摘要:
本文数值模拟并诊断分析了2009年7月华北的一次桑拿天过程,分析了高温高湿天气的环流特征,温度、湿度的水平和垂直分布特征,位涡分布特征等.分析发现,此次桑拿天事件高层为反气旋性环流的高压控制,水平分布图上,低层相对湿度大.垂直剖面上,中低层为下沉气流和暖湿区,有明显的水汽梯度和垂直温度梯度,有倾斜的位涡分布.既然桑拿天发生在夏季普遍高温的大环境之下,因此靠单纯的温度或湿度来动力识别和诊断桑拿天,有较大难度.本文抓住华北地区桑拿天过程高温、高湿、高位涡的特点,引入一个适合于桑拿天的湿热力位涡参数（MTPV,它表示为▽q·（▽θ×▽Q）,这里q是湿度,表示为大气或者云中水汽和所有水凝物的总和,θ是位温,Q是位涡）,对桑拿天进行动力诊断分析,并通过实际个例的计算分析作出简化.个例分析发现,此次高温高湿的桑拿天过程伴随MTPV的异常.虽然2009年7月此次华北地区桑拿天过程有较高的温度,较大的湿度和倾斜位涡发展,但是单个变量的范围远大于我们要研究的华北地区桑拿天的爆发范围.而结合这三个变量引入的MTPV及其简化形式,无论从经向还是纬向剖面图来看,MTPV的异常大值区相对集中在北京及其周边的华北地区对流层的低层,并维持.因而,MTPV及其简化形式均能对此次高温高湿的桑拿天进行较好的动力识别.
Abstract:
A ＂sauna＂ weather event in northem China in July 2009 is numerically simulated and diagnostically analyzed. The atmospheric circulation characteristics, the horizontal and vertical distributions of temperatures and moistures, and the distribution of potential vorticities are studied. It is found that anticyclone dominates the upper troposphere during the ＂sauna＂ weather event. In the horizontal chart, the relative humidity in the lower troposphere is large. From the vertical sections, descending airflow dominates the moist and warm sectors at the middle and lower levels. There are clear humidity gradient, evident vertical gradient of temperature, and slanting distribution of potential vorticity. The ＂sauna＂ weather event occurs in the generally high-temperature environment of summertime, so it is difficult to dynamically identify and diagnose the ＂sauna＂ weather depending on some single factor, e.g., temperature or moisture. According to the high-temperature, large-moisture, and strong-potential vorticity characteristics during the ＂sauna＂ weather in northern China, a moist thermal potential vorticity parameter MTPV, may be expressed as Vq. （V0 x VQ）, where q is the sum of water vapor and all hydrometeors including cloud water, rain water, cloud ice, snow and graupel; 0 is potential temperature; Q is potential vorticity） which is appropriate for ＂sauna＂ weather, is introduced. Then it is used to dynamically diagnose ＂sauna＂ weather event. And it is simplified by calculation analysis in case study. It is found that the MTPV anomaly is accompanied by the ＂sauna＂ weather process. Although high temperature, large moisture and strong slanting potential vorticity development present in the ＂sauna＂ weather process in northern China in July 2009, their coverages for these single variables are larger than ottr target region of this ＂sauna＂ weather event. While for the MTPV and its simplified form combining these variables, their anomalies maintain in the lower troposphere around Beijing and its peripheral areas in northern China, viewing from either zonal or meridional sectionl Therefore, both MTPV and its simplified form can better dynamically identify this high-temperature and high-humidity ＂sauna＂ weather event.
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