Convective Impact on the Global Lower Stratospheric Water Vapor Budget

Water vapor in the stratosphere is primarily controlled by temperatures in the tropical upper troposphere and lower stratosphere. However, the direct impact of deep convection on the global lower stratospheric water vapor budget is still an actively debated issue. Two complementary modeling approaches are used to investigate the convective impact in boreal winter and summer. Convective influence is diagnosed by tracing trajectories through convective cloud top altitude fields derived from global rainfall and brightness temperature data. Backward trajectory model simulations coupled with a detailed treatment of cloud microphysical processes indicate that convection moistens the global lower stratosphere by approximately 0.3 ppmv in boreal winter and summer 2010. The diurnal peak in convection is responsible for about half of the total convective moistening during winter and nearly all of the convective moistening during summer. Deep convective clouds overshooting the tropopause have relatively minor effect on global lower stratospheric water vapor. A forward trajectory model coupled with a simplified cloud module is used to estimate the relative magnitude of the interannual variability of the convective impact. Combining the results from the two models, we find that the convective impact on the global lower stratospheric water vapor during 2006–2016 is approximately 0.3 ppmv with year-to-year variations of up to 0.1 ppmv. An important mechanism of convective hydration of the lower stratosphere is via the detrainment of saturated air and ice into the tropical uppermost troposphere and the subsequent upward transport of some of these moist air parcels across relatively warm and subsaturated tropopause. Plain Language Summary Stratosphere is extremely dry, but small changes in the humidity of the stratosphere can have a big impact on Earth's climate. Water vapor in the stratosphere is primarily determined by temperatures in the tropical upper atmosphere (between the tropospheric and stratospheric layers), but deep convective clouds that rapidly transport humid air up to this region could potentially influence stratospheric water vapor as well. This study uses two complementary modeling approaches to estimate the overall impact of deep convection on global stratospheric humidity. We find that convection moistens the lower stratosphere by about 10% in boreal winters and summers with smaller (by about a third) year-to-year variations during the 2006–2016 period. The daytime peak in convection is responsible for about half of the total convective moistening during boreal winter and nearly all of the convective moistening during boreal summer. Deep convective cloud tops that penetrate into the lower stratosphere have a relatively small effect on stratospheric water vapor. Convection moistens the lower stratosphere by transporting humid air laden with numerous ice crystals to the tropical uppermost troposphere, just below the stratosphere. Some of this humid air subsequently ascends into the stratosphere and ultimately increases the humidity of the lower stratosphere.