Abstract

Recently, observations of electrified oceanic convection and associated cloud-to-ground (CG) lightning were obtained over the tropical western Pacific Ocean during TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment). During COARE, observations of convection were made using a variety of instrument platforms including ship and airborne Doppler radars, an advanced lightning direction finder (ALDF) network, and a shipborne inverted electric field mill. This study focuses on data collected by the COARE ALDF network, fusion of those data with observations, and the methods used to calculate accurate CG return stroke locations.

Analysis of CG lightning data and Doppler radar data indicates that lightning-producing oceanic convection is characterized by deep, vertically developed convective cells with radar reflectivities exceeding 30-dBZ above the height of the −10°C level. In several cases a peak in CG frequency occurred coincident with the descent of precipitation mass bounded by the 30-dBZ reflectivity contour, linking the descent of the hydrometeor mass to the occurrence of CG lightning. The diurnal cycle of oceanic CG lightning, the convective available potential energy (CAPE), and rainfall indicates a peak in all these variables in the early morning hours (local time), approximately 2 h before the peak in cold-cloud area defined by brightness temperatures of less than −65°C. Sounding data indicate a strong positive correlation between CAPE and mixed-layer wet-bulb potential temperature and a weak positive correlation between CAPE and the number of CG lightning flashes observed in a 24-h period. The data also indicate that a highly nonlinear relationship exists between the wet-bulb potential temperature and the number of CG flashes observed in a 24-h period.

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