METEOSAT 4 satellite observations of water vapor show evidence of a rich spectrum of fine scale structure that may be indicative of the transfer of dry, ozone-rich air from the stratosphere deep into the troposphere [Appenzeller and Davies, 1992>. The extent to which air within stratospheric intrusions is irreversibly mixed and remains in the troposphere is of considerable current interest, particularly in respect of pollution effects of increasing numbers of subsonic and supersonic aircraft flights that transverse the lower stratosphere. Here, we diagnose potential vorticity and the corresponding distributions of specific humidity from a numerical weather forecast model and show that the latter field displays many of the characteristics of the Meteosate data. Specific humidity fields on both a pressure surface which corresponds approximately to the Meteosate data sampling and on an isentropic surface are compared.

We show the presence of streamer-like features in specific humidity that are not evident in the PV fields. We also note the prediliction for the streamers to break off, be advected by the flow and to subsequently become 'attached' to other features. Care is therefore required in the interpretation of the Meteosat data for stratosphere tropopshere exchange studies. Nevertheless, we suggest that the importance of specific humidity as an indicator of the dynamical evolution of upper level atmospheric flow has previously been overlooked. ¿ American Geophysical Union 1994