Tropical Cyclone–Like Vortices in the Extratropics: Observational Evidence and Synoptic Analysis

Abstract

In this article two subsynoptic-scale cyclones that developed between 3 and 10 October 1996 over the western-central Mediterranean, causing floods, strong winds, and severe damage, are analyzed. Surface observations reveal that the accumulated rainfall at Santuario di Polsi (southern Calabria, Italy) is more than 480 mm for the first event (cyclone 9610A). The second cyclone (9610B) was characterized by a storm track predominantly over the sea, thus causing less recorded precipitation, but stronger wind. Satellite imagery shows two intensely convective vortices with a scale of 200?400 km and a spiral structure, with the cyclone 9610B displaying a well-defined eyelike feature. The corresponding National Centers for Environmental Prediction analyses, although limited by 1° resolution, confirm the cyclones? positions and intensities, as they can be inferred from satellite imagery, SSM/I data, and observations, but display also the ?signature? of two tropical cyclone?like vortices, including a perfect alignment between the cutoffs at all levels with the surface center, and a warm core. The wind speed cross sections in the meridional and zonal directions through the eyelike feature reveal a virtually motionless column of air. A comparison with the cross sections taken in the same analyses across a named tropical storm in the Atlantic show a strong analogy between the gridded representation of these events. Other remarkable features include very strong horizontal shear in the midtroposphere, and simultaneous lack of vertical shear; increasing low-level vorticity at the expenses of upper-level vorticity; creation of a low-level vorticity maximum; and finally strong low-level convergence and upper-level divergence during the onset and development of each cyclone.