Investigating The Underlying Mechanisms Of Soil Moisture Impacts On Summer Precipitation And Air Temperature Over China

The soil moisture memory can persist for weeks to months in tropics and subtropical regions augmented by precipitation that impacts extreme climate events.Globally the soil moisture coupling,interactions,and feedback have been studied,inferring that soil moisture directly couple with the atmosphere through land-atmosphere interaction and indirectly amplify the sea surface temperature anomalous impacts.This memory and interaction and climate modification probabilistic soil moisture patterns can be predicted through statistical and numerical simulations techniques and tools that can be used as an important source of climate predictability.In this dissertation,the ECV satellite remote sensing soil moisture data of the European Space Agency(ESA),combined with the regional climate model,is used to study the influence of soil moisture on summer precipitation and temperature in China.First,the ECV data in China was verified with in-situ records from selected stations;similarly,the regional climate model Reg CM ability was also checked to simulate China's summer hydrometeorological cycle.Second,the local feedback of summer soil moisture on temperature and precipitation during 1981-2018 was studied.The influence and mechanism of the northsouth oscillation of soil moisture and its impact on the summer monsoon precipitation were checked using the diagnosis and regional climate model.Finally,the spring soil moisture to thermal controls in predicting summer precipitation is explored and discussed.The main conclusions of this paper are as follows:(1)From comparing the agricultural weather station soil moisture and reanalysis soil moisture data(ERA-interim and NCEP)to study the applicability of ECV soil moisture data in China.The results show that the overall ECV data and the other three sets of data have good consistency in the temporal and spatial characteristics.However,there are differences in some regions.In the humid middle and lower reaches of the Yangtze River,ECV compared with the soil moisture data of the agricultural station can capture the seasonal changes;the reanalyzed data shows an inverse phase change compared with the data of the agricultural station.Only ECV soil moisture data in the Yellow River basin can capture the magnitude of soil drying trend as the site data.On the whole,ECV satellite remote sensing soil moisture data has good applicability in China.(2)Using numerical simulation to clarify the feedback of summer soil moisture anomalies in different regions of China to convective precipitation.In North China,“indirect feedback” is the main form;that is,soil moisture changes the conditions of cloud formation in the atmospheric boundary layer of the region.It then causes the water vapor transported by the atmospheric circulation to condense and result in precipitation,which mainly changes the frequency of convective precipitation.In the humid regions of South China,“direct feedback”is the main method;that is,soil moisture provides a direct source of water vapor for atmospheric precipitation.Especially when the soil is wet,it increases the frequency of extreme precipitation.When a high-temperature heat wave occurs,the positive feedback process of soil moistureevapotranspiration-temperature is formed,making the loss of soil moisture self-reinforced and further aggravating the intensity of the heatwave.(3)Through diagnostic analysis,it is found that the south(22°N-31°N,105°E-121°E)and north(35°N-44°N,105°E-121°E)fluctuations of summer soil moisture at the interannual scale in China oscillate the summer precipitation in North China and the middle and lower reaches of the Yangtze River.Abnormal summer soil moisture affects surface energy flux and boundary layer height,which affects the diabatic heating of the atmosphere and the abnormal divergence of the upper atmosphere,causing meridional oscillations that affect the upward movement of the airflow.When the soil moisture presents wet north and dry south,there is a cyclone over the Korean Peninsula with a high-pressure ridge located in the low-pressure trough of North China and the east coast of Japan.The trough of low pressure caused the zonal easterly wind towards China to split into two branches.The northward branch merges into the westerly zone,forming typical frontal weather,and precipitation in the north increases.It descends south to the Yangtze River basin,pushing the subtropical westerly jet into the tropical warm pool area.This pattern has resulted in less precipitation in southern China and more precipitation in northern China.On the contrary,when the soil moisture is dry in the north and wet in the south,more surface net radiation in the northern region becomes sensible heat.Which causes the divergent movement of the upper atmosphere,which triggers the asymmetric phase-shift movement of the subtropical high from north to south,accompanied by geopotential height.And the relative vorticity anomaly changed the regional circulation pattern,resulting in more precipitation in the south and less precipitation in the north.Numerical simulation results also prove the above conclusions.(4)The CMT diagnostic method was used to quantify the contribution of spring soil moisture to summer precipitation in China(15%?30%).The study found that due to the persistence of the “memory” of land surface dryness and wetness and the thermodynamic process of soil moisture in spring.The abnormal changes in energy flux and boundary layer height caused by the meridional oscillation of ascending motion and the diabatic upper atmosphere heating and divergent circulation are abnormal.The positive soil moisture anomalies in spring led to the strengthening of the anticyclone in northern China in summer,resulting in stable stratification and reduced precipitation.The rising subtropical westerly jet in the Yangtze River Basin merged into the high-pressure ridge of the anticyclone in the north of the Yangtze River Basin,and precipitation increased.The negative anomaly of soil moisture in spring causes the asymmetric phase transition of the cyclone over the Korean Peninsula,the formation of frontal weather affect above normal precipitation;while in the south,the water vapor in the Bay of Bengal was hindered,and the precipitation in the Yangtze River Basin decreased.The thermal action of soil moisture in spring in China changed the atmospheric circulation pattern.It caused anomalies in summer precipitation,thus showing its potential for application in seasonal scale climate prediction.The inertia of soil moisture itself and the coupling mechanism of soil moisture-atmosphere can be used as the source of seasonal predictability of precipitation.