Study of the annual courses of frequencies of occurrence of observations reporting thunderstorms series

Abstract

A study is made of series of monthly average frequencies of occurrence of observations reporting thunderstorms from 1989 to 2008 at 69 Cuban meteorological stations. A cluster analysis was performed considering the stations as variables and the monthly frequencies as individuals which yielded three groups consisting of: Summer months with greater occurrences (September, August, July and Jun), transition months (October and may) with moderate occurrence and winter months with low oc- currence. To reveal differences between stations in the annual course which are not forced by the total value of cases, these values are normalized and the values obtained are classified three ranks. Areas under the curves are calculated for these ranked values for each one of the time periods defined by the grouped months off the cluster analysis and then normalized again for the different lengths of these groups. This finally yielded 11 distinct groups of annual course patterns where 6 of these groups hold the majority of the stations (62) and the five remaining contain stations with particular characteristics. The group with the greatest number of stations (23) consisted of a summer with high occurrence, transition months with medium occurrence, and winter with also medium occurrence. The main differences between the annual courses of the stations under study lay on the single (western region) or bimodal (eastern region) shape of the distributions, the lasting and the dates of beginning and end of the season of maximum occurrence.