How Does the Varying Depth of Overshooting Tops Affect the Transport of Water to the Stratosphere?

Overshooting tops (OTs) associated with deep convection are thought to contribute significantly to the rapid transport of tropospheric air and water into the lower stratosphere. Because of their potential influence on chemistry and global radiation budgets, understanding the amount of air and water injected into the stratosphere by OTs is important for future climate research. However, the range of water amounts added to the stratosphere by different OTs, and the mechanisms by which this detrainment occurs, remain uncertain. These uncertainties apply to all OTs, but disparities between numerous OTs within a single storm complex are also possible and should be investigated. Additionally, it is not known whether the convective influence on the total stratospheric water budget is dominated by many small enhancements from storms with weak OTs or just a few storms with very deep OTs.

To investigate these uncertainties, the Bryan Cloud Model (CM1) was used, with horizontal and vertical grid spacing of 100 m, to analyze a case of multicellular convection with many short-lived OTs extending 1–2.5 km above the tropopause over a three-hour simulation period. This behavior was consistent with the multicellular convection and moderate overshooting that developed in an environment sampled during the NASA Dynamics and Chemistry of the Summer Stratosphere mission. A vertical profile from that environment was used to initialize the horizontally homogeneous CM1 simulation. To examine differences between multiple cases occurring in a very similar stratospheric environment, a sample of three OTs from the simulation was analyzed in detail. Separate passive tracers were initialized within progressively more intense regions of each updraft to show where air originating within different regions of the updrafts was detrained and what proportion of the updrafts’ air remained in the stratosphere downstream. Additionally, stratospheric water enhancements were examined, with water vapor increases of up to 500%, relative to the pre-convective base state, found in small plumes downwind of the overshooting. Total water vapor and ice enhancements, both throughout the entire stratosphere and associated with each individual overshoot alone, were also analyzed. This allowed for an exploration of the relative climate impact of these three OTs of varying depth, with a further investigation of whether plumes developed after each overshooting event and a qualitative exploration of their characteristics when they did develop.