Modelling of stratospheric intrusions within a mid‐latitude synoptic‐scale disturbance

Abstract

AbstractThe results of a run of the UK Universities' Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (GCM) is presented, initialized with European Centre for Medium‐Range Weather Forecasts data for 6 October 1990. the dates correspond with observations of a trough made during a Transport of Ozone and Stratosphere/Troposphere Exchange campaign. This anticyclonic wrapping system is compared with a cyclonic wrapping system studied in a previous paper. the fate of stratospheric air is followed using potential vorticity (PV) and specific humidity. the run gives good agreement with radiosonde data for the development of the trough, which involves a tropopause fold and a ‘streamer’, both with regard to the position and evolution of a layer of dry air. Analysis of the height of the tropopause shows that PV values (in PV units) in the range 1‐5 agree well with the thermal tropopause outside the trough, but in order to capture the layer of dry air seen in the radiosonde data, a value of PV of order 1 is required within the trough. Use of PV values in the range 3‐4 may therefore not follow the evolution of dry stratospheric air entering the troposphere, even though such values may on average follow the thermal tropopause. Most of the dry region is visible as a dark band in the Meteosat data, although the lowest levels of dry air (at approx. 800 mb) are too low to be seen by the satellite. Thus Meteosat observations may not capture all processes involved in stratosphere‐troposphere exchange, and the images must be interpreted with caution when using them to follow the movement of dry stratospheric air.