Feedbacks between the worst storms on Earth and lower stratospheric water vapor

The presence of water vapor in the lower stratosphere is enormously consequential for climate. Physically, it helps set the `cold reservoir’ temperature effectively experienced by the tropospheric heat engine, and an increase in water vapor in the lower stratosphere cools the stratosphere and warms the Earth’s surface. Chemically, an increase in stratospheric water vapor speeds ozone destruction. The stratospheric water budget remains poorly constrained, both observationally and theoretically. Strong thunderstorms are known to be an important secondary source of water vapor to the lower stratosphere, and how they may feedback to large scales in a warming climate is almost completely unknown.

The most severe midlatitude supercell thunderstorms typically feature an Above-Anvil Cirrus Plume (AACP), which is a wake of ice and water vapor downstream of overshooting deep convection, several kilometers above the main anvil shield. The AACP is uniquely capable of lofting water high above the tropopause. Using high-resolution large eddy simulations, we show that the AACP is formed by the development of a hydraulic jump at the tropopause, which occurs under specific atmospheric conditions. Immediately upon the jump onset, the simulated water vapor injection rate into the stratospheric overworld increases from less than 1 ton/s to greater than 7 ton/s, accompanied by horizontal windspeeds that top 110 m/s. The presence of a small-scale, dynamical threshold past which only some storms become very effective local hydrators of the lower stratosphere suggests a blind spot in large-scale climate models unable to resolve this behavior. The feedback on the environmental conditions for future storms due to a humidified lower stratosphere is an exciting area of future research.