Analysis of Lightning Strike Variations in Sweden from 2000- 2014

Hewage Nihal, Perera S.S., Boralugoda S.K., Fernando I.M.K., Cooray V.

Lightning is a natural phenomenon; in which massive electric discharge occurs between the electrically charged regions within clouds, clouds to clouds or between a cloud and the Earth's surface. Lightning flash is referred as the temporal equalization of charge regions within the atmosphere. Lightning occurs approximately 40–50 times a second worldwide, resulting in nearly 1.4 billion flashes per year. Present Lightning Location and Protection(LLP) systems used in many countries are capable of studying long-term characteristics of CG lightning flashes with a typical range of 600 km with many sensors interconnected to form lightning detection networks that span thousands of kilometres that can capture lightning in vast land masses.

The ultimate purpose of this study is to assess the possibility of occurrence and variation of cloud-to-ground lightning strikes over the geographical boundary covered by the LLP system of Sweden. Thus, the study measures the cloud density and lightning strike frequencies within the period of the last fourteen years (2000-2014) recorded by Lightning Detection Centers of the LLP system which were holistically dispersed all over Sweden.

Wavelet theories, time frequency analysis and Range Normalization of Signal Strength (RNSS) were used to analysis the results. Wavelet theories have been used to analysis the pattern over the period. Time frequency analysis and RNSS values were used to determine risk status.

According to the analysis, two types of lightning strike patterns were identified in Sweden. One is “Annual Lightning Strike Pattern” which occurred between 0.7-1.4 year and the anther is “Extreme Lightning Strike Pattern” which occurred every 3-5 year period and its frequency increased equator to polar sides of Sweden. Lightning risk was calculated over the period by using RNSS values as magnitude and lightning frequency. The results show that the lightning risk also varied in similar patterns and its return pattern varies from 4 – 3 years from Southern to Northern part of Sweden respectively.