COMPARATIVE STUDY OF CHARACTERISTICS AND THERMODYNAMICAL DEVELOPMENT MECHANISM ON TWO TYPES OF WARMSECTOR HEAVY RAINFALL IN SOUTH CHINACOAST
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摘要: 对2009—2014年4—6月的华南沿海暖区暴雨依据低层环流进行分类：第一类为偏南向型，即珠江口以西经向性偏南向辐合线型，由东南、偏南、西南三支气流汇合；第二类为西南向型，即珠江口以东纬向性西南向辐合线型，由偏西和西南风两支气流汇合。6小时强降水统计显示，6年中偏南向型年平均73.2次，西南向型年平均50.3次。南海夏季风爆发后，两种类型发生频数均明显增加。两类低层辐合线系统对应上层的合成特征显示，500 hPa天气形势偏南向型为宽阔暖脊，温度脊落后，有较明显的暖平流，无锋区；西南向型为弱槽，中纬度温度槽落后，锋区偏北，华南位于锋面前暖区，有弱波动。两类暴雨垂直剖面上有深厚垂直速度中心伸展到400 hPa；对应强烈的辐合层为几个垂直叠置的辐合中心；均为水汽充沛，对流不稳定能量层次深厚，有较强凝结潜热释放，造成气柱增暖拉伸，加强深厚多中心辐合及上升气流，其中西南向型凝结潜热释放更强，偏南向型中高层暖平流强于其凝结潜热释放。探讨热力发展机制的数值模拟显示，凝结潜热释放对气柱增温，大大增强暴雨区垂直速度厚度与强度，并增强暴雨区低层辐合环流，减弱中层辐散环流，其影响力达到环流强度的30%~50%，有利于维持强烈对流，促进暖区暴雨的发展。Abstract: Warm-sector heavy rainfalls in the South China coast during April to June in 2009—2014 can be mainly divided into two types according to low level circulations; one is the southern pattern with a meridional convergence line at the west of Pearl River Estuary, which is formed by the convergence of southeasterly, southerly and southwesterly, and the other is the southwesterly pattern with a zonal convergence line at the east of Pearl River Estuary, which is formed by convergence of westerly and southwesterly. Statistics on 6-hr heavy rainfall events indicate that, during the 6 years, there are on average 73.2 times of the southerly pattern and 50.3 times of the southwesterly pattern per year. After the onset of summer monsoon in the South China Sea, the occurrence frequencies of both patterns increase remarkably. The synthetic diagnosis of pattern circulation show that, at 500 hPa, there is a broad warm high ridge and a temperature ridge is behind the high ridge, which causes an obvious warm advection at the high ridge area. There is no frontal region for the southerly pattern; For the southwesterly pattern, the circulation is a weak trough with a temperature trough falling behind the height trough with weak perturbations, the position of frontal region is near Yangzi River, the South China coast is located in the warm-sctor of the frontal region. At the vertical cross-sections of both categories of heavy rainfall, each has a strong vertical motion center stretching to 400 hPa. The convergence layer in the rainfall region is deep and with several vertical convergence centers overlapping each other. Both types of heavy rainfalls are all with abundant water vapor, with deep convective instability energy layers, and with strong release of latent heat caused by condensation of water vapor. The release of latent heat leads to the warming-up and stretching of air column, thus strengthens deep convergence and vertical velocity upward. There is stronger latent heat release in the southwesterly pattern than in southerly, but there is a stronger warm advection at middle and upper layers which is larger than the latent heat release in the southerly pattern. For investigating in thermo-dynamical developing mechanisms, WRF numerical simulations are made and the results show that, latent heat release warms up the air column, and strengthens and maintains the depth and strength of vertical velocity upward effectively. Also, the release of latent heat strengthens convergent circulation at lower layers and weakens divergent circulation at middle layers over the strong rainfall regions. The influence power can be as strong as 30%~50% of the wind circulation strength of the two types of the warm-sector heavy rainfall over South China coast. The release of latent heat increases and maintains the deep convection and the warm-sector heavy rainfall development.
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图 3 两类暖区暴雨的环流垂直分布与垂直速度的经向剖面s
a.偏南向型的112 °E剖面；b.西南向型的113 °E剖面。阴影区为垂直速度大小，实线为v-w的合成流线。单位：m/s。
图 4 两种类暖区暴雨沿22 °N相对涡度（阴影，单位：10-5 /s）和散度（等值线，单位：10-5 /s）的纬向垂直剖面
a.偏南向型；b.西南向型。散度仅显示＜0的辐合部分。
图 8 两类暴雨过程3个时刻（UTC）的TCTL（a~c）与RML（d~f）的温度差经向垂直剖面
a~c分别为偏南向型2013年5月8日00、03、04时（UTC）沿111.8 °E；d~f分别为西南向型2013年5月21日18、22、23时沿113 °E。单位：℃
图 9 偏南向型（2013年5月8日03时，a）和西南向型（2013年5月21日22时，b）最强降水时刻的850 hPa上TCTL与RML风场差值场
单位：m/s。矢量为偏差风，细实线为风速值偏差中心。
表 1 2009—2014年4—6月两种暖区暴雨类型年分布和月分布样本频数统计
单位：次。 年份与月份 2009 2010 2011 2012 2013 2014 4月 5月 6月 偏南向型 52 45 71 104 92 75 88 153 198 西南向型 24 59 42 61 65 51 64 124 114 合计 76 104 113 165 157 126 152 277 312 
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