A Lagrangian computation of stratosphere-troposphere exchange in a tropopause-folding event in the subtropical Southern Hemisphere

A new Lagrangian technique to calculate the air mass flux across potential vorticity (e) surfaces has been used to perform a case study of a tropopause-folding event in the subtropical Southern Hemisphere. The flux is computed from the rate of change in e along trajectories, which are calculated from ECMWF wind data. Because the tropopause can be defined as a e surface, the method can be used for the calculation of stratosphere-tropopause exchange (STE), A large number of forward and backward trajectories were calculated, starting in the vicinity of the tropopause fold. This fold was observed during the TRACAS (TRAnsport of Chemical species Across the Subtropical tropopause) experiment at La Reunion (21 degreesS, 55 degreesE) in July 1998, With the Lagrangian method the cross tropopause air mass flux can be calculated for the different tropopause levels, that occur in case of a tropopause folding. The computed mass exchange is about -20 x 10(13) kg (i.e., downward) in 4.5 days, which is rather small compared to other (midlatitude) mass exchange studies. The ratio of the fluxes per unit of area in and outside of the fold is about 200:1. An indication of the accuracy of the calculated fluxes has been obtained with the help of the ozone soundings made during the TRACAS experiment.