GAFD Seminar: Charles Powell (University of Cambridge)

We use an idealised fluid dynamical model to explore the processes that control moisture transport by convective overshoots in the tropical tropopause layer. As the gateway to the stratosphere, moisture transport in the TTL directly influences stratospheric water vapour content globally. Numerical simulations of convective penetration events have been performed using realistic and complex meteorological models containing many physical processes, but these are computationally expensive and challenging to interpret. We attempt to gain insight into this process by considering large eddy simulations of a simple fluid dynamical problem representative of the environment, in which overshooting tops are represented by the penetration of a buoyant plume into a strongly stably stratified layer. 

The talk is composed of three parts. First, we introduce a novel method for diagnosing transport of a passive tracer in turbulent flows, using the tracer concentration and fluid buoyancy as coordinates in a 2D phase space. In this space, mixing can be interpreted geometrically. We show that the mixing between a buoyant plume and the surrounding stratified environment can be separated into three mixing 'stages', each corresponding with coherent regions of the plume. We then explore the generation of internal gravity waves, showing that existing theories cannot explain the observed gravity wave spectra in our numerical simulations. Using Dynamic Mode Decomposition and ray tracing, we show that waves originate inside the turbulent plume, and are modified as they propagate into the environment, yielding the observed wave spectrum. Finally, we use a minimal moisture scheme that represents only the essential physical processes that contribute to hydration of the TTL. The transport of vapour into the environment above the moist plume relies on mixing of ice from the plume into the warm and dry environment, where it sublimates to form vapour. Downward settling of ice modulates this process. We show that the convective intensity of the plume directly controls moisture transport by allowing penetration to a greater height, hence access to warmer environmental air, as well as driving more energetic turbulent mixing. Large-scale vertical shear in the stratified layer increases moisture transport by enhancing turbulent mixing via critical-layer wave breaking and shear instabilities.