| Based on the daily mean total cloud data for the period 1980-2015 from 549 meteorological stations in China.daily precipitation data between April and October and NCEP/DOE-2 monthly mean and daily reanalysis,the study is performed of the distribution,annual variation,climate tendency rate and spatial and temporal variation characteristics of total cloud in China.And then we obtain its corresponding relation with the north boundary belt of East Asian subtropical summer monsoon by combining its mean position,and discuss their advance and retreat process over time and decadal changes of their locations and the possible mechanisms of the anomalies of subtropical summer monsoon north boundary.The main conclusions are as follows:(1)The monthly evolution of total cloud amount in China is continuous, it is mainly performed as the variation of greater cloud amount value range and the location area.The cloud annual variation in autumn and winter is greater than spring and summer on the whole.Cloud of areas influenced by East Asian subtropical summer monsoon in China is stable over the summer monsoon initial stage,and its annual variation gets greater gradually from June to August.(2)In spring,cloud cover in most regions of China shows decreasing trend, while the summer, autumn two season show the same distribution with the annual average climate tendency rate ( increasing in the North and decreasing in the South ) , winter is the most significant cloudier season. Over the past thirty-six years, China's cloud changes can be divided into: consistent with the whole region,opposite anomalies in the south and north and opposite anomalies in the middle and northern(southern) regions.(3)July and August monthly mean 50%-65% total cloud band can be used to characterize the East Asian subtropical summer monsoon north margin, 50% line can describe the characteristics of the northern boundary of subtropical summer monsoon north edge while 65% lines can be used to express the southern boundary. The characteristic cloud line and summer monsoon north boundary have a synchronous promoting process and decadal variation characteristics.(4)During the past thirty-six years the north edge belt of summer monsoon has not a obvious northern or southern movement, the position variation trend of the northern boundary is not obvious while the southern boundary shows a weak northward trend. Compared with the north boundary partial southern years,in partial northern years, the strength and range of high and low pressure center get greater gradually from low level to high level.performs as several profound systems.From medium-low level to high level,the process of water vapor is from convergence to divergence, the convergence position is norther and this situation is to the benefit of the precipitation gets to higher latitudes.(5)The possible mechanism of abnormal north edge of summer monsoon is : the ridge of Subtropical High shrinks back to east and south,the water vapor transported from Indo-China Peninsula and India Peninsula to China increases,the cold air of Lake Baikal western side depression leads to the decreasing of geopotential height which is in favor of the strength of cyclonic circulation.At the same time,the temperature difference gets greater for both sides of the edge belt which could promote the mixture of northwest cold air and the southeast warm air,and there are intensive isotherms such as a frontal zone in the edge belt.The fierce atmosphere barocline in the north edge belt of summer monsoon leads to the increase of precipitable water vapor in downstream of the Yellow River,Huang-Huai and Jianghuai region and the water vapor convergence generated in middle and downstream area of the Yellow River results in the increase of cloud amount in edge belt and the partial northern position of the north boundary. |
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