PERSISTENT PRECIPITATION ANOMALIES AND QUASI-BIWEEKLY OSCILLATION DURING THE ANNUALLY FIRST RAINY SEASON OVER SOUTH CHINA IN 2010
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摘要: 利用中国台站逐日降水资料以及NCEP-DOE逐日再分析资料，分析了2010年华南汛期降水异常的低频特征及大气环流的影响。结果表明，2010年华南汛期降水异常呈显著的低频振荡特征，其中前汛期以10~20天振荡（准双周振荡）为主，后汛期以20~50天振荡（季节内振荡）为主。重点讨论了准双周尺度上前汛期持续性降水异常与中高纬和热带地区大气低频振荡的关系。中高纬地区的低频环流可通过Rossby波能量沿着低频遥相关波列的频散影响华南低频环流的变化。波活动通量分析显示，西西伯利亚作为Rossby波源，其波能量沿着横跨欧亚大陆的低频遥相关波列向我国东部地区频散，引起该地扰动加强，从而引起华南低频环流及垂直运动的变化进而造成华南降水的异常。热带东印度洋的准双周振荡是影响华南前汛期降水的另一低频来源。当赤道东印度洋对流旺盛（抑制），其上空为强上升（下沉）气流，低层辐合（辐散）高层辐散（辐合），而华南上空盛行下沉（上升）运动，不利于（有利于）华南降水。来自中高纬和低纬的低频信号的叠加并配合低频水汽输送共同影响了华南环流异常的低频变化，从而引起华南的低频降水异常，有利于华南持续性降水异常的发生。Abstract: Based on the in-situ daily precipitation from Chinese Meteorological Administration (CMA) and the National Centers for Environmental Prediction-Department of Energy (NCEP-DOE) daily reanalysis data, the low-frequency characteristics of precipitation anomalies and the impact of atmospheric circulation during the rainy seasons of South China in 2010 are investigated. The results show that the precipitation anomalies during this period have obvious low-frequency characteristics. Specifically, the annually first rainy season is controlled by a 10~20-day oscillation (quasi-biweekly oscillation), while the annually second rainy season is controlled by a 20~50-day oscillation (intraseasonal oscillation). With focus on the quasi-biweekly oscillation, the relationship between the persistent precipitation anomalies and the atmospheric low-frequency oscillation at mid-high latitude and tropical zone during the annually first season is discussed. The low-frequency circulation at the mid-high latitude can influence the low-frequency circulation over South China by the dispersion of Rossby wave along the low-frequency wave train. The analysis of wave activity flux indicates that the wave energy originated from West Siberia disperses toward the eastern China along the low-frequency teleconnection wave train which stretches across the Eurasian continent, leading to the strengthening of the disturbance over eastern China and the precipitation anomalies by changing the low-frequency circulation and vertical movement over South China. The quasi-biweekly oscillation over the tropical eastern Indian Ocean (IO) is another low-frequency origin influencing the annually first season of South China. When the convection over the eastern equatorial IO is strong (inhibited), there are strong ascending (descending) movements in this region accompanied with convergence (divergence) at lower layer and convergence (divergence) at upper atmosphere. At the same time, descending (ascending) wind prevails over South China, suppressing (promoting) the precipitation. The superposition of low-frequency signals from mid-high and low latitude, along with the transportation of anomalous moisture, determined the low-frequency variations of circulation anomalies over South China, causing the low-frequency precipitation anomalies and favoring the persistent rain anomalies.
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图 3 2010年4—6月华南降水原始序列（黑色曲线），异常序列（直方图）和10~20天滤波序列（红色曲线)
单位：mm。虚线为正/负1个标准差，数值表示准双周振荡位相。降水量/(mm/d)
图 5 2010年4—6月华南降水10~20天振荡位相合成的沿107.5~120 °E平均的低频位势高度（等值线，单位：gpm）和低频纬向风（阴影，单位：m/s）及低频垂直环流（流线，垂直速度（单位：-10-2 Pa/s）和经向风（单位：m/s）的合成矢量）的经向剖面图
a.最干位相；b.最湿位相。
图 6 2010年4—6月华南降水10~20天振荡位相合成的最湿位相200 hPa低频流函数（等值线，单位：106 m2/s）、波活动通量（矢量箭头，单位：m2/s2, x方向分量大于10 m2/s2）及其散度（阴影，单位：10-5 m/s2）
图 8 2010年4—6月华南降水10~20天振荡位相合成的对流层高低层10~20天低频速度势（等值线，单位：10-6 m2/s）、低频辐散风场（矢量，单位：m/s）及其散度（阴影，单位：10-6 m/s2）
a. 200 hPa最干位相；b. 200 hPa最湿位相；c. 850 hPa最干位相；d. 850 hPa最湿位相。
图 9 2010年4—6月华南降水10~20天振荡位相合成的沿着点（90 °E，5 °N）到点（117.5 °E，25 °N）的10~20天滤波的垂直速度（阴影，单位：-10-2 Pa/s）和垂直环流场（矢量，垂直速度，单位：-10-2 pa/s和经、纬向风（单位：m/s）的合成）随位相演变情况
图 10 2010年4—6月华南降水10~20天振荡位相合成的整层积分的水汽通量输送（矢量, 单位：kg/(m·s)）及其散度（阴影区表示水汽辐合区, 单位：10-5 kg/(m2·s)）
a、c为最干位相，b、d为最湿位相。a、b为原始水汽输送，c、d为低频风对平均水汽的输送。
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