Gaoyuan qixiang (Aug 2024)
Study on Characteristics and Causes of Regional Meteorological Drought in Yunnan from April to June in 2020
Abstract
Based on the daily MCI （Meteorological Drought Composite Index） of 125 stations in Yunnan province from April to June 2020， and the monitoring standard of regional drought process， the spatial-temporal distribution characteristics are analyzed， and the circulation causes of meteorological drought are also investigated.The results demonstrate that： （1） Two meteorological drought processes occurred from April to June 2020， one is a severe drought in southwestern area of Yunnan from April 1 to 25， and another was from May 9 to June 30 which is a severe drought with a long duration in the whole province.（2） From April 1 to 25 in 2020， the distribution of Mid-high latitude 500 hPa height field are two troughs and one ridge， a trough of low pressure in the Ural Mountains and eastern East Asia， and a ridge of high pressure in the area from Lake Balkhash to Lake Baikal， forming a "negative-position-negative" wave train which is conducive the cold air to the southward and impact of Yunnan.The low latitudes are controlled by a zonal positive abnormal height field， and there is south wind anomaly of 700 hPa in Yunnan， so the cold air mainly affects eastern of Yunnan.The Arabian-Sea-Bay of Bengal-South China Sea of 500 hPa height field is positive anomaly， which is not conducive to the transport of southwest warm and humid air to Yunnan.70% of the water vapor transport tracks to Yunnan come from the westerlies， so the regional meteorological drought process occurs in western Yunnan with less precipitation and higher temperature.（3） From May 9 to June 30， the seasonal transition of South Asian High is later than normal， and the establishment of the regional monsoon circulation in the Bay of Bengal is late also.Before the establishment of monsoon， all of the water vapor transport tracks to Yunnan region come from the westerly belt， and the specific humidity is mainly negative anomaly.After the establishment of the monsoon， 97% of the water vapor transport tracks to Yunnan come from the Indian Ocean， and the specific humidity was mainly positive anomaly.However， the circulation distribution in the middle and high latitudes of 500 hPa is not conducive the cold air to the southward and impact Yunnan.Meanwhile， the distribution of low latitude 500 hPa is positive anomaly， and the subsidence movement is strengthening， the correspondence between water vapor transport and the convergence rising motion was not good， so a sever large-scale meteorological drought occurred in Yunnan province.
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