Numerical diagnosis of a small, quasi-tropical cyclone over the western Mediterranean: Dynamical vs. boundary factors

A small, quasi-tropical cyclone occurred on 12 September 1996 over the western Mediterranean. Intense convective activity over the region during this period also produced a tornado outbreak in the Balearic Islands and torrential precipitation over eastern mainland Spain. Mesoscale model runs properly simulate the cyclone formation and show convection and heavy precipitation following the cyclone trajectory during its eastward progression. A sensitivity study examining the upper-level dynamic forcing, latent- and sensible-heat fluxes from the sea, and orography is conducted. A potential-vorticity (PV) inversion technique is used to reduce the amplitude of the upper-level trough in the model initial conditions. The results show that neither the orography nor the sensible-heat flux from the sea play a significant role during this particular cyclone development. Conversely, both the latent-heat flux and the upper-level trough are shown to be crucial for low-level cyclogenesis. Features common to hurricane-like polar lows are found for the cyclone, and an analysis of the precise role of the upper-level structures and the convective development is conducted. A factor-separation technique is used to determine the individual effects of the aforementioned factors, as well as their interaction. At the first stage of the cyclogenesis, the upper-level PV anomaly enhanced the low-level circulation of the synoptic-scale low and enhanced the latent-heat flux from the sea. During its mature stage, the circulation associated with the small-scale cyclone enhanced the latent-heat flux from the sea, thereby helping to maintain the development of deep convection, and inducing further cyclone deepening by diabatic heating. This scenario has many similarities with the air-sea interaction instability mechanism. Thus, the joint action of the upper-level anomaly, as a spin-up agent, and the latent-heat flux, as a sustainer of convection, emerges as the primary factor for the genesis and evolution of the small quasi-tropical cyclone.