Figure 5. (a) Raw, un-geolocated ISS LIS background image from
approximately 2044 UTC on 1 June 2017. (b) The same ISS LIS background
image but geolocated, showing ISS LIS events detected in the domain of
the image during the overpass. The suspect LIS flash not seen near
CATS-inferred cloud also is indicated, and LIS and CATS boresight ground
tracks are shown.
Of the 8246 LIS/CATS matchups, 105 (1.3%) have no CATS-identified cloud
within 50 km along the CATS ground track (further constrained by the
flash centroid needing to be within 25 km distance perpendicular to the
CATS ground track). Of these 105 candidate false alarms, 65 occurred
during daytime (777.4 nm backgrounds during night have limited utility
for this paper’s analysis) and have nearly coincident ISS LIS
backgrounds (within approximately 30-60 seconds, the frequency with
which LIS provides background imagery, as discussed in Section 2.3).
Manual review of these geolocated backgrounds, with LIS lightning
overlaid, was performed. This analysis found that the vast majority of
candidate false alarms were similar to Fig. 5, where obvious convection
(in this case, over Hispaniola) is in the LIS field of view and the
presence of lightning is reasonable to infer. This case demonstrates the
fundamental limitations of co,paring 2D horizontal lightning
observations with lidar nadir curtains: Sometimes, obvious connections
are just missed. Tightening distance thresholds obviously would improve
on this, at the cost of reducing the total size of the already small
LIS/CATS dataset. At the same time, the fact that only
~1% of LIS/CATS comparisons with the chosen distance
thresholds couldn’t find at least some cloud in the vicinity of
lightning is highly encouraging for the approach adopted in this study.