Characteristics of lightning signals over the Tibetan Plateau and the capability of FY-4A LMI lightning detection in the Plateau

Because of its topographic features, lightning flashes are unique over the Tibetan Plateau. The Lightning Mapper Imager (LMI) on board the launched China's Fengyun (FY)-4A geostationary satellite is a useful tool for studying these lightning flashes. In this study, firstly, based on data acquired by the Lightning Imaging Sensor (LIS) on board the Tropical Rainfall Measuring Mission (TRMM) satellite in the period 1998-2013, the optical radiation characteristics of lightning flashes over the Tibetan Plateau were compared with those over the central and eastern land region (CELR) of China at the same latitude. The results showed that lightning flashes over the Tibetan Plateau had a mean optical radiance of 422.35 mu J sr(-1) m(-2) nm(-1), a mean duration of 210.79 ms and a mean radiation area of 217.33 km(2), whereas the lightning flashes over the CELR had a mean optical radiance of 692.83 mu J sr(-1) m(-2) nm(-1), a mean duration of 295.29 ms and a mean radiation area of 290.85 km(2). Evidently, lightning flashes over the Tibetan Plateau had significantly lower radiance, shorter life cycles, and smaller radiation areas than those over the CELR. Secondly, the data for lightning events over the Tibetan Plateau acquired from the FY-4A LMI and International Space Station (ISS) LIS were compared from 21 March 2017 to 25 September 2017, and from 20 March 2018 to 18 September 2018. The correlation between the frequency distribution of the optical radiance of lightning events observed by the two lightning imagers was 99.33%, but due to the footprint area of a LMI pixel is larger than that of a LIS pixel, the mean radiance of lightning events over the Tibetan Plateau measured by the FY-4A LMI was only 1/5 of that measured by the ISS LIS. Based on the statistical results, a time/space coincidence window of 2.1 s/33 km was selected. A resulting mean coincidence ratio (CR) of 3.66% was found for the FY-4A LMI and ISS LIS lightning events for the Tibetan Plateau. The spatial distribution of the CR values was correlated with the optical radiance of lightning events and the duration of lightning flashes as well as the number of events in flashes, with the Pearson's correlation coefficients (r) of 0.80, 0.82 and 0.78, respectively. These results suggest that the optical radiation characteristics of lightning flashes over the Tibetan Plateau have a great impact on their detection by the FY-4A LMI. For observation of lightning flashes over the Tibetan Plateau using the FY-4A LMI, it is necessary to enable the Real-Time Event Processor (RTEP) algorithm to more finely distinguish various regions based on lightning flash characteristics and to set a more flexible and smaller threshold for extracting lightning events.