Development of an Alpine lee cyclone during MAP D-PHASE: forcings for cyclogeneis

The development of a subsynoptic-scale cyclone in the Gulf of Genoa during the Mesoscale Alpine Project (MAP) Demonstration of Probabilistic Hydrological and Atmospheric Simulation of flood Events

in the Alpine region (D-PHASE) project is investigated using analyses and model simulations. Rapid cyclogenesis is triggered by the passage of a synoptic-scale trough and associated cold front across the Alps on 15 November, 2007. An embedded coherent tropopause disturbance (CTD) is shown to have played an important role in promoting the development of the lower-level vortex by simultaneously providing quasigeostrophic ascent forcing and reducing the bulk column stability over warm Mediterranean waters.

In addition to this upper-level forcing, a pair of diabatically-generated lower-level potential vorticity (PV) features are shown to be important to the cyclogenetic process in this case. The first, a warm surface potential temperature anomaly, is generated by pre-frontal upslope precipitation on the Alpine north-side, followed by parcel descent in the lee. The second PV feature is a mountain-scale PV banner that extends southward from the southwestern tip of the Alps as frontal progress is slowed by the barrier.

A set of numerical simulations that include both orographic and initial state modifications show that the cyclogenesis event is sensitive to the presence of both lower-level PV features. When the Alpine orography is removed but the lee-side warm perturbation is retained, a cyclone develops in the eastern Gulf of Genoa. Conversely, when the Alps are present but the warm anomaly is removed, a storm forms on the cyclonic shear side of the PV banner. These results suggest that, in the presence of cyclogenetic forcing generated by the CTD, the environment responds to the combined influence of these two orographically-induced PV features by promoting cyclogenesis in a localized area favored by their implied circulations.