源(潜热)和云量均与降水呈现出高度的时间相关性,而且云南区域大气热源和云量随夏季风雨季的时间变化呈现出与相邻的南亚热带季风区相似的单峰年变化特征。(3)云南夏季风雨季爆发时间存在明显的年际变化,雨季爆发偏早(晚)年的合成结果表明:在偏早年雨季爆发时,来自孟加拉湾东南部的低层西南气流可直达云南区域,该区域上空为辐散气流,有利于区域上升运动,云南区域大气总热源(潜热)和云量明显强于偏晚年;偏晚年同时段的孟加拉湾南支槽偏弱,西太平洋副高位置明显偏西,不利于云南区域上升运动和降水。在雨季盛期,偏早和偏晚年的区域降水率接近,偏晚年的区域大气总热源(潜热)和高云量值略高于偏早年。

This study investigates the climatological characteristics of the atmospheric heat source and cloud amount over Yunnan before and after the onset of summer monsoon rainy season using meteorological field of ERA5(European Center for Medium-Range Weather Forecasts fifth generation global atmospheric reanalysis dataset)and satellite-retrieved precipitation and cloud amount data during 2001-2020.The results show the following:(1)The atmospheric heat source and cloud amount over Yunnan are strongly influenced by summer monsoon circulation and exhibit significant monthly variations.In June,the transport of water vapor by the southwesterly from the eastern Bay of Bengal causes Yunnan summer monsoon rainy season.After the onset of the summer monsoon rainy season,the regional atmospheric heat source(especially latent heat released by precipitation condensation)and cloud amount significantly increase with respect to those in April-May,while surface sensible heat flux and net radiation flux of air column are reduced.(2)According to the method reported by Wang and LinHo(2002),the average onset time of the summer monsoon rainy season over Yunnan is approximately pentad 31 during 2001-2020.The regional atmospheric heat source(latent heat)and cloud amount correlate well with the total precipitation rate.The changes in the atmospheric heat source and cloud amount with the summer monsoon rainy season over Yunnan present annual variations similar to those over the adjacent South tropical monsoon region,with a single peak structure.(3)The onset time of the Yunnan summer monsoon rainy season demonstrates an evident interannual variability.Synthetic analyses reveal that in the early-onset years,the low-level southwesterly wind from the southeastern Bay of Bengal can reach the Yunnan region while the high-level divergence exists,which is conducive to regional ascending motion,and atmospheric heat source(latent heat)and cloud amount are much higher than those in the late-onset years.The southern branch trough in the Bay of Bengal is weaker,and the position of the Western Pacific subtropical high extends more toward the west in late-onset years,which are unfavorable for regional ascending motion and precipitation over Yunnan.In the peak period of the Yunnan summer monsoon rainy season,the precipitation rate is similar in the early-and late-onset years,but the atmospheric heat source(latent heat)and high cloud amount in the late-onset years are slightly higher than those in the early-onset years.