Abstract

We use an idealised fluid dynamical model to explore the processes that control moisture transport by convective overshoots in the tropical tropopause layer, which influences stratospheric water vapour content globally. Numerical simulations 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. We study turbulent mixing in the flow, explore internal wave generation, and construct a simplified model of convective hydration in the atmosphere.

The talk is composed of three parts. First, we introduce a novel method for diagnosing transport of a passive tracer in turbulent flows using a 2D phase space where mixing can be interpreted geometrically and 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 formulate a minimal moisture scheme that represents only the essential physical processes that contribute to hydration of the TTL . Here we focus on the influence of large-scale vertical shear in the stratified layer, which increases moisture transport by enhancing turbulent mixing via critical-layer wave breaking and shear instabilities.