Spatiotemporal Characteristics And Occurrence Mechanism Of Sudden Drought In Northern Chin

With the intensification of global climate and the reduction of water resources,the frequency and impact scope of drought events have increased.In general,drought is a long-term phenomenon with slow development,which takes months or even longer to reach the maximum intensity and coverage.However,recent studies have found that drought can occur rapidly under anomalous atmospheric circulation and underlying surface conditions,the drought that develop faster than traditional one is defined as“flash drought”.According to the definition of related literature,this study defines the whole flash drought event as follows:the flash drought event begins when the percentile of pentad average soil moisture decreases from 40%to 20%,with no less than a 5%decrease in soil moisture percentile per pentad,and the duration of the event is no less than three pentads.When the percentile of pentad average soil moisture restores to 20%again,the flash drought event ends.Compared with the traditional drought,flash drought is characterized by strong flashness,rapid development,high intensity and special difficulty in predicting.With the intensification of global warming and human activities,the risk of high temperature heat wave increases,and the shortage of precipitation will not be enough to reveal the change of drought intensity,and the enhancement of evapotranspiration caused by high temperature will become the main driving factor of drought intensification.Flash drought is more sensitive to global warming than traditional drought,and may become a common drought phenomenon in the future,which requires in-depth research.Based on meteorological station data and GLDAS grid data from 1978 to 2020,this paper studies the temporal and spatial characteristics and mechanism of flash drought in northern China.Firstly,the flash drought in the study area was identified,its spatiotemporal characteristics and evolution was analyzed;Secondly,the driving role of meteorological factors and water vapor changes in the process of flash drought was discussed by correlation analysis;Finally,based on the land-atmosphere coupling parameters,the relationship between flash drought and land-atmosphere coupling is discussed.The main conclusions are as follows:（1）Spatiotemporal evolution characteristics of flash drought.Firstly,the frequency of flash drought in the year is the highest in WNW,with a rate of 14.2%,followed by ENW,NC and NE,with the rates of 11.9%,11.4%,and 7.6%,respectively.While the frequency in the plant growing season is the highest in the south of NE and the south of NC,and low in the other regions.Secondly,the hotspot areas of flash drought shows an obvious eastward trend from WNW to ENW in northern China from the 1980s to 2010s.Thirdly,the proportions of affected sites are always high in WNW and ENW,both above 60%,and they have an obvious upward trend in NC and NE,with the proportion increasing from 44%and 28%to 82%and65%,respectively,as confirmed by a significance test of p<0.01.Lastly,flash drought could occur all through the year in WNW and NC,while it occurred mainly in the summer half-year and summer in EWN and NE.（2）The driving of meteorological elements and water vapor during the flash drought process.Firstly,the occurrence of flash drought is influenced by both precipitation and temperature in WNW,ENW and NE（arid and semi-arid areas）,while it is influenced mainly by temperature in NE（humid areas）.Secondly,precipitation mainly showed a downward trend,and the decrease of precipitation,corresponded to the decrease of soil moisture during flash drought.Temperature showed an upward trend,but its impacts on soil moisture is not direct,which is also constrained by the changes in precipitation Thirdly,the main high occurrence period of flash drought in each region is in late spring and early summer.When the temperature is high,it is prone to flash drought in the event of precipitation deficit.In addition,there is a second high occurrence period in WNW,which occurs in the late summer and early autumn,mainly affected by the precipitation deficit.Lastly,flash drought in WNW and ENW is mainly affected by water vapor advection.The input of water vapor is less than the output of water vapor,which can not form effective precipitation in the local area,making the soil moisture decrease.Flash drought in NC and NE is mainly affected by wind field divergence.A large amount of water vapor diffuses outward and increases evaporation,resulting in rapid loss of local soil moisture.（3）The relationship between flash drought and land-atmosphere coupling.Firstly,summer and autumn are during the plant growth season,with wet soil and high vegetation coverage,the averageTCI_SM-LHFin in northern China is 4.2 and 2.9,while the average TCI_LHF-LCLis-3.2 and-2.1,and the intensity of land-atmosphere coupling is high.In the spring and winter,the averageTCI_SM-LHFin northern China is 0.5 and 1.3,while the average TCI_LHF-LCLis-1.5 and-1.2,the coupling intensity is weak,especially in winter,where flash drought events are difficult to occur.Secondly,in northern China,except for some humid areas in NE,it is characterized by“soil moisture limited”,while in the humid areas in NE,it is characterized by“energy limited”.Therefore,the occurrence mechanisms of flash droughts in the two regions are different.Thirdly,during flash drought,the probability of low evapotranspiration under low soil moisture conditions becomes higher than that during non-flash drought based on joint probability density function,the percentage growth of each sub-region is around 6%-18%,indicating that evapotranspiration is more easily to be controlled by soil moisture,that is,the region is transitioning from an“energy limited”to a“soil moisture limited”.Finally,compared to non-flash drought period,there is also a transition of evapotranspiration from a transitional type to a"soil moisture limited"type in WNW,ENW and NC,and a transition from an"energy limited"type to a"soil moisture limited"type in NE during flash drought period.