Morgenstern, Olaf (Atmospheric Science, ETH Zurich, Switzerland)
Davies, Huw C. (same affiliation)
Severe precipitation events in the Alpine region often have a distinct PV signature. On the one hand, certain tropopause-level PV configurations can contribute to triggering these events, and on the other hand significant tropospheric PV anomalies ensue as a result of large latent heat release. The latter PV anomalies are diagnostically retrieved from NWP model outputs using a technique that evaluates diabatic Lagrangian PV tendencies. A comparison with model-generated PV shows that the representation of PV tendency is capturing the dominant source of PV adequately, and also that in the cases under consideration much of the tropospheric PV structure is associated with diabatic heat release. A 3D animation of these PV anomalies provides a useful guide to their geographic origin and movement. These PV anomalies (positive or negative in sign) tend to form at the upstream side of the orography and are subsequently advected across the mountain chain to develop into a vortex which is no longer diabatically driven, or they may reinforce a lee vortex by adding considerably to its cyclonic motion. Additional insight is sought from 3D animations of the Convectively Available Potential Energy (CAPE) showing how conditional instability translates itself into convection and PV formation in the vicinity of the Alps. The examples considered are MAP- or MAP-like cases.