Lightning detection with strong convection
The animation above shows a strong convective system between Austin and San Antonio moving southeastward on 8 June 2023 . SPC storm reports note that this complex dropped hail in Bulverde (in Comal County, 13 miles northeast of San Antonio) around 1900 UTC, and in various locations in Bexar County (of which San Antonio is the county seat) between 1900 and 1930 UTC). Why are GLM observations of Flash Extent Density sparse in this severe storm with obvious overshooting tops and a warm trench?
GLM observations are clustered on the trailing edge of the system at the beginning of the animation. It’s likely that optical detection of lightning flashes is being inhibited by the thickness of the cloud (and suspended hydrometeors) in this case. GLM observations of Total Optical Energy (in this case with a 1-minute sampling) at 1840 UTC, below, from GOES-16 shows very little activity in this convective complex. Compare that to GOES-18 observations of the GLM TOE at the same time, at bottom (click here for a toggle)!
When using the GLM, remember that it is detecting an optical signal of lightning. Consider what might lie between the lightning emissions and the satellite. For this case, absorption by thick cloud, rain and perhaps suspended hail was preventing the optical signal from reaching GOES-16 (over the Equator at 75.2o W, to the east of the storm). GOES-West (over the Equator at 137o W) had a view of the lightning from the west that was less affected by the thick cloud and whatever was inside it.
Thanks to John Cintineo, CIMSS, and Kevin Thiel, SPC for alerting me to this event during the Hazardous Weather Testbed. Blog posts created by NWS forecasters that discuss HWT demonstration products can be found here.
Added, 11 July 2023: GLM Engineers have determined that the GOES-16 GLM did detect flashes during this event in the region where GOES-18 also detected flashes. However, a blooming filter in the GLM processing screened out the signal (erroneously). The blooming filter activates when the BGMSB (background most significant bits) that is related to background brightness increases too quickly. That can happen during explosive development.