摘要:
本文利用最近12年的TRMM（Tropical Rainfall Measuring Mission)卫星资料, 分析了亚洲季风区积云降水和层云降水的时空分布特征。结果表明: 从多年平均角度看, 亚洲季风区积云降水和层云降水空间分布主要呈现出随纬度变化的特征: 25°N以北的副热带季风区以层云降水方式为主, 其所占比例在50％以上, 而且表现为由南向北层云降水比例逐渐增加的特征; 25°N以南的较低纬度地区则以积云降水为主, 其比例在55％左右, 而且表现为全区比较一致的空间分布特征。积云和层云降水观测数比例的空间分布表明整个亚洲季风区降水云系以层云为主（层云平均占60%以上)。在季节时间尺度上, 东亚季风区特别是25°N以北的副热带季风区, 积云降水和层云降水都表现出显著的季节变化特征（季节变化标准差都在10%以上): 冬季以层云降水为主, 所占比例在85%左右, 随着夏季的到来和季风雨带的向北推进, 层云降水比例逐渐减小而积云降水比例逐渐增加, 并在7、8月份层云降水比例达到最低值55%左右而积云降水比例达到最高45％左右, 此后随着时间的推移, 层云降水比例又逐渐增加而积云降水比例逐渐降低并恢复到冬季的状态; 南亚季风区以及东亚季风区25°N以南的热带季风区, 积云降水和层云降水季节变化特征不太显著（季节变化标准差在3%左右): 一年四季都以积云降水为主, 其比例在50%～65%之间变化。并且, 本文从动力因子和热力因子对上述积云降水和层云降水的时空分布成因进行分析, 其结果表明, 亚洲季风区积云降水和层云降水的时空分布主要受季风环流的风场垂直切变动力因子所支配, 而温度、水汽、相当位温和对流有效位能等热力因子对积云降水和层云降水的强度和范围及其持续时间也起着一定的作用, 并且这些动力因子和热力因子对积云降水和层云降水的时空分布的影响存在一个阈值标准。
Abstract:
Based on the Tropical Rainfall Measuring Mission (TRMM) data in the latest twelve years, the characteristics of spatial and temporal distributions of convective rainfall and stratiform rainfall in the Asian monsoon region are analyzed. The results show that: on the multi-year mean timescale, the spatial distributions of convective rainfall and stratiform rainfall in the Asian monsoon region show the characteristics of meridional variation, such as, the stratiform rainfall dominating over the subtropical monsoon regions to the north of 25°N, whose percentiles are above 50% and increase with latitudes, however, the convective rainfall dominating over the regions to the south of 25°N, whose percentiles are about 55% and show homogeneous spatial distribution characterstics. The spatial distributions of convective and stratiform rainfall pixels percentiles show that the stratiform cloud is the main rain cloud all over the Asian monsoon region (the area mean of stratiform rainfall pixels percentiles is above 60%). On the seasonal timescale, the convective rainfall and stratiform rainfall show distinguished seasonal variation (their standard deviations are above 10%) over the northern part of the East Asian monsoon region (to the north of 25°N) which is called the subtropical monsoon region. The stratiform rainfall dominates over this region, whose percentiles are about 85% in winter, but the percentiles of the stratiform (convective) rainfall decrease (increase) gradually and reach the minimum 55% (maximum 45%) with the coming of summer and the northward shift of monsoon rainband. With the advance of time, the percentiles of the stratiform (convective) rainfall increase (decrease) gradually to the winter status. However, the convective rainfall and stratiform rainfall show little seasonal variation (their standard deviations are below 3%) over the South Asian monsoon region and the southern part of the East Asian monsoon region (to the south of 25°N) which is called the tropical monsoon region. The convective rainfall dominates over this region, whose percentiles are from 50% to 65% all the year round. The analyses of dynamic factors and thermodynamic factors show that the spatial and temporal distributions of the convective rainfall and the stratiform rainfall in the Asian monsoon region are controlled by the vertical wind shear dynamic factor of the monsoon circulation. However, the thermodynamic factors, such as temperature, moisture, equivalent potential temperature, and convective available potential energy, influence the intensity, extension, and duration of the convective rainfall and the stratiform rainfall. These dynamic factors and thermodynamic factors seem to have their critical values.
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