[1] The Optical Transient Detector (OTD) is a space-based instrument specifically designed to detect and locate lightning discharges as it orbits the Earth. This instrument is a scientific payload on the MicroLab-1 satellite that was launched into a 70°inclination low Earth orbit in April 1995. 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 Â 1300 km 2 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 À1 that was derived in 1925 from world thunder day records. The range of uncertainty for the OTD global totals represents primarily the uncertainty (and variability) in the flash detection efficiency of the instrument. The OTD measurements have been used to construct lightning climatology maps that demonstrate the geographical and seasonal distribution of lightning activity for the globe. An analysis of this annual lightning distribution confirms that lightning occurs mainly over land areas, with an average land/ocean ratio of $10:1. The Congo basin, which stands out year-round, shows a peak mean annual flash density of 80 fl km À2 yr À1 in Rwanda, and includes an area of over 3 million km 2 exhibiting flash densities greater than 30 fl km À2 yr À1 (the flash density of central Florida). Lightning is predominant in the northern Atlantic and western Pacific Ocean basins year-round where instability is produced from cold air passing over warm ocean water. Lightning is less frequent in the eastern tropical Pacific and Indian Ocean basins where the air mass is warmer. A dominant Northern Hemisphere summer peak occurs in the annual cycle, and evidence is found for a tropically driven semiannual cycle.

ABSTRACT:The Geostationary Operational Environmental Satellite (GOES-R) is the next series to follow the existing GOES system currently operating over the Western Hemisphere. Superior spacecraft and instrument technology will support expanded detection of environmental phenomena, resulting in more timely and accurate forecasts and warnings. Advancements over current GOES capabilities include a new capability for total lightning detection (cloud and cloud-to-ground flashes) from the Geostationary Lightning Mapper (GLM), and improved capability for the Advanced Baseline Imager (ABI). The Geostationary Lighting Mapper (GLM) will map total lightning activity (in-cloud and cloud-to-ground lighting flashes) continuously day and night with near-uniform spatial resolution of 8 km with a product refresh rate of less than 20 sec over the Americas and adjacent oceanic regions. This will aid in forecasting severe storms and tornado activity, and convective weather impacts on aviation safety and efficiency among a number of potential applications. In parallel with the instrument development (a prototype and 4 flight models), a GOES-R Risk Reduction Team and Algorithm Working Group Lightning Applications Team have begun to develop the Level 2 algorithms (environmental data records), cal/val performance monitoring tools, and new applications using GLM alone, in combination with the ABI, merged with ground-based sensors, and decision aids augmented by numerical weather prediction model forecasts. Proxy total lightning data from the NASA Lightning Imaging Sensor on the Tropical Rainfall Measuring Mission (TRMM) satellite and regional test beds are being used to develop the pre-launch algorithms and applications, and also improve our knowledge of thunderstorm initiation and evolution. An international field campaign planned for 2011-2012 will produce concurrent observations from a VHF lightning mapping array, Meteosat multi-band imagery, Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor (LIS) overpasses, and related ground and in-situ lightning and meteorological measurements in the vicinity of Sao Paulo. These data will provide a new comprehensive proxy data set for algorithm and application development.

[1] We describe the clustering algorithm used by the Lightning Imaging Sensor (LIS) and the Optical Transient Detector (OTD) for combining the space-based observations of lightning pulse data into events, groups, flashes, and areas. Events are single pixels that exceed the LIS/OTD background level during a single frame (2 ms). Groups are clusters of events that occur within the same frame and in adjacent pixels. Flashes are clusters of groups that occur within 330 ms and either 5.5 km (for LIS) or 16.5 km (for OTD) of each other. Areas are clusters of flashes that occur within 16.5 km of each other. The flash data from LIS/OTD are currently being used for lightning and thunderstorm processes and climatological studies; therefore we test how variations in the algorithms for the event-group and group-flash clustering affect the flash count for a subset of the LIS and OTD data. We divided the subset into areas with low (1-3), medium (4-15), high (16-63), and very high (64+) flash counts to see how changes in the clustering parameters affect the flash rates in these different sizes of areas. We found that as long as the cluster parameters are within about a factor of two of the current values, the overall flash counts do not change by more than about 20%. Therefore the flash clustering algorithm used by the LIS and OTD sensors are robust and create flash rates that are relatively insensitive to reasonable variations in the clustering algorithms.

[1] We have combined analyses of high-altitude aircraft observations of electrified clouds with diurnal lightning statistics from the Lightning Imaging Sensor (LIS) and Optical Transient Detector (OTD) that are carried aboard low-Earth-orbiting satellites to reproduce the diurnal variation in the global electric circuit. Using basic assumptions about the mean storm currents as a function of flash rate and location (i.e., land or ocean) and the global electric circuit, our estimate of the current in the global electric circuit matches the Carnegie curve diurnal variation to within 4% for all but two short periods of time, in which the difference was 11% in one time period (0430 UTC) and 6% in the second period (1830 UTC). This excellent agreement with the Carnegie curve was obtained without any tuning or adjustment of the satellite or aircraft data. We assume that (1) the mean values for current and flash rates in the aircraft storm overflight data set (1.7 A and 0.8 flashes min −1 for oceanic thunderstorms, 1.0 A and 2.2 flashes min −1 for land thunderstorms, 0.41 A for oceanic electrified shower clouds (i.e., electrified but no lightning detected), and 0.13 A for land electrified shower clouds) and (2) the diurnal variations in lightning rates over land and ocean found in the satellite data set are universally applicable. Mean contributions to the global electric circuit from land and ocean thunderstorms are 1.1 kA (land) and 0.7 kA (ocean). Contributions to the global electric circuit from electrified shower clouds are 0.22 kA for ocean storms and 0.04 kA for land storms. The mean total conduction current for the global electric circuit is 2.0 kA. The means that for the number of storms contributing to the global electric circuit, 1100 are land storms with lightning, 530 are ocean storms without lightning, 390 are ocean storms with lightning, and 330 are land storms without lightning. A closer fit to the Carnegie curve is possible if the contributions from electrified shower clouds are increased by a factor of 3 or 4.Citation: Mach, D. M., R. J. Blakeslee, and M. G. Bateman (2011), Global electric circuit implications of combined aircraft storm electric current measurements and satellite-based diurnal lightning statistics,

Two approaches are used to characterize how accurately the north Alabama Lightning Mapping Array (LMA) is able to locate lightning VHF sources in space and time. The first method uses a Monte Carlo computer simulation to estimate source retrieval errors. The simulation applies a VHF source retrieval algorithm that was recently developed at the NASA Marshall Space Flight Center (MSFC) and that is similar, but not identical to, the standard New Mexico Tech retrieval algorithm. The second method uses a purely theoretical technique (i.e., chi-squared Curvature Matrix Theory) to estimate retrieval errors. Both methods assume that the LMA system has an overall rms timing error of 50 ns, but all other possible errors (e.g., anomalous VHF noise sources) are neglected. The detailed spatial distributions of retrieval errors are provided. Even though the two methods are independent of one another, they nevertheless provide remarkably similar results. However, altitude error estimates derived from the two methods differ (the Monte Carlo result being taken as more accurate). Additionally, this study clarifies the mathematical retrieval process. In particular, the mathematical difference between the first-guess linear solution and the Marquardt-iterated solution is rigorously established thereby explaining why Marquardt iterations improve upon the linear solution.

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[1] Using rotating vane electric field mills and Gerdien capacitors, we measured the electric field profile and conductivity during 850 overflights of clouds and thunderstorms. The measurements were made with NASA ER-2 and Altus-II aircrafts. Peak electric fields, with lightning transients removed, ranged from À1.0 kV m À1 to 16. kV m À1, with a mean value of 0.9 kV m À1. The median peak field was 0.29 kV m À1 . Flash rates ranged from 0 to over 27 flashes min À1 with the mean flash rate of 1.2 flashes min À1 . The median flash rate for an overpass was 0.25 flashes min À1 . The positive plus negative conductivity ranged from 0.6 pS m À1 to 3.6 pS m À1 at the nominal flight altitudes of 15 to 20 km. The mean and median total conductivity was 2.2 pS m À1 . Peak current densities during the overpasses ranged from À2.0 nA m À2 to 33. nA m À2 . The mean peak current density was 1.9 nA m À2 , and the median value was 0.6 nA m À2 . Using the peak electric fields, a median field falloff with distance based on all overflights, and cylindrical storm symmetry, the total upward current flow from storms in our data set ranges from À1.3 to 9.4 A with a mean value of 0.8 A. The median total current was 0.27 A. The contributions from lightning field changes do not significantly affect the total derived currents. We found that 7% of the storms were producing current flows above the storms that were opposite in polarity from the standard role that thunderstorms play in the global electric circuit. Approximately one third of the storms had no detectable lightning during the overpasses but still had significant electric fields. Owing to a possible sampling bias, the fraction of nonlightning storms with electric fields may not reflect the global probability of these clouds.