Statistical analysis of the shear lines and torrential rains over the Yangtze-Huaihe river region during June-July in 1981-2013

Using the ERA-Interim wind data and China's 756 meteorological station daily precipitation data, the shear line days, torrential rain days and shear line torrential rain days during June-July over the Yangtze-Huaihe river region in 1981-2013 have been selected by the 3 criteria of the meridional shear of zonal wind, relative vorticity, zero line of zonal wind. It is shown that during June and July from 1981 to 2013, 30.2 days (nearly 1 month) are torrential rain days, 33.2 days are shear line days, and 22.0 days are shear line torrential rain days. The shear line torrential rain days has accounted for nearly 2/3 of the shear line days and nearly 3/4 of the torrential rain days. The numbers of the shear line days, as well as the torrential rain days during June-July are on insignificant annual increase, and its increase rate is one order of magnitude larger than that of the shear line torrential rain days which is basically unchanged in the past 33 years. Earlier than 2000, there are more frequency fluctuations of shear line day, torrential rain day and shear line torrential rain day compared with that after 2000.There are some similar decadal variances in torrential rain days, shear line days and shear line torrential rain days over the Yangtze-Huaihe river region, and those decadal variances of the 3 series are also similar with the decadal variance of precipitation amount over the Yangtze-Huaihe river region in the period of 1981-2007.The shear line torrential rain mostly occur between late June and early July, the meiyu period. The number of shear line torrential rain days during June-July shows 2-3 year period prior to 1995 and no significant period later than 1995.Climatologically, the numbers of shear line torrential day show a 2-4 day period in early-middle June and middle-late July, while it shows no significant period from late June to early July, indicating that shear line torrential rain days maintain a stable frequency during that period, and the precipitation caused by shear line makes up most of the Meiyu precipitation.