| A statistical comparative analysis of cyclonic storms activity over the Bay of Bengal(Bo B)and Arabian Sea has been implemented using best-track data from 1977 to 2018 issued by JTWC.And then based on the daily precipitation data of national stations,NCEP/NCAR reanalysis data and hourly Black Body Temperature data of GMS satellite,the precipitation in China under the impact of storms over the Bo B from 2000 to 2018 is extracted to analyze its statistical characteristics and the main physical processes.The influence of storm activities on the rainfall during the pre-flood season in South China and the relationship between storm activity and the onset of monsoon in South China Sea are further explored.Results have shown that:The annual variation in the frequency and duration of cyclonic storms has a significant increasing trend over the Arabian Sea and an insignificant decreasing trend over the Bo B.The monthly frequency of cyclonic storms in both two Seas shows a notable bimodal character,with peaks occurring in May and October–November,respectively.The maximum frequency of cyclonic storms occurs in the second peak as a result of the higher moisture content at mid-levels in the autumn.However,the largest proportion of strong cyclones(H1–H5 grades)occurs in the first peak as a result of the higher sea surface temperatures in early summer.Cyclonic storms in the Arabian Sea are mainly generated in the eastern basin,whereas in the Bo B the genesis locations have a meridional(zonal)distribution in May–June(October–November)as a result of the seasonal movement of the low-level positive vorticity belt.The mean intensity and size of cyclonic storms over the Arabian Sea are stronger and larger,respectively,than those over the Bo B,and the size of cyclonic storms over the North Indian Ocean in early summer is larger than that in autumn.The asymmetrical structure of cyclonic storms over North Indian Ocean is affected by the topography and the longest radius of the 34 kt surface wind often lies in the eastern quadrant of the cyclonic storms circulation in both sea areas.There are two ways that cyclonic storms over the Bo B affect precipitation in China,one is the direct impact of the northward movement of storm clouds,the other is the indirect impact of long-distance water vapor transportation.According to the statistics,there are 71 cyclonic storms in Bo B from 2000 to 2018,of which 41(58%)have affected China.Among them,4 storms have only direct precipitation,19 storms have only indirect precipitation,and the remaining 18 storms have both direct and indirect effects.Precipitation directly affected by storm is mainly distributed over the Qinghai-Tibet Plateau and Southwest China,while the precipitation centers indirectly affected by storm are located in the south of the Yangtze River and Yunnan province,respectively.The avearge annual precipitation days and amount in May is greater than that in October-November.Under the influence of storm,the rainfall center is located in South China and western Yunnan in early summer,and the average daily rainfall is more than twice as much as that in autumn.However,in the southern region of the Tibet Plateau,precipitation in autumn is stronger than that in early summer.The storm causes heavy rainfall in China mainly through the circulation pattern of the "trough in north and vortex in south" in early summer and the "confluence of two high pressures" in autumn.Early summer is one of the peak periods of storm activity in the Bo B,and it is also during the pre-flood season in South China.Further study shows that the impact of storm on the precipitation in the pre-flood season in South China mainly occurs from the third pentad to the sixth pentad of May.In the the fourth pentad of May,the percentage of storm precipitation in the amount precipitation during this pentad in South China is 20%,and the large value mainly occurs in Hunan Province and the northern Guangdong,which exceeds 30%.Under the impact of storm,heavy rain and torrential rain are mainly distributed in the north of 22°N in South China,the frequency of downpour(>100mm)accounts for more than 30% of the total frequency of that in South China.It is also found that the correlation coefficient between the generation date of the first storm and the outbreak date of the summer monsoon in South China Sea is 0.6,and on average,the monsoon of South China Sea breaks out about 7 days after the generation of storm.During the storm activity,the low level southwest jet is established,the western Pacific subtropical high retracts eastward,the convection develops in the southern South China Sea,and the condensation heating is favorable for the meridional temperature gradient to change from negative to positive and the outbreak of the summer monsoon in South China Sea,thus influencing the precipitation during the pre-flood season in South China. |
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