GLOBAL LIGHTNING ACTIVITY H. J. Christian National Space Science and Technology Center Huntsville, Alabama ABSTRACT: Our knowledge of the global distribution of lightning has improved dramatically since the 1995 launch of the Optical Transient Detector (OTD), followed in 1997 by the launch of the Lightning Imaging Sensor (LIS). Together, these instruments have generated a continuous seven-year record of global lightning activity. These lightning observations have provided a new global perspective on total lightning activity. For the first time, total lightning activity (CG and IC) has been observed over large regions with high detection efficiencies and accurate geographic location. This has produced new insights into lightning distributions, times of occurrence and variability. It has produced a revised global flash rate estimate (46 flashes per second) and has lead to a new realization of the significance of total lightning activity in severe weather. Accurate flash rate estimates are now available for large areas of the earth (+/- 72 o latitude). Ocean-land contrasts as a function of season are clearly revealed, as are orographic effects and seasonal and interannual variability. The data set indicates that air mass thunderstorms, not large storm systems dominate global activity. The ability of LIS and OTD to detect total lightning has lead to improved insight into the correlation between lightning and storm development. The relationship between updraft development and lightning activity is now well established and presents an opportunity for providing a new mechanism for remotely monitoring storm development. In this concept, lightning would serve as a surrogate for updraft velocity. It is anticipated that this capability could lead to significantly improved severe weather warning times and reduced false warning rates. 1. INTRODUCTION LIS and OTD data have been combined to provide a 5-yr OTD average annual and seasonal worldwide distribution of total lightning activity. The global flash rate is further decomposed into its seasonal cycle, and the variation of flash rate is compared between land and ocean, northern and southern hemispheres, tropics and sub-tropics, and the three major continental land masses- Africa/Europe, the Americas, and Asia/Maritime Continent. The global annual average total (intracloud and cloud-to-ground) flash rate is found to be 46 fl s -1 [Christian et al., 2003, revised with LIS data]. The annual global flash rate ranges from a maximum of 55 fl s -1 (in Northern Hemisphere summer) to a minimum of 35 fl s -1 (in Northern Hemisphere winter). These lightning flash rate estimates are less than half the traditional estimate of 100 fl s -1 , proposed by Brooks [1925]. Additionally, the OTD/LIS derived global flash rate is significantly less than recently published estimates of the average global flash rate, given that these prior estimates were all greater than 60 fl s -1 [Mackerras et al., 1998; Kotaki and Katoh, 1983; Orville and Spencer, 1979]. Moreover, these prior estimates were reported with a range of uncertainty due to instrument and sampling limitations that approached a factor of two, which is the likely explanation for the discrepancy between the prior estimates and the results obtained from the OTD/LIS data. The range of uncertainty associated with the OTD/LIS derived global flash rate is estimated to be well less than 20%, a value that primarily reflects uncertainty in the flash detection efficiency of the instrument. The observed OTD/LIS flash rate also implies that mean flash dipole moment change is somewhat lower than that used by Heckman et al. (1998). The annual average global distribution of total lightning flash rate is shown in Fig. 1. Flash rate is contoured in units of fl km -2 yr -1 , based on a 0.5 o ×0.5 o compositing grid smoothed with a 2.5 degree spatial moving average operator. The geographical distribution of flash rate density is in general qualitative agreement with the climatological distribution of thunderstorm days [WMO, 1953], being dominated by the diurnal heating of the major land masses. The peak mean annual planetary flash density exceeds 80 fl km -2 yr -1 and occurs just west of Kamembe, Rwanda, a place that averages 221 thunderstorm days per year. Figure 1. The annualized distribution of total lightning activity (in units of fl km -2 yr -1 ).