3.3 Use as a tool for GLM calibration and validation
A high-priority effort at NASA MSFC has been to properly validate the GLM instruments on GOES-16 and GOES-17. This includes examining the GLM flash detection efficiency (DE), the flash false alarm rate, the lightning location/timing accuracy, maximum data rate capability, and long-term instrument degradation. Specific GLM instrument requirements have to be validated to confirm that GLM performance is acceptable for critical operations and decision making. GLM validation often makes use of several different ground-based lightning detection networks. These are high-quality data sources, but they are all land-based. A large part of the GLM FOV, especially GLM-17, is over the ocean where the quality of ground-truth data is highly variable. Fortunately, ISS LIS has provided vital data of uniform quality out over the oceans.
Another problem with comparing GLM to ground sources is that the comparison is not truly one-to-one. The GLM is an optical sensor, whereas all of the ground-based networks consist of RF sensors. The RF sensors look at fundamentally different physics and different parts of the lightning flash, making one-to-one comparisons difficult. Since ISS LIS is an optical sensor very similar to GLM (though with approximately 4x better spatial resolution), it is the only source of direct comparison for GLM. Indeed, because ISS LIS is a heritage sensor of GLM, with similar operation and data structure, it has provided particularly easy/efficient inter-comparisons (i.e., both optical, both spaceborne, and both detect lightning over land/ocean).
GLM-16 has been observed to have a substantially depleted flash DE over the northwestern CONUS (e.g., Washington and surrounding states). A detailed plot of GLM-16 flash DE for the period January 2018 to December 2019 is shown in Fig. 10. The GLM flashes were compared with observations from ISS LIS (Fig. 10 left), as well as data derived from two ground-based RF lightning detection networks - ENGLN (Fig. 10 middle) and GLD360 (Fig. 10 right). All three comparison datasets agree on the basic structure of GLM-16’s northwestern CONUS DE depletion (reduced to as low as 20-40%). The fact that both optical spaceborne (ISS LIS) and RF ground-based (ENGLN and GLD360) measurements agree provides increased confidence that the DE depletion is the result of GLM instrument effects near the edges of its FOV.
ISS LIS continues to provide important one-to-one comparisons with GLM data, and will remain a key dataset for current and future GLM validation [e.g., Zhang and Cummins , 2020], as well as potentially other future geostationary lightning observations, such as from the forthcoming Meteosat Third Generation (MTG) Lightning Imager (LI) [Kokou et al. , 2018]. ISS LIS further provides a single observation system that will enable direct intercomparisons between GLM, LI, and other space-based observations.