Abstract: Based on the conventionally observational data, the NCEP reanalysis data and satellite cloud image, the formation reasons of heavy rain weather on July 31, 2011 in the west of Heilongjiang province were diagnosed. The weather system characteristics, atmospheric instable conditions, water vapor condition causing this process and dynamically triggering mechanism were discussed. The results indicate that the heavy rain is caused by the interaction of low vortex, warm-wet airflow in front of low vortex trough and cold air. The strong southerly airflow sets up a water vapor channel where water vapor can be transported rainfall areas. The increase of temperature and humidity in the low level makes atmospheric stratification unstable. The strong northwest airflow converges with the strong southeast warm-wet airflow on low level and generates the strong wind shear, so convergence and ascending motion augment, which provide the dynamic condition for the heavy rain. At the same time, it is favorable to release the unstable energy. Upper divergence and lower convergence go together, which is favorable to the development and maintenance of ascending movement. Mesoscale low pressure and mesoscale convergence line on the ground provide the conditions for the development and maintenance of mesoscale cloud mass. The mesoscale cloud mass still stays about 21 hours in the areas of the heavy rain areas, which is the main reason causing the heavy rain.

Key words: The heavy rain, Water vapor transportation, Atmospheric instable condition, Jet stream, Mesoscale