The Mechanisms Leading to a Stratospheric Hydration by Overshooting Convection

Abstract

Overshoots are convective air parcels that rise beyond their level of neutral buoyancy. A giga-large-eddy simulation (100-m cubic resolution) of “Hector the Convector,” a deep convective system that regularly forms in northern Australia, is analyzed to identify overshoots and quantify the effect of hydration of the stratosphere. In the simulation, 1507 individual overshoots were identified, and 46 of them were tracked over more than 10 min. Hydration of the stratosphere occurs through a sequence of mechanisms: overshoot penetration into the stratosphere, followed by entrainment of stratospheric air and then by efficient turbulent mixing between the air in the overshoot and the entrained warmer air, leaving the subsequent mixed air at about the maximum overshooting altitude. The time scale of these mechanisms is about 1 min. Two categories of overshoots are distinguished: those that significantly hydrate the stratosphere and those that have little direct hydration effect. The former reach higher altitudes and hence entrain and mix with air that has higher potential temperatures. The resulting mixed air has higher temperatures and higher saturation mixing ratios. Therefore, a greater amount of the hydrometeors carried by the original overshoot sublimates to form a persistent vapor-enriched layer. This makes the maximum overshooting altitude the key prognostic for the parameterization of deep convection to represent the correct overshoot transport. One common convection parameterization is tested, and the results suggest that the overshoot downward acceleration due to negative buoyancy is too large relative to that predicted by the numerical simulations and needs to be reduced.

reply to referee's comments

“…The convective overshoots have the potential to increase the humidity in the stratosphere via rapid sublimation of convectively lofted ice and mixing with dry stratospheric air. This has been demonstrated in previous studies in both modelling and measurement (Dessler and Sherwood, 2004;Chaboureau et al, 2007;Jensen et al, 2007;Homeyer et al, 2014Homeyer et al, , 2017Khaykin et al, 2016;Rysman et al, 2016;Smith et al, 2017;Dauhut et al, 2018;Funatsu et al, 2018;among others). Even a small volume of tropospheric air can carry a significant quantity of water in the condensed phase.…”

Review of "Simulation of convective moistening of extratropical lower stratosphere using a numerical weather prediction model" by Z. Qu et al.

“…As shown by previous cloud-resolving model studies (e.g. Dauhut et al, 2018) the magnitude of irreversible hydration in the lower stratosphere increases as the maximum heights of overshoots extending into the stratosphere increase. It would be helpful for the authors to discuss this issue within the context of the current simulations.…”

The diurnal cycle of the clouds extending above the tropical tropopause observed by spaceborne lidar

“…Since any CATS L2O layer entirely above the tropopause is labeled as an aerosol layer (like in CALIPSO; Pan and Munchak, 2011), our study will not include clouds with their base in the stratosphere. Davis et al (2010) noted that lidars in space may miss the thinnest subvisible cirrus clouds, but with enough spatial averaging cirrus clouds with optical depths near 0.001 can be detected (Martins et al, 2011). Lidar cloud detections also suffer from a lower sensitivity in the presence of sunlight, which induces significant additional noise in the lidar signal, but climatologies are still relevant (Noel et al, 2018).…”