Influence of the apennines on track deflection of Genoa cyclones

Observations of the MAP special observing period reveal two different track patterns for Genoa cyclones as they impinge on the Apennines. In some cases, the cyclones appear to be completely blocked by the orography and are deflected southward (e. g. IOP-1). In other cases, the cyclones do cross over the Apennines, but experience a period wherein two surface cyclones exist, one on the upstream side and a new, secondary cyclone on the downstream side of the mountains. Aloft, the system propagates without stalling. With time, the upper level trough phases with the secondary cyclone and the secondary cyclone becomes the dominant feature. Such a track is referred to as a discontinuous, but not blocked track for the purposes of this research. One such case during MAP belonging to this category is IOP-8. The aim of this study is to understand the control parameters that dictate whether a cyclone is blocked and deflected or if it is discontinuous, but not blocked. Examination of observations as well as ECMWF reanalysis data shows there is a dependency on the vortex Froude number (Fvortex) with smaller values of Fvortex indicating a greater likelihood for cyclone blocking and deflection. However, numerical sensitivity experiments indicate there are other, more important control parameters that dictate the degree of track deflection.