TRANSPORT INTO THE TROPOSPHERE IN A TROPOPAUSE FOLD

A tropopause fold developed to the west of the British Isles on the we stern flank of a trough in the 300 mb flow on 6 October 1990. Radioson de ascents over western Europe showed very dry stable layers beneath t he jet stream in the potential temperature range 310-315 K. These were evident on profiles from 12h 6 October to 00h 8 October. The European Centre for Medium-range Weather Forecasts (ECMWF) model assimilations were examined for this period to determine how well they represented the radiosonde observations. Good agreement was found, especially for the humidity distribution. The model overestimated the vertical extent of the fold by a factor of about 2, so its representation of potentia l vorticity was less successful. Nevertheless, agreement was sufficien tly good for the model to be used to evaluate the amount of stratosphe re-troposphere exchange in the event. Isentropic trajectories were cal culated for the air in the fold as represented by the ECMWF assimilati on at 00h on the 7th. Those on the western edge of the fold split from the jet stream and transferred to the troposphere, while those on the eastern side ended up in the cut-off low. The model fields were used to estimate a lower bound of 1.1 x 10(14) kg and 2 x 10(32) molecules for the amount of stratospheric air and ozone, respectively, transferr ed into the troposphere by this fold. Owing to the model's incomplete representation of the event, the actual amount transferred may be at l east twice this estimate.