Quantitative Evaluation Of Land-Air Coupling Strength Over China

The land surface-air coupling strength refers to the influence of land surface state changes on precipitation and surface air temperature,which is used to characterize the impact of land surface on the climate system.Quantitative assessment of land-air coupling strength is of great significance to the study of climate change.In this study,firstly the summer land-air coupling strength over China from 1979 to 2018 is statistically evaluated according to different land-air coupling strength indices using ERA-Interim reanalysis data set.Then,“hot spots” of land-air coupling strength is identified and the characteristics of spatio-temporal variation of land-air coupling strength over hot spots area is analyzed.Thirdly,the key land surface factors that have essential impact on the land-air coupling strength in hot spots area is identified,and their influences on surface air temperature and precipitation are quantitatively evaluated.Finally,the physical mechanisms of the land-air coupling process in the "hot spots" is analyzed.The following conclusions are drawn:(1)Four different land surface-air coupling indices are used to evaluate the spatiotemporal variations of land-air coupling strength over China in summer.The results show that the strong land surface-air coupling zones in summer are concentrated in western Xinjiang,western Qinghai-Tibet Plateau,Yunnan,central Inner Mongolia and the surrounding areas in China.Among them,the middle Inner Mongolia is the strongest region,with the regional average coupling strength and the density of coupling "hot spots" significantly higher than other strong land surface-air coupling zones.During 1979-2018,the coupling strength between land surface and precipitation demonstrates a significant upward trend,while the coupling strength between land surface and air temperature exhibits oscillating characteristic.(2)The Liang-Kleeman information flow method are used to identify the key land surface factors influencing the summer coupling strength around the middle Inner Mongolia area.It is found that local soil moisture has a remarkable impact on precipitation in July,and local soil temperature significantly affects surface air temperature in August.The statistic equilibrium feedback method is further applied to quantify the coupling strength in this area.The results showe that the coupling strength of soil moisture and precipitation is 3.17mm/std?SM in July.The soil temperature-surface air temperature coupling intensity is 0.77?/std?ST in August.(3)The key physical processes of land surface-air coupling in the middle Inner Mongolia area are revealed.The results show that soil moisture significantly controls the process of evapotranspiration in this area,and the increase of soil moisture leads to the local enhancement of evapotranspiration and increase of the relative humidity in the lower troposphere,associated with positive anomalies of cloud fraction and precipitation.On the other hand,increased local soil temperature leads to enhanced of local upward heat transfer from land surface,resulting in increased surface air temperature.In addition,the increase of soil temperature also have a significant effect on the change of local radiation processes: increased soil temperatures leads to positive anomaly of surface air temperature,which in turn will lead to a reduction of relative humidity and making it difficult for water vapor to condense.The reduction of cloud cover in the lower troposphere contribute to positive anomaly of surface incoming solar radiation,and the surface air temperature increases accordingly.