Abstract: Based on the conventional meteorological observational data and the NCEP reanalysis data, the 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 north China and low pressure inverted 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 southwesterly 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.

Key words: Tianjin area, Heavy snowstorm, Return-flow weather, Diagnostic analysis