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

Autor: Meloen, J., Siegmund, P.C., Sigmond, M.
Zdroj: Tellus - Series A; January 2001, Vol. 53 Issue: 3 p368-379, 12p
Abstrakt: A new Lagrangian technique to calculate the air mass flux across potential vorticity (Q) surfaceshas been used to perform a case study of a tropopause-folding event in the subtropical SouthernHemisphere. The flux is computed from the rate of change in Q along trajectories, which arecalculated from ECMWF wind data. Because the tropopause can be defined as a Q surface, the method can be used for the calculation of stratosphere–troposphere exchange (STE). Alarge number of forward and backward trajectories were calculated, starting in the vicinity ofthe tropopause fold. This fold was observed during the TRACAS (TRAnsport of Chemicalspecies Across the Subtropical tropopause) experiment at La Reunion (21°S, 55°E) in July 1998.With the Lagrangian method the cross tropopause air mass flux can be calculated for thedifferent tropopause levels, that occur in case of a tropopause folding. The computed massexchange is about — 20×1013kg (i.e., downward) in 4.5 days, which is rather small comparedto other (midlatitude) mass exchange studies. The ratio of the fluxes per unit of area in andoutside of the fold is about 200 : 1. An indication of the accuracy of the calculated fluxes hasbeen obtained with the help of the ozone soundings made during the TRACAS experiment.
Databáze: Supplemental Index