Vorticity, deformation and divergence signals associated with stratosphere-troposphere exchange

Abstract

The dynamical tropopause of the extratropics, defined as an isosurface of potential vorticity, acts as a barrier to stratosphere–troposphere exchange (STE). Therefore, any transport across this barrier is necessarily associated with diabatic or frictional processes such as turbulence, radiative forcing or condensational heating. Most of these processes are linked to distinct flow phenomena and hence with distinct signatures of the velocity field. The focus of this study is on the characteristics of the velocity field at the time and location of the tropopause crossing, as given by the flow-components divergence (DIV), deformation (DEF) and vorticity (VORT). Firstly, the prevalence of distinct signals is shown for three case studies. Then a Lagrangian climatology of STE is used to assess the climatological aspects of this link for the year 1990. Methodologically, the flow-field components are traced along the STE trajectories, both derived from ERA-40. Some key results of this study are as follows. (1) DEF, positive values of VORT and negative values of DIV are markedly correlated with stratosphere-to-troposphere transport (STT). Therefore, the typical flow field associated with STT is deformative, cyclonic and convergent. (2) For troposphere-to-stratosphere transport (TST), the link with the flow-field components is less pronounced. DEF, VORT and DIV all show small, but distinct local maxima just before the exchange. The flow field during TST is therefore rather divergent and less cyclonic. (3) Sensitivity studies reveal that these findings are robust with respect to temporal and spatial variability. Copyright © 2009 Royal Meteorological Society