Analysis of instability indices during the development of a Mediterranean tropical-like cyclone using MSG-SEVIRI products and the LAPS model

Abstract

A tropical-like cyclone that affected southeastern Italy in September 2006 is analyzed by means of instability parameters. The Lifted Index and K-Index generated by two different tools have been tested: the “Global Instability Index” (GII) developed at the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), mainly based on Meteosat Second Generation (MSG) observed radiances, and the Local Analysis and Prediction System (LAPS), a numerical diagnostic model developed at the National Oceanic and Atmospheric Administration (NOAA). The indices are very similar in the regions where they are both available. However, as the GII algorithm works only in cloud-free conditions and provides data with a coarse resolution (about 60–75 km over the Mediterranean regions), the LAPS products are selected as more appropriate for the analysis of this small scale cyclone. The analysis of the K-Index and Lifted Index during the evolution of the cyclone suggests that the areas identified as favorable for the development of severe convection are different according to the two indices. In fact, very large values of the K-index are found in the vicinity of a frontal system crossing the central Mediterranean Sea, and in particular in its northern, most active side. At the same time, the Lifted Index identifies as area potentially favorable to the development of severe convection a region of thermodynamic instability between the warm and moist air at low levels and the cold and dry air aloft. These two areas overlap just before the explosive deepening of the cyclone. Thus, at least for this case study, the co-location of the extreme values of the two indices may be regarded as an indication for the occurrence of severe convection.