Influence of the Apennines and Other Factors on Genoa Cyclone Movement During MAP

In this study, tracks of two cyclones occurred during MAP IOP-8 and 1 exhibited dramatically different characteristics as they passed over the Apennines. Both cyclones formed over the Gulf of Genoa, propagated eastward, and impinged on the Apennines over the Italian Peninsula. The surface cyclone associated with IOP-8 slowed down along the upstream (west) side of the mountain and accelerated eastward over the mountain range. However, the track of the cyclone appears to be rather straight, with only minor deflections. Similar to some tropical cyclones crossing over Taiwan's Central Mountain Range, midlatitude cyclones over Appalachians and Rockies, a lee cyclone formed near surface concurrently with the parent cyclone, which then develops by coupling with the upper-level cyclone and eventually replaces the parent cyclone. This type of cyclone track is classified as cross-over track with single or twin cyclones. The track for the surface cyclone associated with IOP-1 behaved completely different. The surface cyclone was deflected rightward (southward) in early stage, while a secondary (Adriatic) cyclone formed over the Adriatic Sea. The Adriatic cyclone co-existed with the Genoa cyclone for 18 h. In both IOP-8 and IOP-1, the cyclones in the mid- and upper-troposphere were able to keep a continuous track as they passed over the Apennines. The track deflection and the associated flow circulation are investigated by performing MM5 numerical experiments. The theory of Lin et al. (2005 JAS) on track deflection is hypothesized to explain the differences of track deflection between the IOP-8 and IOP-1 cyclones. Based on ECMWF reanalysis data and MM5 modeling results, it is found, however, that the basic-flow and vortex Froude numbers for both cyclones are almost the same. Thus, the track deflection cannot be explained simply by the orographic blocking effects. Instead, we propose that the track deflection of the IOP-1 surface cyclone is mainly controlled by synoptic forcing. Specifically, it was noted that the upper-level cyclone was able to cross over the Appalachians due to the relaxed locking of a downstream high, which then set an environment to steering the low-level cyclone track. Lin et al.'s theory will also be briefly reviewed to help explain track deflection on tropical and extratropical cyclones when they cross over other mountain ranges.