采用NCEP再分析资料、Micaps常规观测资料和中天山近30 a气象观测资料,对2011年3月发生在中天山北坡区域性暴雪过程形成的气候背景、环流形势和影响系统进行了分析,同时对其水汽条件、动力条件等物理特征进行了诊断。结果表明:此次区域暴雪500 hPa环流形势场为南北两支锋区汇合型;水汽通量场、水汽通量散度场都反映出该区域内有大量水汽输送和水汽辐合,红外云图显示南北两个不同性质的冷暖云团交绥至中天山,西南暖湿气流沿着阿拉伯海和孟加拉湾形成水汽输送带,为此次暴雪提供了充沛的水汽;物理量诊断表明,非地转湿Q矢量散度辐合及锋生函数的最强区与暴雪区有较好的对应关系,同时非地转湿Q矢量散度最大辐合区基本位于400~600 hPa,对降雪强度预报有指示意义;正差动假相当位温平流加剧了大气层结的不稳定,热力强迫作用增强了上升运动,有助于降雪的增强。
A regional snowstorm occurred over northern slope of the central Tianshan Mountains on March 16, 2011. The climatic background, circulation situation and physical diagnostic analysis of this snowstorm were conducted using the NCEP data, Micaps data and observed data on northern slope of the central Tianshan Mountains during the period from 1981 to 2010. The results showed that the 500 hPa circulation situation of the snowstorm happened was the convergence pattern of north and south frontal zones. When the snowstorm occurred, there was the sufficient water vapor supply and strong water vapor convergence, which was validated by the water vapor flux and divergence of water vapor flux field. The cold and warm cloud clusters joined up over the central Tianshan Mountains, which could be seen clearly from the infrared cloud image. The southwesterly warm-humid air along the Arabia Sea and the Bay of Bengal formed water vapor transport belt that provided plentiful water vapor for the snowstorm. Physical diagnosis revealed that both convergence area of ageostrophic wet Q-vector from 600 hPa to 400 hPa and the strongest frontogenesis were accorded with snowfall area. This finding may be of very importance in predicting snowfall intensity. The unstable stratification was intensified by the advection of positive differential pseudo-equivalent potential temperature. Thermal forcing also enhanced the development of ascending motion and was favorable for the intensification of heavy snowfall.
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