Water Vapor Exchange between Atmospheric Boundary Layer and Free Troposphere over Eastern China: Seasonal Characteristics and ENSO Anomaly

Abstract. This study develops a quantitative climatology of water vapor exchange between the atmospheric boundary layer (ABL) and free troposphere (FT) over Eastern China. The exchange flux is estimated for January, April, July, and October over 7 years (2011 and 2014–2019) based on a water vapor budget equation using simulated meteorological data. The spatial pattern of the ABL-FT water vapor exchange flux is closely related to the topographic distribution. The seasonal variation shows that the water vapor exchange in the northern region is downward in January and October with the flux being 37 %–72 % of the surface evaporation to maintain the ABL moisture, while it is weak upward in April and July; the southern region presents persistently water vapor output from the ABL to the FT, with the ratio of exchange flux to surface evaporation increasing from 10 % in January and October to 60 %–80 % in April and July. Three physical processes determine the total water vapor exchange, among which the ABL diurnal variation drives large magnitude exchange flux within the one-day cycle, but for the net monthly mean flux, the vertical motion at the ABL top is the main contributor. The anomaly of water vapor exchange in ENSO years illustrates triple antiphase distribution: strengthening in the middle area and weakening in the north and south zones of Eastern China in La Niña year, and vice versa in El Niño year. It agrees with the spatial pattern of anomalous precipitation, implying the crucial role of ABL-FT water vapor exchange in atmospheric water cycle.