Evaluation of Stability Indices for Thunderstorm Prediction in Mendoza Argentina

BUSTOS C.; VIDELA H.; SANTOS R.; ARANEO D. C.; SIMONELLI S.

Helsinki

Conferencia; 7th European Conference on Severe Storms; 2013

European Severe Storms Laboratory

The Province of Mendoza has been described as a hot spot in severe convection. Many thermodynamic parameters and indices are currently being used as thunderstorm predictors because of their high correlations with the onset and development of convection. Some of these indices have been developed for one specific area and their forecasting accuracy has generally been assessed in that zone and not in others. It is a highly intriguing question whether there are parameters or indices that may function adequately as thunderstorm predictors, as far as the Probability of Detection (POD) is concerned, irrespective of the latitude of the study zone. The pre-convective environment on thunderstorm days in Mendoza has been investigated during a 5-yr period (1999?2004) from October to May. Thermodynamic parameters calculated from the atmospheric radiosounding in Cruz Negra, Mendoza (started at 15 UTC) were used to characterize the initiation of convection. The effectiveness of a number of forecasting indices for non-frontal thunderstorm activity has been investigated for Mendoza. The indices include the Total Total index (TT), the Showalter index (SHI), K index (KI), Lifted index (LI), S index (SI) and CAPE. The best parameters were evaluated by using two methods: skill scores and discriminant analysis. For every thermodynamic parameter, six skill scores were derived, but only the values of the TSS (True Skill Statistics) were considered to verify the success of the thunderstorm prediction. A guidance value had to be determined for every decision variable to receive the single components of a contingency table. That value was found at the point where TSS reached a maximum. This optimized guidance value divided the dataset into two groups. One group contained the values larger than the determined value. This means for an index like the KI, which ideally increases with increasing thunderstorm activity, that thunderstorms were predicted as long as the guidance value was exceeded. In contrast, if the values were smaller than the determined value, no thunderstorms were predicted. In order to assess the occurrence or non-occurrence of storms in the study zone additional data were used from an S-band radar network in the province of Mendoza. For the decision whether a thunderstorm day was expected or not, the best results were obtained with the original SHI which showed the highest TSS skill score (0.506), POD= 0.85. The guidance value for SHI was determined to be 4.3. In this case thunderstorms were predicted as long as this value was not exceeded. The next highest TSS skill score for the same prediction was reached by using the TT with the guidance value 44.7, TSS(0.49) and POD (0.73). The CAPE was not found to be a successful predictor of air mass thunderstorms TSS (0.418), POD (0.67).