Climatic Statistical Analysis And Composite Analysis Of Shear Line Over Yangtze-Huaihe River Region During Meiyu Period

Using ERA-Interim wind data and China’s station daily precipitation data, the shear line days, torrential rain days and shear line torrential rain days during June-July over Yangtze-Huaihe river region(28°~34°N,110°~122°E) in 1981-2013 have been selected with a combination of objective analysis and subjective analysis. The shear lines causing torrential rains are then classified into warm shear line, cold shear line, quasi-static shear line and vortex shear line. The frequencies, precipitations, spatial distributions and system intensities of 4 kinds of shear lines are analysed. Lastly, 10 typical cases of 4 kinds of shear line torrential rains are selected, by composite analysis, precipitation systems distribution, 3-dimensional dynamical structure, thermal structure and water vapor transportation of 4 kinds of shear lines are summarized and compared.Firstly, the shear line days, torrential rain days and shear line torrential rain days during June-July over Yangtze-Huaihe river region in 1981-2013 are selected. The result shows, during June and July(61days) from 1981 to 2013,30.2 days are torrential rain days, 33.2 days are shear line days,22.0 days are shear line torrential rain days. Shear line torrential rain days has accounted for nearly 2/3 of shear line days and nearly 3/4 of torrential rain days. This result has updated the traditional conception. Shear line and torrential rain have a positive correlation. Shear line torrential rain day maintain a stable frequency during late June and early July, which concentrates most of the shear line torrential rain days.The classification result shows that, warm shear line is the most frequent shear line with nearly 40% share of the total shear line frequency, while the other 3 kinds of shear lines account for 20% more or less respectively. The proportions are also applicable in shear line torrential rain frequencies. Vortex shear line has the strongest system intensity and precipitation intensity, followed by cold shear line and quasi-static shear line, warm shear line has the weakest system intensity and precipitation intensity but warm shear line contributes the biggest share to total shear line precipitation volume and is rising over the 33 years significantly. In the middle and western parts of Yangtze-Huaihe river region, shear line precipitation is mainly caused by warm shear line and vortex shear line, while in the eastern parts of the region, shear line precipitation is caused by warm shear line and cold shear line.Composite analysis of shear lines shows that, shear line over Yangtze-Huaihe river region mainly occur below 500 hPa, shear line leans northward with the increase of altitude.Around the lower level shear line,there is upward motion,while around the higher level shear line,there exists downward motion. Lower level shear line has “cold wet” structure and convective instability. The upward motion and convergence around middle and eastern parts of shear line are stronger than those around western part of shear line. The water vapor transportation around middle and eastern parts of shear line is in higher altitude than that around western part of shear line. Up over the south of shear line, there always exist a indirect heat circulation, the warm and moist air is crucial component of upward flow around the shear line, besides, for the cold shear line and vortex shear line, there also exist a direct heat circulation over the north of shear line, the upward flows of the secondary circulation overlap each other over the shear line, which is an important factor for torrential rain. It also shows that warm shear line,cold shear line,quasi_static shear line and vortex shear line have differences as for the weather systems distribution of different levels,especially the position and intensity of upper jet flow have strong impact on the formation and development of shear line and shear line torrential rain.