Stratosphere-troposphere exchange (STE) and mesoscale dynamics
Dr Jeffrey Chagnon
Dr Suzanne Gray
Deep convection often extends to the height of the tropopause. Vertical transport can take place across the otherwise stable interface when convective updrafts come in contact with the tropopause. Some of this transport can be accomplished within the overshooting convective plumes, and some can be accomplished by high frequency gravity waves and the turbulent motions they induce. The purpose of this work is to examine the characteristics of mixing along the tropopause above mesoscale convective systems. Case studies in which deep convection occurred over the UK are being simulated using the Met Office Unified Model (UM) in standard operational configuration (12 km horizontal grid spacing) and at very high resolution (1 km horizontal grid spacing). A passive stratospheric tracer is integrated in the simulations in order to identify episodes of cross tropopause transport and to quantify their depth and magnitude. The evolution of the tracer field is then correlated to properties of the gravity wave field. It is shown that a small amount of mixing of stratospheric into the mid troposphere takes place which is largely independent of model resolution. Additionally, some shallow mixing takes place within a few kilometres of the tropopause in the high resolution simulations that resolve high frequency gravity waves.
Chagnon, J. M., and S. L. Gray, 2007: Stratosphere-troposphere transport in a numerical simulation of midlatitude convection. Journal of Geophysical Research - Atmosphere, 112, D06314, doi:10.1029/2006JD007265.