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Causal Analysis of Consecutive Torrential Rains in Guangdong Province Before the Onset of South China Sea Monsoon
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摘要: 2010年5月上中旬南海季风尚未爆发，广东一周内出现罕见的连续3场区域性暴雨 (下称连场暴雨)。利用常规气象观测资料和NCEP分析资料，从降水时间特征和环流形势对比了连场暴雨和持续性暴雨的异同，并应用局地经向环流数值模式诊断探讨其可能形成机制。结果表明:中高纬度地区阻塞形势建立对广东5月连场暴雨和6月持续性暴雨发生均尤为关键，连场暴雨期间阻塞高压位于乌拉尔山附近，降水与中纬度短波槽南下密切相关；而持续性暴雨期间阻塞高压偏东位于亚洲大陆中部，降水主要受热带西南季风北推影响。尽管大尺度环流背景相似，但3场暴雨过程天气系统配置差异较大。数值诊断结果进一步表明：激发连续3场暴雨的主要物理因子为潜热加热、温度平流和西风动量输送。潜热加热是此次连场暴雨的正贡献和正反馈的最直接因子，而西风动量输送和温度平流对暴雨发生有一定触发作用和指示意义 (超前0~1.5 d)。因此，分析和预报季风爆发前的连场暴雨过程，应注意中高纬度地区西风动量输送、冷暖平流活动和相应的天气形势演变。Abstract: Guangdong suffers from consecutive torrential rains before the onset of South China Sea Monsoon (SCSM) in May 2010, which is rarely seen in South China. Based on meteorological observations, regional automatic weather station data and NCEP data, the characteristics of the consecutive torrential rains are analyzed through a comparative analysis of consecutive torrentials and persistent rains. And the possible mechanism of the consecutive torrential rains are analyed with a complete (including dynamic and thermodynamic mechanisms) linear-diagnostic model for the local-meridional circulation.The results show that the blocking high plays an important role in both consectutive torrential rains and persistent rains. The consectcutive torrential rains are associated with the south-eastward propagation of short wave troughs and the blocking high located over Mount Ural, while the persistent torrential rains are associated with the northward movement of the SCSM and the blocking located over central of Asian continent. There are obvious differences among the weather patterns of the three consecutive torrential rains, although the large-scale background circulations are similar. "5.7" torrential rain occurs behind upper-level trough, lower-level shear line and cold front far from the rain belt, which is rarely observed in South China. "5.9" torrential rain is relatively typical since it occurs near the front and in the south of lower-level shear line, but upper-level short-wave trough is not clearly indentified. "5.14" torrential rain featured typical rainfall pattern in South China, heavy rainfall occurs in front of upper-level trough, in the south of low-level shear line and near the front.Numerical quantitative diagnosis shows that the contributors to the local meridional circulation associated with the consecutive torrential rains are mainly latent heating, horizontal temperature advection and westerly momentum transport. Latent heating is the major contributor and provides positive feedback to the torrential rain, while horizontal temperature advection and westerly momentum transport play an important role in triggering consecutive torrential rains (0—1.5 d prior to the torrential rains). The upper-level westly jet is in favor of the rising motion and upper-level divergence of Guangdong through the horizonal westerly momentum transport and vertical air mass adjustment. On the other side, the vertical westerly momentum transport and the upper-level trough provide favorable conditions for the southward moverment of cold air mass, which triggering stronger rising motion over Guangdong and more moisture convergence (latent heat release). Therefore, it is necessary to pay more attention to the evolution of mid-latitude synoptic circulation associated with westerly momentum and horizontal temperature advection in forecasting the consecutive torrential rains before the onset of SCSM.
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图 1 2010年5月上中旬广东3场暴雨过程累积降水量分布
(a)“5.7”大暴雨，(b)“5.9”暴雨，(c)“5.14”暴雨
Fig. 1 Distribution of heavy rain processes on 7 May (a), 9 May (b) and 14 May (c) in 2010
图 2 2010年5月上中旬 (a) 和2005年6月中下旬 (b) 区域平均 (22°~24.5°N, 112°~116°E) 逐日降水量
Fig. 2 Area-averaged (22°~24.5°N, 112°~116°E) daily precipitation from 1 May to 17 May in 2010(a) and from 9 Jun to 26 Jun in 2005(b)
图 3 2010年5月连场暴雨主降水期 (5月6日、5月9日、5月14日)(a) 和2005年6月持续性暴雨过程 (19—23日)(b) 的500 hPa位势高度 (单位：dagpm) 合成图 (粗黑线为槽线)
Fig. 3 500 hPa mean geopotential height (unit:dagpm) during the consecutive torrential rains in May 2010(a) and persistent torrential rains in June 2005(b)(the thick line denotes the trongh)
图 4 2010年5月上中旬500 hPa位势高度场 (单位：dagpm) 和上升运动 (a) 及2005年6月中下旬850 hPa风场 (矢量) 和500 hPa上升运动中心 (b) 沿112°~116°E平均的时间-纬度图 (阴影为上升运动中心, 单位:10-2Pa·s-1)
Fig. 4 Latitude-time cross-section of 500 hPa mean geopotential height (unit: dagpm) and ascending motion from 1 May to 17 May in 2010(a) and 850 hPa mean horizontal wind (vector) and 500 hPa ascending motion from 9 Jun to 28 Jun in 2005(b) for 112°—116°E (shaded areas represent the significant upward motion, unit: 10-2Pa·s-1)
图 5 2010年5月上中旬3场暴雨过程天气形势图 (阴影区为强降水区)
(a)“5.7”大暴雨，(b)“5.9”暴雨，(c)“5.14”暴雨
Fig. 5 Schematic diagram of weather pattern of heavy rain processes on 7 May (a), 9 May (b) and 14 May (c) in 2010 (shaded areas represent the heavy rain area)
图 6 2010年5月1—17日广东连场暴雨期间500 hPa垂直运动的时间演变 (单位：10-2Pa·s-1)
(a) 实测，(b) 模拟，(c) 潜热加热单独激发，(d) 水平温度平流单独激发
Fig. 6 Time series of zonal averaged (from 107° to 117°E) 500 hPa ωφ(unit:10-2Pa·s-1) in Guangdong consecutive torrential rains during 1—12 May 2010
(a) observed, (b) simulated, (c) effect of latent heating, (d) effect of horizontal temperature advection
图 7 2010年5月1—17日广东连场暴雨期间沿113°E的850 hPa水汽通量 (矢量，单位：g·cm-1·hPa-1·s-1) 和925 hPa经向风 (阴影) 的纬度-时间演变图
Fig. 7 Latitude-time cross-section along 113°E of 850 hPa water vapor flux (vector, unit: g·cm-1·hPa-1·s-1) and 925 hPa meridional wind component (shaded) in Guangdong consecutive torrential rains during 1—17 May 2010
图 8 2010年5月3次暴雨过程期间沿113°E的假相当位温 (等值线，单位：K) 和垂直环流 (矢量) 分布
(黑色三角形为广州所在纬度，阴影为小于340 K的区域)
Fig. 8 Height-latitude cross-sections of pseudo-equivalent potential temperature (contour, unit: K) and vertical circulation (vector) along 113°E (shaded areas represent pseudo-equivalent potential temperature less than 340 K) in May 2010
图 9 2010年5月1—17日主要物理因子在暴雨区 (24°N) 单独激发的ωψ|500与所有因子共同激发的总体ωψ|500的相关系数
(绝对值大于0.33的相关系数达到0.01显著性水平)
Fig. 9 Time-lag correlation between total ωψ|500 and ωψ|500 attributed to the main effects (24°N) during 1 May to 17 May in 2010
(correlation coefficients greater than 0.33 are significant for the 0.01 level)
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