G. Vaughan et al., TRANSPORT INTO THE TROPOSPHERE IN A TROPOPAUSE FOLD, Quarterly Journal of the Royal Meteorological Society, 120(518), 1994, pp. 1085-1103
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.