天津地区一次大到暴雪天气的诊断分析

应用常规探测资料和NCEP再分析资料， 对2011年2月下旬典型华北回流形势下天津地区一次大到暴雪天气进行了诊断分析。结果表明：回流降雪过程中，华北上空西风环流以纬向型为主，冷空气主体偏北，主要影响系统为华北回流冷高压和低压倒槽。同时，回流降雪中有浅薄的冷空气垫，其上有暖湿气流在爬升，爬升高度大约为650 hPa。回流降雪期间有来自西南和东北两个方向的水汽在天津地区交绥，西南方向的水汽较为暖湿，东北方向的水汽相对干冷，低空和超低空为一致的东北气流，900 hPa附近有超低空急流，700 hPa以上为西南暖湿气流。降雪过程中对流层低层到高层均为一致的强上升运动，上升高度可达200 hPa，对应于低空和超低空有强的辐合。降雪开始前天津及其周边地区有较强的对流不稳定能量和对称不稳定性，有利于对流的发展。

Diagnostic analysis of a heavy snowstorm process in Tianjin area

Based on the conventionally meteorological observation data and the NCEP reanalysis data, a heavy snowstorm process in the last ten-day period of February of 2011 in Tianjin area was diagnosed. This process was under the situation of typical return-flow weather in the north China. The results show that the westerly circulation over the north China is mainly the zonal type during the return-flow snowfall process. Cold air mainly leans to northward, and the main influencing systems are the return-flow cold high in the north China and low pressure in- verted trough. At the same time, there exists a thin cold air cushion in return-flow snowfall accompanied by the climbing of warm-wet air that can reach about 650 hPa. During the process of the return-flow snowfall, the south- westerly warm-wet air and northeasterly cold-dry air converges in Tianjin area, and it is the northeast air flow at low or ultra low levels. Ultra low level jet stream and southwest warm-wet air flow appear near 900 hPa and above 700 hPa,respectively. There is the uniform strong ascending motion from the low to high troposphere during the snowfall and the ascending height can reach 200 hPa, which corresponds to the strong low-level and ultra-low-level convergence. The convective instability energy and symmetry instability are strong before snowfall in Tianjin and its surrounding area, which is favorable to the development of convection.