A multi-stage evolution of an ALPEX cyclone

Abstract

The rôles of topography, convection, sensible and latent heat fluxes in Alpine lee cyclogenesis are investigated. The adoption of a newly developed factor separation method allows the identification of the contributions of each of these processes as well as their synergistic effects. Topographical blocking is the dominant factor in the first and most rapid phase of the cyclone deepening. This is followed by the convection contribution at the 2nd but slower phase. Local moisture flux is dominant in the 3rd phase accompanied by a significant cyclolytic contribution by the Alps. Between the 1st and the 2nd phases of deepening, the synergistic effect of convection induced by the mountains plays an important rôle in the deepening at a time when the convection independent of mountains has not become yet so active. Coarser horizontal resolutions still capture these features, although to a lesser extent. Model simulation results are shown to be strongly dependent upon the chosen set of factors. As the number of factors increases, a specific contribution diminishes because synergistic terms with the new factors are extracted from the contribution of the specific factor under investigation. Another interesting conclusion is that the elimination of an important factor from the investigation will not remove its contribution. It will reappear and be attributed to another factor which is the most synergistic to the important factor. The spread of the cyclones’ centers in the model simulations is shown to be a powerful tool in understanding the effects of different factors on the evolution of the model solutions. For instance, convection moves the cyclone to the east northeast while topography ties the cyclone to the lee of the Alps. The sea moisture fluxes tend to move the cyclone toward the warm bodies of water.