Observational Evidence of the Moistening of the Lower Stratosphere by Tropical Deep Convection

The effect of overshooting convection on the lower stratospheric humidity has been an unsettled subject. While the detrainment of extremely cold and dry air from convective cores may dehydrate the Tropical Tropopause layer (TTL), the convectively lofted ice crystals may moisten the TTL and the lower stratosphere. We seek to quantify the impact of overshooting deep convection on the lower stratospheric water vapor by combining the deep convective clouds (DCC) measurements from the Atmospheric Infrared Sounder (AIRS) with atmospheric sounding data from the Microwave Limb Sounder (MLS). We compare the MLS water vapor, temperature, relative humidity and ice water content measurements coincident with AIRS DCC against those not coincident with DCC during the period from 2004 to 2012. We find that deep convection cools and dehydrates the TTL layer from 147 hPa to 121 hPa, but can moisten the 100 hPa and 83 hPa by more than 10-15% over several tropical deep convective regions within 20°S to 20°N, including Central America, South America, Southeast Asia and western Pacific. The cloud ice amount and relative humidity (with respective to ice) are persistently higher throughout 147 hPa and 83 hPa for the averaged profiles with DCC than the averaged profiles without DCC. Using different thresholds to define DCC yields qualitatively similar results, with stronger DCC corresponding to a greater moistening effect. Our study suggests tropical deep convection acts as an “express way” to moisten the lower stratosphere, although the rarity of overshooting convection makes its impact on the annual mean water vapor concentration rather small.