Effects Of Land-atmosphere Coupling On Summer Climate And Extreme Events

Land-atmosphere interaction can affect the exchange of energy,heat and matter in the planetary boundary layer.Also,it modulates the changes of climate elements on multiple space-time scales,which is of great significance for the prediction of climate change,especially extreme events.Soil moisture is a key hydrologic factor on land surface,and the relationship between soil moisture and climate factors is usually used as an important index to measure land-air coupling.Soil moisture is a key predictor of seasonal to interannual climate change,and is crucial to the prediction of extreme weather and climate events such as drought,flood and extreme high temperature.High temperatures,low rainfall and low relative humidity,however,often occur simultaneously,and compound extreme events often cause far more damage to human and natural systems than any independent event.The underlying surface of land in Asia is complex and diverse with high spatial heterogeneity,and it is both tropical and subtropical monsoon regions and "hot spot" of land-atmosphere coupling.Therefore,the climate change in the Asian land region is jointly influenced by large-scale circulation systems and land-surface processes.It is a common method combining dynamics and statistics to predict the anomalies of atmospheric circulation on the monthly scale by atmospheric circulation model and then predict the anomalies of air temperature and other factors by statistical methods.The quantitative differentiation between large-scale atmospheric circulation and local evapotranspiration caused by soil moisture is of guiding significance for improving summer climate prediction in Asia,and is also an important scientific problem to be solved urgently.In this thesis,a dynamic adjustment approach based on constructed circulation analogs(DA-CCA)is used to investigate the summer climate elements by taking a wide range of 500 h Pa geopotential height as the atmospheric circulation index.The components caused by large-scale advection transport are determined and separated from the original field,and then the mechanism of residual component related to land surface processes is analyzed and studied.The analysis reveals that the difference between the total and circulation components is closely related to the soil moisture in the previous period and same period,which can be used to reflect the influence of local land surface processes on summer climate.Hence,the quantitative contribution of large-scale advection transport and local soil moisture to water vapor sources in the precipitation over the middle and low latitudes in Asia was investigated.On this basis,the contribution mechanism of local soil moisture to the record-breaking heavy rainfall in Henan in July 2021 and the severe dry heat and low rainfall in southern China in summer 2022 was studied respectively.The main research results and conclusions are summarized as follows:(1)Direct water vapor sources of terrestrial precipitation include large-scale atmospheric circulation transport and local land surface evapotranspiration.It is important to discuss and isolate the contribution of these two processes for climate prediction.In this study,the DA-CCA approach was used to quantitatively isolate the contribution of atmospheric circulation to summer terrestrial precipitation in the Asian middle and low latitudes from 1980 to 2019,taking 500 h Pa geopotential height as the circulation factor.The difference between the total precipitation and the component caused by circulation is called the residual component.Through the correlation analysis between the residual and the land surface thermodynamic dynamic elements,it is found that the residual is closely related to the soil moisture change in the previous period,which mainly affects the near-surface atmospheric humidity through the increase of evapotranspiration,thus affecting the precipitation.The key influence areas of circulation factors on terrestrial precipitation anomalies show a "positionnegative-positive" tri-pole distribution in the mid-latitudes.While the key areas of soil moisture-precipitation feedback are mainly located in northeastern China and northern Indian Peninsula.Atmospheric circulation determines the magnitude of summer terrestrial precipitation,while the residual reflecting land-atmosphere coupling effect dominates the trend of precipitation although its magnitude is small.At the same time,the effects of circulation and soil moisture on the interannual variability of precipitation are similar.These results indicate that,in addition to the effect of largescale circulation,it is important to consider local soil water and heat changes to improve the prediction of summer extreme precipitation.This conclusion is helpful for us to better understand the evolution mechanism of summer climate in the middle and low latitudes of Asia,and also has reference value for improving climate prediction.(2)Using ERA5 reanalysis data and dynamic adjustment method,diagnosis and attribution analysis were carried out for the extreme heavy precipitation that broke the historical extreme value in Henan Province in July 2021(referred to as "21·7" Henan heavy precipitation event).Firstly,the summer precipitation events in eastern China with similar circulation background to July 2021 were identified using ERA5 data.The results show that 2011 is a similar 500 h Pa circulation year to July 2021,but the corresponding precipitation fields are significantly different from North China to the middle and lower reaches of the Yangtze River.Then,the DA-CCA approach was used to separate the large-scale dynamic effects of circulation and local land-surface effects from the precipitation anomalies over eastern China during 1979-2021.The results show that the residual component of precipitation is the main reason for the heavy precipitation anomaly from Jianghuai to the lower reaches of the Yangtze River in July 2021.Attribution analysis shows that the residual is mainly caused by the change of local thermal factors,such as increased evaporation,decreased sensible heat flux,increased atmospheric humidification and enhanced boundary layer instability,thus increasing precipitation.The "21·7" Henan heavy precipitation event is related to significant sensible negative anomaly and strong convective instability energy.Generalized to the evolution of precipitation in the whole study period,the extreme precipitation in the whole eastern China and Henan Province in July from1979 to 2021 is mainly reflected in the residual,while the stable circulation component dominates the situation with small anomalies.The results emphasize the important influence of local thermal effect on the extreme precipitation in summer.(3)In this study,the southern China region was taken as the research area,and the monthly atmospheric reanalysis data from 1959 to 2022 were used to comprehensively consider the three extreme events of high temperature,low rainfall and low relative humidity,which were defined as the complex event of dry heat and low rainfall when the anomaly exceeded their inter-annual standard deviation during1959-2022.By means of synthetic analysis and similar circulation-dynamic adjustment method,it is revealed that the region with high variabilities in summer temperature,precipitation and relative humidity in 2022 in southern China are basically consistent with the local coupling hot spots of spring soil moisture and summer climate during 1959-2022.When the soil in the Yangtze River Basin and Huang-Huai area was dry in spring,and the southeast area was wet,the probability of the combined extreme events of dry heat and low rainfall increased in summer.Through the case analysis of 2022 summer,it can be found that after the removal of large-scale circulation,the residual of air temperature in eastern China is generally higher,the residual of precipitation is significantly deficient in the Yangtze River basin,and the atmospheric dry phenomenon in the northern Hunan and southern China of Henan is only caused by the residual contribution.The results show that the spring soil moisture anomaly significantly intensifies the intensity of the compound extreme events of dry heat and low rainfall over southern China in the summer of2022.Besides,attribution analysis shows that in the spring of 2022,soil wet anomaly in southern China in spring is able to last to early summer,which is not conducive to the uplifting of air mass and the formation of precipitation.Meanwhile,the surface albedo decreases,the strengthening of long-wave radiation causes the high temperature and the decrease of near-surface relative humidity.This study shows that the spring soil moisture over southern China is an important indicator of the occurrence of summer extreme climate events and has practical application value to improve the prediction of dry heat and low rainfall compound events.