Comparison of instability indices from COSMO-I7 and ECMWF-IFS analyses over the Piedmont Region, Italy, and new modifications to theKIndex

Abstract

This paper presents a statistical analysis of heavy rain events recorded by the high resolution network of ARPA Piemonte (Piedmont Region Environmental Protection Agency) rain gauges between 2007 and 2014. In order to define convective heavy rain events over the region, a rainfall threshold of 10 mm in 20 min was chosen. Events were sampled at intervals of 6 h for the warning areas of the Piedmont Region, Italy, and their distribution was studied in relation to the values of instability indices obtained from the European Centre for Medium Range Weather Forecasts Integrated Forecasting System and the COnsortium for Small-scale MOdeling (COSMO)-I7 operational analyses. The performances of the indices of the two meteorological models were investigated for the different warning areas in a pseudo-operational weather forecasting context, obtaining useful information in the phase of issuing thunderstorm early warnings. The second part of the work had the goal of improving the performance of these indices, modifying the one that showed better verification results in terms of receiver operating characteristic (ROC) area increase and the K Index (KI). This was done by introducing dependences on the thickness of geopotential height and wind direction at different levels. KI modifications that displayed better results were called reduced K Index (KR), obtained by dividing KI by the thickness of geopotential height between 500 and 850 hPa, and KW, obtained by adjusting the KR value with additive constants depending on the wind direction at different levels. KW in particular led to a net improvement of the performance of the original KI over the region.