A comparative analysis of the atmospheric circulation in summertime rainy days with different precipitation intensity in eastern Northwest China during 1981-2012

Using a daily precipitation dataset that consists of observations at 198 stations in eastern Northwest China and ERA-Interim reanalysis data during 1981-2012, the tendencies of rainy days and total amount of summer precipitation of different types are investigated. Moreover, the similarities and differences in atmospheric circulation corresponding to precipitation of different types are compared. Major results show that light rain and moderate rain are the two major types of precipitation in the summer, which account for more than 90% of summer rainy days and about 70% of total amount of summer precipitation. Spatially, the proportion of light rain days increases from southeast to northwest, while the proportion of moderate rain days, heavy rain days and torrential rain days decreases from southeast to northwest. Days of light rain and moderate rain as well as the amount of total precipitation during these days both experienced a significant decline during the study period. Large decline is found in the 1980s and 1990s, while rainy days and total amount of precipitation have slowly declined in recent several years. On the other hand, days and amount of torrential rain showed a slightly increasing trend. Analysis of several cases in 1981 and 2003 indicates that when the Ural blocking high and the Mongolian cyclone strengthened while the subtropical high extended westward, cold airflow from the north and warm airflow from the south both intensified. Such a pattern was favorable for the convergence of cold and warm airmasses in eastern Northwest China, leading to increases in precipitation. It is also found that under the condition when both cold airflow and warm airflow enhanced, light rain was mainly controlled by the cold airflow from the north while the boundary line between cold and warm airmasses was located to the south and the water vapor transport in the southern boundary was relatively weak. Moderate rain, heavy rain and torrential rain were mainly influenced by warm and humid air from the south when the boundary line was located to the north, and the water vapor transport and moisture convergence in the southern boundary strengthened. In this circumstance, rainfall intensity was dependent on the input of moisture flux and the height the ascending motion could reach.