Synergism of upper-level potential vorticity and mountains in Genoa lee cyclogenesis - A numerical study

The interaction of topography and upper-level potential vorticity (PV) anomaly in intensive case of Alpine lee cyclogenesis (3-6 March 1982) is investigated. The factor separation method is used in conjunction with the PV inversion technique to isolate individual roles of topography and upper-level PV as well as their synergic nonlinear effect. The application of the factor separation method allows to separate low and upper tropospheric dynamics in the real case of lee cyclogenesis and to estimate quantitatively the pure and interactive contributions of topography and upper tropospheric PV anomaly to the pressure deepening in the lee of the Alps. The PV-topography interactive effect was found to be strong and comparable to the pure PV advection contribution. It is shown that the synergic contribution is responsible for the dipole structure oriented exactly as predicted by theory and as found in the observations. The "pure" topography contribution is small during the "trigger" phase but becomes strongly cyclolytic (i.e., assisting cyclone decay) in the second phase of the lee development. Superposition of the pressure change patterns produced by the two factors along with their interaction, results in a strong deepening in the right location. The joint cyclogenetic action is proposed as the explanation for the fast pressure fall during the "trigger" phase.