The Role of Stratospheric Air in a Severe Weather Event: Analyses of PV and Total Ozone

Severe weather events are typically associated with dry tropospheric air originating from the Southwest desert and Mexican plateau that overrun the warmer moist low level air. The resulting capping inversion often delays the release of CAPE until very unstable conditions have developed. This study examines the role of dry stratospheric air in a severe weather outbreak that occurred on April 8, 1999 in the Midwest. The data used to examine this relationship is from the NCEP Eta model and total ozone data from the Earth Probe Total Ozone Mapping Spectrometer (EP/TOMS).

Backward trajectories will be used to show the origin of parcels and the complex nature of the airstreams involved in the system. The dry air that descends is envisioned to play a role in the convection that develops along the Iowa and Nebraska border. A closer examination of the trajectories show stratospheric air descending to 800 hPa just west of the convection. It is believed that the near tropopause and stratospheric air aided in the development of severe weather by minimizing the mid to upper level cloud cover allowing solar insolation to increase thermal instabilities.

The total ozone data supports parcel trajectories from the stratosphere descending to 2 km above the surface. The small-scale features observed in the high-resolution total ozone data compare favorably with the geopotential heights and potential vorticity data. Detailed vertical structure in the potential vorticity appears to be captured by small-scale variations in the total ozone data. A comparison of coarse-grid analyses with fine-grid Eta model output shows that poor horizontal resolution significantly degrades the fine-scale structure within the potential vorticity field. The capability of the total ozone to identify mesoscale features may be beneficial in model verification. The total ozone is also useful in determining whether potential vorticity is of stratospheric origin or if it is diabatically generated in the troposphere. Another potential benefit of the total ozone data is that it could be applied in short-range forecasting provided that measurements are retrieved from a geostationary platform.