Tuesday, 11 January 2005
VHF lightning detection and storm tracking from GPS orbit
Lightning is a direct consequence of the electrification and breakdown processes that take place during the convective stage of thunderstorm development. Not surprisingly, lightning occurrence has been shown to be statistically correlated to the strength of the convective updraft and can be used as a proxy for locating and nowcasting strong convection and severe weather. This paper demonstrates the basis for and explores the feasibility of using very high frequency (VHF) radio receivers in Global Positioning System (GPS) orbit to detect lighting occurrence and track convective storms on a global scale. Ground-truth for the GPS-detected events is provided by the Los Alamos Sferic Array (LASA) for lightning activity in the Florida area and by the World Wide Lightning Location Array (WWLL) for storm activity in the central and western Pacific Ocean. Analysis of GPS/LASA correlated events indicates that the majority of GPS-detected VHF emissions (about 80 %) are associated with positive-polarity narrow bipolar events (NBEs). The remaining 20 % are due to intense VHF return stroke attachment transients occurring primarily over sea water. The detectability of VHF lightning from GPS orbit is found to be a function of the receiver sensitivity, the source strength and the source radiation pattern. GPS/LASA analysis is also used to demonstrate sustained GPS-based tracking and characterization of Tropical Storm Edouard in 2002. Edouard's NBE flash rates are found to be correlated to the total lightning flash rate in (and convective strength of) the storm. Simultaneous, two-satellite observations of lightning/storm activity over the western Pacific Ocean, ground-truthed with the WWLL array, demonstrate the ability of GPS-based VHF receivers to routinely identify, locate and track more generic storm activity. We conclude that GPS-based VHF lightning detection, specifically NBE detection, presents a viable means of mapping global deep convection and severe weather from space.