Development and Tropical Transition of an Alpine Lee Cyclone. Part II: Orographic Influence on the Development Pathway

Abstract

The development and subsequent tropical transition of a subsynoptic-scale cyclone over the Gulf of Genoa (GoG) on 15 November 2007 led to the rapid onset of tropical storm-force winds near the islands of Corsica and Sardinia. This study evaluates the influence of two key ingredients on the cyclogenesis event: a near-surface warm potential temperature perturbation in the lee of the Alps and a mountain-scale potential vorticity (PV) banner. A high-resolution modeling system is used to perform a set of attribution tests in which modifications to the Alpine orography control the presence of the cyclogenetic ingredients. When either feature exists in the initial state, a GoG cyclone develops even when the Alpine barrier is removed; however, when neither the warm perturbation nor the PV banner is present, there is insufficient lower-level PV to couple with the upper-level trough to promote cyclogenesis. A conceptual model involving the complimentary interaction of the two PV features is presented that accurately describes the development location of the cyclone beneath a midlevel vorticity maximum. Despite development in most of the attribution tests, the energy sources for the cyclones vary widely and represent a spectrum of cyclogenetic pathways from baroclinically to convectively dominant. Removal of the Alpine barrier allows for a stronger thermal wave and a baroclinic mode of development, rather than the diabatically generated hurricane-like vortex seen in the control and available observations. Similarly, insufficient flow interaction with the low-resolution representation of the Alps in the global-driving model is shown to favor a baroclinic mode of cyclogenesis in that integration. Adequate resolution of both the Alpine terrain and the incipient cyclone itself are shown to be important to correctly predict the evolution of the system from both structural and energetic perspectives.