Performance assessment of the Optical Transient Detector and Lightning Imaging Sensor

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.