Low-Level Potential Vorticity and Cyclogenesis to the Lee of the Alps

Abstract

High-resolution numerical model simulations over the Alpine region are presented that reveal the presence of low-level elongated bands of potential vorticity (PV) downstream of high topography. These PV streamers (or PV banners) occur when the synoptic-scale wind turns into a direction across the Alps. Individual pairs of banners with anomalously positive and negative values of PV can be attributed to flow splitting, either on the scale of the whole of the Alps (primary banners), or on that of individual massifs and peaks of the model topography (secondary banners). The PV bands have amplitudes of up to ?2.5 and +5 pvu, a width of 50?150 km, can attain a length of up to 1500 km, and extend in the vertical from the surface up to the 500-hPa level on occasions. The PV banners are associated with zones of enhanced horizontal wind shear. The analysis of daily output from the operational NWP model run of the Swiss Meteorological Institute also demonstrates that such PV streamers are a frequent feature and occur whenever there is appreciable flow past the Alps. Low-level PV streamers may interact with the larger-scale flow, and thereby represent an intermediary between the (unbalanced) formation of an orographic vortex, and its (approximately balanced) interaction with the synoptic-scale environment. This process is analyzed for one particular case of Alpine lee cyclogenesis. Simulations show that PV streamers may wrap up and subsequently contribute to the low-level PV anomaly within the developing cyclone. It is suggested that the two phases of Alpine lee cyclogenesis can be interpreted in this way, that is, as the rapid formation of a low-level orographic vortex followed by its baroclinic and diabatic interaction with an approaching upper-level trough. To test this interpretation, sensitivity studies with dry dynamics, reduced surface friction, and idealized terrain are conducted.