Figure 1: Map of age of recent lightning strikes in Texas on 19 September 2019 at 2110 UTC (map created through analysis of lightning detection sensors) (https://www.weathertap.com)
Cloud Height and Reflectivity Act as Maps of Lightning for Imelda
Imelda made landfall in Houston, TX as a tropical depression on Wednesday, 18 September 2019. It brought heavy precipitation and severe weather, including multiple tornado reports. Maps of lightning strikes associated with the storm within the hours before 2110 UTC 19 September 2019 display the effectiveness of using satellite imagery and radar imagery to identify areas of severe weather (Figure 1). The locations of lightning strikes within the 12-30 minutes prior to 2110 UTC (marked in white) display concentrations of lightning to the southwest, south, southeast, and northwest of Houston, as well as further north over Austin, TX. Satellite imagery of Texas within the IR Cloud Top channel display high clouds in the same areas, including significant cloud heights off the coast south of Galveston, TX, which may be too far from land to detect lightning strikes as there is not much recent lightning displayed in that location (Figure 2). Lightning is caused when deep convective clouds have large quantities of ice which aids in the polarization of the cloud structure. These ice particles result from water droplets high in the atmosphere freezing, but first they must reach altitudes cold enough. This is why deep clouds with significant heights are associated with lightning causing dynamics.
Figure 2: GOES-16 Infrared Cloud Top Imagery on 19 September 2019 at 2116 UTC (https://www.star.nesdis.noaa.gov/GOES/)
The base radar reflectivity is a less effective method of predicting the locations of large quantities of lightning strikes in the case of Imelda on 19 September 2019, with more areas of significant rainfall identified than had lightning strikes, such as to the north of Houston (Figure 3). The largest cell associated with the storm with respect to horizontal scale, just south of Houston and over Galveston, is easily apparent on the radar reflectivity image as a large area of reflectivity values over 40 Dbz. It is less easy to identify cells further west that match up with the individual cells identifiable by the red colors indicating tall clouds on the satellite imagery. Radar also has the disadvantage of limited range and the fact that radar signals can be blocked from reaching regions on the opposite side of heavy precipitation from the radar. This indicates that in the case of Imelda, radar may have been unable to detect precipitation in regions with frequent lightning strikes; satellite imagery of cloud height acted as a fairly accurate map of regions with a high volume of lightning strikes.
Figure 3: Radar reflectivity map from KHGX (Houston, Tx) on 19 September 2019 at 2111 UTC (https://radar.weather.gov/ridge/radar.php?product=NCR&rid=HGX&loop=yes)