Feb 16 2007
Global Frequency and Distribution of Lightning
This is a summary and review of the paper by Christian et al. titled Global Frequency and Distribution of Lightning as Observed from Space by the Optical Transient Detector. Because the actual abstract is so long, I’ve cherry-picked the parts that I think are most important. See the entire thing at the link above.
Parital Abstract: The Optical Transient Detector (OTD) is a space-based instrument specifically designed to detect and locate lightning discharges as it orbits the Earth. Given the orbital trajectory of the satellite, most regions of the Earth are observed by the OTD instrument more than 400 times during a 1 year period, and the average duration of each observation is 2 min. The OTD instrument optically detects lightning flashes that occur within its 1300 × 1300 km2 field of view during both day and night conditions. A statistical examination of OTD lightning data reveals that nearly 1.4 billion flashes occur annually over the entire Earth. This annual flash count translates to an average of 44 ± 5 lightning flashes (intracloud and cloud-to-ground combined) occurring around the globe every second, which is well below the traditional estimate of 100 fl s−1 that was derived in 1925 from world thunder day records.
This paper is obviously one of the first papers published about the Optical Transient Detector, as most of the paper is on the methodology of the instrument rather than on science gathered from its data. The introduction to the paper motivates the need for a lightning detector that can view a large percentage of the globe, and also be able to distinguish diurnal, and seasonal cycles. According to the paper, much of the previous lightning detection from space was an unintended bonus. Previous methods were unable to accurately quantify the frequency and distribution of the lightning flashes.
The section on methodology contains subsections on instrument description, characteristics, measurement details, and instrument sensitivity. I’m going to skip over this entire section as it appears that it was fairly well designed. On to the results, discussion, and conclusion.
Results from this paper mainly include organizing the lightning flash data and presenting it in different ways. There are several tables throughout the text which report the mean annual flash density for cities in different continents. It is unclear how they chose the particular cities. As already known, the greatest flash density is over central Africa, where it exceeds fl km-2 yr−1. The eastern gradient in flash densities in the Congo basin is attributed to the topography of the Mitumba mountains. The Congo and Amazon basins are compared and contrasted with respect to lightning flash density. Obviously lightning over the Amazon basin is less than over the Congo. Christian references McCollum et al. [2000] which offers several reasons why there could be differences. Central Africa has about 50% smaller cloud droplet radii, more CCN, fewer large CCN, lower column water vapor, less precipitable water, drier subcould air, higher cloud bases, and greater topographic relief. Most of these changes can be attributed to topographic (mountains), dynamic (air flow patterns), and geographic (land cover) changes between the two areas.
The section on the differences between lightning over land and lightning over oceans is not too surprising. There is much more lightning - as well as thunderstorms - over continents. As learned in meteorology 101, this is because of the differential heating of the surface. Because of the differences in heat capacities, the land is able to warm faster than the ocean. Hot surface temperatures and topography (of which there is none over the ocean) combined with conditionaly unstable air is a good recipe for thunderstorms. It is pointed out that there are some areas of the ocean have high lightning flash densities. These all occur in regions where the water temperature is hot, such as the ITCZ and the Gulf Stream, or downwind of continents, such as off the southeastern coast of South America.
This paper offers a good introduction to the instrumentation used to monitor lightning from space. The techniques used to accurately discriminate lightning from atmospheric and sensor noise is dealt with in detail in the paper (but not in this review). It offers some basic conclusions which were not surprising. Perhaps the biggest contribution of this paper is that it greatly reduces the total world number of flashes ber second that was derived in 1925 from thunder day records to an average of 44 ± 5 lightning flashes (intracloud and cloud-to-ground combined) occurring around the globe every second.
References:
Christian, H. J., et al., Global frequency and distribution of lightning as observed from space by the Optical Transient Detector, J. Geophys. Res., 108(D1), 4005, doi:10.1029,2002JD002347, 2003.
McCollum, J. R., A. Gruber, and M. B. Ba, Discrepancy between gauges and satellite estimates of rainfall in equatorial Africa, J. Appl. Meteorol., 39, 666-679, 2000.
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