A lightning climatology of the South-West Indian Ocean

C Bovalo,N Bègue,C Barthe

doi:10.5194/nhess-12-2659-2012

Abstract

Abstract. The World Wide Lightning Location Network (WWLLN) data have been used to perform a lightning climatology in the South-West Indian Ocean (SWIO) region from 2005 to 2011. Maxima of lightning activity were found in the Maritime Continent and southwest of Sri Lanka (>50 fl km−2 yr−1) but also over Madagascar and above the Great Lakes of East Africa (>10–20 fl km−2 yr−1). Lightning flashes within tropical storms and tropical cyclones represent 50 % to 100 % of the total lightning activity in some oceanic areas of the SWIO (between 10° S and 20° S). The SWIO is characterized by a wet season (November to April) and a dry season (May to October). As one could expect, lightning activity is more intense during the wet season as the Inter Tropical Convergence Zone (ITCZ) is present over all the basin. Flash density is higher over land in November–December–January with values reaching 3–4 fl km−2 yr−1 over Madagascar. During the dry season, lightning activity is quite rare between 10° S and 25° S. The Mascarene anticyclone has more influence on the SWIO resulting in shallower convection. Lightning activity is concentrated over ocean, east of South Africa and Madagascar. A statistical analysis has shown that El Niño–Southern Oscillation mainly modulates the lightning activity up to 56.8% in the SWIO. The Indian Ocean Dipole has a significant contribution since ~49% of the variability is explained by this forcing in some regions. The Madden–Julian Oscillation did not show significative impact on the lightning activity in our study.

Highlights

The World Meteorological Organization (WMO) has long been the only available source of information about thunderstorm days (World Meteorological Organization, 1953).Lightning activity was recorded by human observers based on the sound of thunder
It is important to note that the flash density reported by Collier et al (2006) is higher than in Christian et al (2003) due to a higher detection efficiency with Lightning Imager Sensor (LIS) compared to Optical Transient Detector (OTD)
Even if convection is enhanced by the Madden–Julian Oscillation (MJO), El Nino–Southern Oscillation (ENSO) or Indian Ocean Dipole (IOD), it does not mean that updrafts will be vigorous enough to generate supercooled liquid water and graupel, and separate charges (Zipser and Lutz, 1994)

Summary

Introduction

Lightning activity was recorded by human observers based on the sound of thunder. This kind of data is very sparse and may not be representative of a specific region. A better knowledge of location and frequency of global and regional lightning activity has been possible with the emergence of ground-based detection networks and satellite observations. The highest flash rate (82.7 fl km−2 yr−1) was obtained in the equatorial Congo Basin. As in Christian et al (2003), the highest flash density was found in the Congo Basin (107 fl km−2 yr−1), but Madagascar and South Africa exhibit high flash density (32.1 fl km−2 yr−1 and 26.4 fl km−2 yr−1, respectively). It is important to note that the flash density reported by Collier et al (2006) is higher than in Christian et al (2003) due to a higher detection efficiency with LIS compared to OTD (see Fig. 3 of Christian et al, 2003)

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Discussion

Conclusion