| Central Africa is a global hotspot for lightning and severe thunderstorms favorable for Transient Luminous Events(TLEs).The Imager of Sprites and Upper Atmospheric Lightning(ISUAL)instrument on board the FORMOSAT-2 satellite observed 35165 TLEs globally between June 2004 and June 2014.This study revealed a dense concentration of TLEs in the Central African region,including 485 blue jets/starters,five gigantic jets,and 306 sprites.Compared to other global lightning hotspots conducive to thunderstorms that produce TLEs,little to no research has been done in the Central African region to fully comprehend the meteorology and lightning features of the TLE-producing storms ISUAL instrument recorded.In this study,we report the TLEs that include the sprites(positive and negative),blue jets,and the gigantic jets observed by ISUAL and the characteristics of their corresponding thunderstorms and lightning.We used satellite and reanalysis-based datasets from the Tropical Rainfall Measuring Mission(TRMM),the Meteosat Second Generation SEVIRI(MSG SEVIRI),the Geostationary IR Channel Brightness Temperature(BT)-Grid Sat-B1 Climate Data Record(CDR),the World Wide Lightning Location Network(WWLLN),and the ERA5 to examine the characteristics of the thunderstorms and lightning TLE-producing storms in Central Africa.The main contents and conclusions of this paper are as follows:The study on the Upward jets(blue jets/starters and gigantic jets)showed that the blue jets/blue starters occurred in storms with very high cloud tops reaching 12-16 km and coldest cloud tops with low brightness temperatures below 265 K,accompanied by overshooting tops that penetrated the tropopause layers.We also observed very high rain rates above 2 dBZ near the blue jets or blue starter location and also from the vertical profile viewed very high altitude and high reflectivity values.From our observation,we speculate that all the blue jets or blue starters occurred in deep convective storm systems with powerful updrafts and overshooting tops that penetrated the tropopause layer.Regarding the gigantic jets,we only observed five gigantic jet cases from ISUAL datasets in Central Africa.Four of the five events had CAPE readings recorded at 20:00 UTC exceeding 1000 J/kg,but one of the gigantic jet events had CAPE values taken at 20:00 UTC of 139.85 J/kg.All the gigantic jets occurred with the lowest brightness temperatures of less than 260 K and the highest cloud top heights of 8-16 km.We also observed very low lightning flashes using WWLLN,especially when the gigantic jet occurred.These gigantic jets occurred in intense convective storms,probably accompanied by overshooting tops that penetrated the tropopause layer.The last study was about the sprites using ISUAL datasets,we observed 306 sprites in this region between 2004 and 2015.We categorized the sprites into 288 positive and 18 negative sprites.We observed the clouds near the negative sprites with the lowest brightness temperatures below 260 K and very high cloud top heights of～12-16 km.We also observed very high lightning flashes,especially when the negative sprite occurred,an indicator of cloud-to-ground lightning.The negative sprites occurred in intense convective regions with strong updrafts capable of driving the cloud top to penetrate the tropopause.On the other hand,for the positive sprites we only selected 11 cases out of 288.We conducted a case study for the positive sprite within the TRMM passage window on 4th Feb 2014.We observed this positive sprite to occur around regions with the lowest brightness temperatures of below 255K and very high cloud top heights～13-15 km.The vertical reflectivity profile from TRMM had zero reflectivity values from 0 to 10 km and exceptionally high reflectivity values from 10 to 20 km,indicating stratiform convection.The remaining positive sprite cases occurred around clouds with the lowest brightness temperature below 260K and highest cloud top heights between 12-16 km.From our comprehensive study,the findings suggested that blue jets,gigantic jets,and negative sprites preferably occurred in thunderstorms in deep convective systems.These severe convective systems appear to be accompanied by overshooting tops entering the tropopause,which generate a high and strong upper positive layer,a key component in producing favorable charge structure conditions for developing blue jets,gigantic jets,and negative sprites.The positive sprites detailed in our study also occurred in areas with the coldest cloud tops and highest cloud top heights,even though the vertical profile in the case study shows weak convection.Therefore,the positive sprites most likely happened in stratiform convection or a dying deep convective environment.These results will help the world learn more about the meteorological parameters and lightning conditions of the thunderstorms that caused all these Transient Luminous Events in the Central African region. |
