Numerical forecast and analysis of a tropical-like cyclone in the Ionian Sea

S Davolio,F Pacifico,R Rotunno,A Moscatello,M M Miglietta,A Buzzi

doi:10.5194/nhess-9-551-2009

Abstract

Abstract. A forecasting system has been implemented for operational weather prediction over southern Italy in the framework of the RISKMED (INTERREG IIIB) project, which aims at weather-risk reduction in the central and eastern Mediterranean area. The system comprises two different regional forecasting chains, one based on BOLAM and MOLOCH models, the second based on the Weather Research and Forecasting (WRF) model. On 26 September 2006, a subsynoptic-scale vortex developed over the Mediterranean Sea and affected south-eastern Italy. The cyclone was documented by radar reflectivity maps, Meteosat Second Generation satellite images and surface stations data. The observational analysis showed that the cyclone possessed the typical characteristics of a Mediterranean tropical-like cyclone. All the limited-area models employed in RISKMED were able to properly forecast the formation of the small-scale vortex, although with differences in intensity, trajectory and evolution. The predictability of the event has been evaluated employing a multi-model, multi-analysis approach. The simulation results show strong sensitivity to the specification of the initial and boundary conditions. Sensitivity experiments were performed in order to analyse the mechanisms responsible for the development and the maintenance of the cyclone over the sea. The life cycle of the vortex turned out to be characterized by different phases: the orographic cyclogenesis on the lee side of the Atlas Mountains; the initial phase of development, characterised by the critical role played by the surface heat fluxes; the transition to a tropical-like cyclone mainly as a consequence of the latent-heat release associated with strong convective activity over the Ionian Sea; finally, the maintenance of the vortex strength due to both the surface fluxes and the release of latent heat.

Highlights

Due to its peculiar morphology, physiographic characteristics and geographical location, the Mediterranean basin is an area prone to high-impact weather events, such as heavy precipitation or intense cyclogenesis and cyclones often associated with wind storms, floods, high waves and storm surges, which can adversely impact human activities leading to economic damages and even casualties
The two modelling systems used were the hydrostatic model BOLAM together with the non-hydrostatic model MOLOCH, which is one-way nested in BOLAM twelve hours after initialization, and the non-hydrostatic model Weather Research and Forecasting (WRF)/ARW, version 2.2, implemented in a two-way nesting
The results in terms of forecast trajectory and cyclone intensity concerning the limited area models are presented in Fig. 6a and b for BOLAM, Fig. 9a and b for MOLOCH and Fig. 12a and b for WRF

Summary

Introduction

Due to its peculiar morphology, physiographic characteristics and geographical location, the Mediterranean basin is an area prone to high-impact weather events, such as heavy precipitation or intense cyclogenesis and cyclones often associated with wind storms, floods, high waves and storm surges, which can adversely impact human activities leading to economic damages and even casualties. During the pre-operational phase of the project, on 26 September 2006, an intense mesoscale vortex displaying features typical of tropical cyclones, on a much smaller scale, affected the target area The formation of this vortex was forecast by the RISKMED models, with differences in intensity, evolution and trajectory, as discussed in this paper. Since this kind of phenomena, known as a tropical-like cyclone (TLC hereinafter; Billing et al, 1983) or Mediterranean hurricanes – “Medicanes” – (Emanuel, 2005), are observed only occasionally over the Mediterranean basin (Lagouvardos et al, 1999; Pytharoulis et al, 2000; Fita et al, 2007) and the mechanisms leading to their intensification are not yet fully understood, this event represented an opportunity to explore different aspects related to dynamics and forecasting of this TLC.

Observational evidence

Control forecast

Global models

Limited area models

Conclusions