| On the basis of the self-calibrating palmer drought severity index(sc PDSI),the average daily maximum temperature of more than 2400 observation stations of the National Meteorological Information Center,the soil moisture and evapotranspiration data of the Global Land Data Assimilation System(GLDAS 2.0/2.1),and fifth generation of ECMWF atmospheric reanalysis of the global climate(ERA5)from 1979 to 2017,the traditional drought and flash drought indexes of the grid are defined.After analyzing the climatic characteristics of traditional droughts and flash droughts in growing seasons in China and the empirical orthogonal expansion spatial modalities of their respective indices,areas with frequent occurrences and high variability of two types of droughts are identified.And the identification criteria of regional traditional drought events and flash drought events are given and the synthetic analysis of their respective evolution processes are carried out.Then the elements and circulation fields of their respective typical cases are diagnosed,and finally the similarities and differences between the two types of drought are compared.The results are as follows:(1)From 1979 to 2017,traditional droughts mostly occur in the semi-arid and semi-humid regions of China.The areas with frequent occurrences and large variability are located in the central-northern North China-the central-eastern part of Inner Mongolia and the Yunnan-Guizhou region;Flash droughts mostly occur in the summer of southern China,with the most frequent in July,and Hunan and Zhejiang are the most frequent regions.The two regions are also the regions with the highest climate variability in flash droughts.Both traditional droughts and flash droughts show obvious characteristics of interdecadal changes,and their frequency in recent ten years is obviously higher.(2)Synthetic analysis of traditional droughts shows that the decrease in precipitation is the main driving factor for its occurrence.From 2009 to 2014,two traditional drought events occur in Yunnan-Guizhou region.Before the first event,this region is controlled by the anticyclonic anomalous northwest airflow and strong sinking movement on the southeast side of the Qinghai-Tibet Plateau.Therefore,the warm and humid water vapor in the Bay of Bengal is difficult to transport to Yunnan-Guizhou region,and the precipitation is low;Before the second event,this region is mainly affected by the flat and weak westerly air currents,the southern branch trough is weaker than usual and the westerly water vapor transportation is southerly and weaker,therefore,the precipitation is less.(3)Synthetic analysis of flash droughts shows that,unlike traditional droughts,high temperature is the main driving factor for its outbreak: one pentad before the drought,temperature rises rapidly,evapotranspiration increases rapidly,and soil moisture decreases significantly.Two flash droughts break out in Hunan from June to July 2013.Before the first event break out,the turbulent wind causes the average specific humidity field water vapor to transport to the south,and the 500 h Pa geopotential height in Hunan increases rapidly.The abnormal downdraft causes a decrease in precipitation and adiabatic increase in temperature.Evapotranspiration increases with the increase in temperature,which reduces soil moisture.This event is driven by high temperature;At the beginning of the second event,the obviously west,north and stable Northwest Pacific subtropical high increases the surface net solar radiation flux and the disturbance wind field controlled by the subtropical high continues.Water vapor in this area is transported northward,and evapotranspiration changes with precipitation.This event is driven by the decrease in precipitation,and the high temperature plays a promoting role.Both flash droughts and traditional droughts are essentially droughts.When precipitation is low,if driving factors such as high temperature occur within a short period of time,flash droughts will break out. |
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