Figure 1. (a) Time-height plot of CATS feature mask with ISS
LIS flashes overlaid for half an ISS orbit on 15 March 2017. ISS LIS
flash times are plotted vs. distance from CATS ground track (not flash
height, which LIS does not measure). (b) Time-height plot of CATS cloud
phase with ISS LIS flashes overlaid for half an ISS orbit on 15 March
2017. ISS LIS flash times are plotted vs. distance from CATS ground
track (not flash height, which LIS does not measure).
In addition to the above statistical analysis, a manual review of
combined LIS/CATS quicklooks (e.g., Fig. 1) was performed. A major focus
of this review was to identify LIS lightning that did not appear to
occur near CATS-identified clouds. These potential false alarms were
further studied using manual inspection of daytime LIS/CATS matchups
using geolocated ISS LIS backgrounds, which are available every 30-60
seconds from LIS. These backgrounds (Blakeslee, 2020b) were geolocated
using the ISS_Camera_Geolocate open-source software (Lang,
2019), originally developed in support of the Schultz et al. (2020)
study.
2.4 Other datasets used
To assess the distribution of radar-observed echo-top heights, for
comparison to CATS-measured thunderstorm cloud-top heights, we examine
TRMM and Global Precipitation Measurement (GPM) Precipitation Features
radar precipitation features (RPFs; Liu et al., 2008) for March through
October 1998-2014 (TRMM) and 2014-2020 (GPM). The RPFs are defined as
contiguous areas of at least 4 pixels of precipitation as detected by
the TRMM Precipitation Radar (PR) or the GPM Dual-Frequency
Precipitation Radar (DPR) Ku-band radars. These radars have minimum
threshold reflectivities of 17 dBZ and 12 dBZ, respectively. Pixel size
is roughly 20 km2. For each RPF, radar and passive microwave
characteristics (e.g., echo-top heights, reflectivity profiles, and
brightness temperatures) are provided, but in this study the focus was
primarily on echo-top heights. In order define features with lightning,
the coincident LIS data were used for TRMM RPFs. The GPM Core satellite
does not have a lightning imager onboard, and therefore GPM RPFs were
co-located with World Wide Lightning Location Network (WWLLN; Virts et
al., 2013) data within a +/- 10-minute window in the boundary of the
feature (Liu, 2020).
3 Results
3.1 Histograms of cloud- and echo-top height
Figure 2 shows a 2D histogram of CATS-measured maximum cloud-top-height
vs. latitude within 50 km along the CATS ground track of at least one
LIS-detected flash (Fig. 2a). The histogram is normalized but no
adjustments for sampling frequency have been made due to the short time
period of analysis (< 8 months). March-October 2017 primarily
covers the spring, summer, and early fall seasons (i.e., warm season) in
the northern hemisphere. Within 10° S to 20° N latitude, thunderstorm
maximum cloud-top height was most frequently 16-17 km MSL. A downward
sloping in maximum cloud-top height occurred toward the northern
mid-latitudes, reflecting the general downward sloping of the tropopause
toward the poles (Santer et al., 2003). Of course, as is well known
thunderstorm heights are not fully constrained by the tropopause (Liu &
Zipser, 2005). Regardless, within 35-50° N the thunderstorm cloud-top
height most commonly ranged between 10 and 14 km MSL. Note the
relatively few samples south of 10° S, reflecting the LIS/CATS dataset’s
focus on the northern hemisphere’s warm season.
To help validate the conclusions implied by Fig. 2a, histograms for TRMM
(Fig. 2b) and GPM RPF (Fig. 2c) maximum 20-dBZ echo-top height vs.
latitude are shown for all RPFs with at least one LIS- or WWLLN-detected
lightning flash during March through October (1998-2014 for TRMM,
2014-2020 for GPM). Due to the longer time periods for analysis, the
TRMM and GPM histograms were normalized by sampling frequency. Note that
TRMM was inclined at a much lower orbital angle (35°) than either ISS
(57°) or GPM (65°), so latitudinal coverage was reduced relative to the
other two platforms. Regardless, both TRMM and GPM suggest that
thunderstorm 20-dBZ echo-top heights are lower by approximately 2 km
than lidar-inferred cloud-top heights. The downward sloping of the
echo-top heights toward the poles (with approximately the same slope as
Fig. 2a), as well as the preference for thunderstorms in the northern
hemisphere during May-October, are also apparent. This suggests the
added value of using a lidar over a radar to obtain a more accurate
measurement of cloud-top height, but also implies a way to use lidar
observations to help scale radar echo-top heights to cloud-top heights
if only a radar is available (at least from an average global
perspective). Moreover, this analysis also helps validate the matching
criteria for the LIS/CATS comparison.