Abstract:
Based on the daily rainfall data of the middle and lower reaches of the Yangtze River and the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data from 1965 to 2016,the decadal change of the persistent heavy precipitation (PHP) and its circulation feature were analyzed.Results show that the peak stage of the PHP during the summer season is from June 21 to July 10.The frequency of the PHP in this period shows a similar decadal change with that in whole summer.The frequency of the PHP increases first and then decreases from 2004 to 2015.The high frequency of PHP events occur during 1989-2003.The circulation characteristics are different for the low and high frequency periods.During the period of 1965-1988,the airflow is straightness in the high-latitude region and the cold air is not active.Meanwhile,the Western North Pacific High (WNPH) is weak.It results in a weak transportation of water vapor.The perceptible water is less than the normal level and the vertical velocity is weak as well.During the period of 2004-2015,the high-latitude circulation field shows a pattern that is high in the west and low in the east at 500 hPa geopotential height.The transportation of the cold air is weak,and the Bay of Bengal trough is not strong.The WNPH is strong but located southward,so the water vapor is difficult to be transported far away to the middle and lower reaches of the Yangtze River.The preciptible water is less than the normal level as well.The high frequency of PHP exists in the period of 1989-2003,but the circulation patterns are completely opposite between the yeas with high frequency PHP and years without PHP.The high of Okhotsk Sea is strong during the frequent PHP year,and the cold air is active.The WNPH is strong and shifts westward,which enhances the water vapor transporting into the middle and lower reaches of the Yangtze River from its northwest side.Its preciptible water is sufficient.The cold and warm air intersection makes the vertical motion energetic.So,the decadal change of the PHP in this region is closely related to that of the large-scale circulation.