CONTOUR LENGTHENING RATES NEAR THE TROPOPAUSE

We present the results of contour advection calculations using winds d erived from the UK Universities Global Atmospheric Modelling Programme (UGAMP) model. Contours are initialised using PV values on a set of p otential temperature surfaces which span the stratosphere and troposph ere in order to examine the mixing properties of stratospheric and tro pospheric air on a time-scale of a few days. We show that the contour stretching rates do not convey any information about the location of t he tropopause, although the lower layers of the stratosphere are vigor ously mixed with the troposphere over this time. The results suggest t hat the stratosphere becomes more isolated from the troposphere with i ncreasing potential temperature, with mixing confined to a narrow regi on in the subtropics at the higher levels (greater than 330K).