Schmid, W. (Atmospheric Science, ETH, Zurich, Switzerland)
Wernli, H. (same affiliation)
Linder, W. (same affiliation)
The Swiss midlands north of the Alps are often affected by severe thunderstorms, causing hail, floods, severe winds, and occasionally even small tornadoes. Such events develop in an airmass with substantial moisture in low levels. Where does this moisture come from? The Alps are an obstacle for moist air moving northward from the Mediterranean sea. The moist airmasses are either deflected to the left or right before reaching northern Switzerland, or the moisture reaches northern Switzerland from elsewhere. A good understanding of the moisture transport is needed for accurate thunderstorm predictions and warnings.
Here, two severe thunderstorm cases are investigated (21 July 1992 and 22 July 1995). Backward trajectories are computed using initialized analysis data from the ECMWF T213L31 model (mesh size 80 km), and the evolution of relevant parameters (pressure, potential temperature, mixing ratio) along the trajectories are analysed.
On 21 July 1992 (most severe hail day in Switzerland since 1971), the moist airmass originated over the Ligurian sea (gulf of Genova) and reached Switzerland by moving westward and then northward along the western edge of the Alps. Moisture was mixed from layers near the ground up to 700 hPa in the potentially unstable airmass.
Similar trajectories were also found on 22 July 1995 but the associated airmass remained relatively dry, compared with another airmass entering Switzerland from western France. These two branches of trajectories converged over Switzerland, leading to severe thunderstorms (spawning one damaging tornado) in parts of northern Switzerland.
In summary: the Mediterranean sea is an important source of moisture for some but not for all severe thunderstorms in northern Switzerland. It is planned to repeat the trajectory analysis with a high-resolution limited-area model (mesh size 15 km), in order to gain gurther insight into the mesoscale moisture transport.