Atmosphere Ocean Science Colloquium
Hydraulic jump-like dynamics above supercell thunderstorms
Speaker: Morgan E O'Neill, Stanford University
Date: Wednesday, December 2, 2020, 3:30 p.m.
The most severe 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 large anvil shield. Thunderstorms are known to be an important secondary source of water vapor to the lower stratosphere, with substantial implication for climate. It has been long agreed that the occurrence of the AACP is coincident with breaking gravity waves, but a more detailed study of its dynamics and lifecycle has not been undertaken. Using 50-m resolution large eddy simulations, we show that the overshooting top of a supercell acts as a topographic obstacle and drives a hydraulic jump downstream – if the lower stratospheric winds are strong enough. Previous work has shown that the occurrence or lack of an AACP is a function of the strength of the lower stratospheric winds. A serendipitous airborne radar observation of a storm with weaker upper winds in 2011 corroborates our finding that weak upper winds induce flow separation in the lee of the overshooting top, preventing the formation of a hydraulic jump. In the simulated stronger winds case, we find that some stratospheric air crests the effective topography, plummets smoothly down the lee side at speeds exceeding 110 m/s, and then quickly transitions to highly turbulent in a rapidly-evolving hydraulic jump. This jump injects around 8 tons/s of water vapor and ice into the lower stratosphere irreversibly, several kilometers above the top of the anvil cloud, forming a distinct, realistic AACP. We provide the first study of a large-scale hydraulic jump in the absence of solid topography.