3.2 Lightning climatology
The ISS LIS lightning climatology has been completed for the first three
years of data (March 2017 - Feb 2020; Fig. 6a). Within ± 38° latitude,
these results are broadly similar to the more than 13-year (after orbit
boost; September 2001 - December 2014) TRMM LIS climatology (Fig. 6b).
During this period, TRMM had a nominal orbit altitude of 402.5 km - very
similar to the ISS orbit altitude range of 400-405 km. (The TRMM
pre-boost orbit altitude was substantially lower, at 350 km, and thus is
not considered here to keep the comparison more direct.)
Because ISS LIS has a shorter period of record (3 years vs 13 years),
the lightning density maps are not as smooth despite the 0.5° gridding
(Fig. 6a). There are also sampling limitations over low flash rate
regions, such as the global oceans. However, notable hotspots
[Albrecht et al. , 2016] from the TRMM LIS climatology (Fig.
6b), such as central Africa, Paraguay/northern Argentina/Rio Grande do
Sul (Brazil), Lake Maracaibo (Venezuela), the Himalayas/Indian
Subcontinent, and the Maritime Continent stand out, and feature
comparable flash rate densities between ISS and TRMM LIS. For example,
peak flash rate densities over central Africa exceed 80
km-2 yr-1 in both datasets. The
well-known stark contrast between land and ocean lightning flash rate
densities also stands out in both plots, as do the coastal enhancements
in lightning over the Gulf Stream, near west Africa, the Caribbean and
near Central America, near southeastern Brazil, the Bay of Bengal, etc.
Despite the more limited sampling, ISS LIS also has been able to observe
the small enhancements in lightning over the open Pacific, between ± 30°
latitude, west of -120° (south Pacific) and -150° (north Pacific)
longitude (including the Intertropical Convergence Zone, ITCZ). This is
similar to TRMM LIS (Fig. 6b), but the patterns are more diffuse due to
fewer samples. Note that portions of this more active ocean region are
now under continuous observation by GLM-17. Additional notable lightning
features that weren’t fully observed by TRMM LIS include enhancement
over the Tien Shan mountain range near northwest China, and a slight
enhancement over New Zealand (especially the north island).
In the global aggregate, lightning flash rates (between ± 38° latitude)
are comparable between ISS LIS and TRMM LIS (Fig. 7). Both datasets show
globally averaged flash rate ranging between 25 and 55
s-1. In addition to significant annual and interannual
variability, both datasets also appear to show semiannual variability in
the global flash rate, which is consistent with Williams[1994]. This, along with Fig. 6, demonstrates that ISS LIS is making
fundamentally similar observations to TRMM LIS, and thus is capable of
extending the TRMM LIS dataset over the tropics and subtropics for a
longer time period.
ISS LIS enables coverage of higher latitudes (± 55°) compared to TRMM
LIS (± 38°), providing renewed viewing of regions not observed by
spaceborne global lightning sensors since the OTD mission ended in 2000.
For example, ISS LIS reenables more complete viewing of the Great Plains
of the United States (US), which features flash rate densities
~30 km-2 yr-1extending as far north as the border with Canada (Fig. 6a). The improved
coverage of the continental US is a particularly important advantage of
ISS LIS, because this coverage allows for a more robust examination of
lightning/climate relationships within ongoing National Climate
Assessment (NCA) studies [Koshak , 2017]. Another mid-latitude
hot spot over Manchuria is also observed by ISS LIS, and the coastal
enhancement of lightning near eastern South America is seen to extend
further south. ISS LIS also provides coverage of most of Europe,
including the lightning enhancement near the Alps. Lightning enhancement
over Turkey is observed by ISS LIS.
The combined TRMM LIS and OTD dataset [Cecil et al. , 2014]
provides a useful point of comparison for ISS LIS. OTD was in LEO orbit
at 70° inclination and 740-km altitude [Christian et al. ,
2003], so it provided coverage at higher latitudes than ISS LIS, but
with reduced spatial resolution and geolocation accuracy. Table 1 shows
a comparison of globally averaged flash rates from ISS LIS relative to
the OTD and TRMM LIS climatology published by Cecil et al.[2014]. ISS LIS is measuring slightly lower flash rates, but the
numbers are generally within 5-10% of the previous climatology, which
is well within the magnitude of expected offset from the reduced
effective detection efficiency in version 1 ISS LIS data (Section 3.1),
the sampling limitations of the 3-year ISS LIS record, as well as
interannual variability (e.g., Fig. 7). Even the relatively larger
discrepancies seen between TRMM LIS/OTD and ISS LIS during
December-February are reasonably attributed to the above causes as well,
since the differences are still within ~15%. Note also
that the ISS LIS global flash rates in Table 1 are not smoothed, unlike
the TRMM LIS/OTD values.
Relative to Cecil et al. [2014], ISS LIS has observed
potentially higher flash rate densities in notable mid-latitude areas -
such as Turkey and the Middle East, southern Canada, Manchuria, Europe
and Northern Africa (Fig. 6a). However, caution in interpreting the
3-year ISS LIS dataset is required, since the relative impacts of
individual storms may be influencing these differences. Integration of
ISS LIS observations into the full LIS/OTD gridded dataset, which will
enable detailed quantitative comparisons for individual regions, is
planned for a future study. This planned analysis should be able to
determine if, and to what extent, lightning has increased globally at
higher latitudes as a result of climate change, relative to the OTD era
(1995-2000) [e.g., Veraverbeke et al. , 2017; Williams ,
2020].
The seasonal distribution of lightning from ISS LIS also follows
expectations established by previous global climatologies (Fig. 8).
Globally, lightning is maximized during June-August (Fig. 8b); however,
both March-May and September-November also have significant activity
(Fig. 8a, c). Notably, in boreal autumn the northern Great Plains of the
US can remain active, even as similar latitude locations in Europe, for
example, see a substantial decrease from the summertime peak (Fig. 8c).
The Manchuria lightning peak is primarily a boreal summertime
phenomenon, with a significant decrease in both spring and fall.
Lightning in the Middle East is most prevalent during boreal spring and
fall, providing evidence for a semiannual signal in lightning in certain
regions of the globe [Williams , 1994]. Turkey reaches its
maximum in summer. Boreal winter leads to a significant reduction in
northern hemisphere lightning (Fig. 8d); however, there are noticeable
hot spots remaining in the US Gulf Coast. The lightning peak near
Paraguay is most distinctive during austral spring (Fig. 8c). These
basic seasonal patterns are also observed in the TRMM LIS/OTD dataset
[Cecil et al. , 2014].
Globally averaged diurnal variability of lightning (Fig. 9) follows the
typical patterns observed in previous climatologies [Virts et
al. , 2013; Blakeslee et al. , 2014; Cecil et al. , 2014].
Namely, the diurnal cycle over land drives the overall global diurnal
variability in lightning, with the ocean flash rate essentially flat
throughout the day and night. On average, lightning peaks in the local
afternoon (3-4 pm Local Solar Time, LST), and reaches a minimum near 10
am LST (Fig. 9a). The timing and the approximate dynamic range in the
LST reference frame (15-100 s-1) are comparable to the
analysis of Blakeslee et al. [2014] for TRMM LIS/OTD. Viewed
in UTC time coordinates (Fig. 9b), lightning follows the classic
Carnegie-like curve structure [Mach et al., 2011], peaking
during 18-19 UTC. The timing and the 30-60 s-1 dynamic
range are very close to Blakeslee et al. [2014] as well. Note
that the approximately 2x larger global diurnal variability in flash
rate, versus the diurnal variability of electric field in the Carnegie
curve, is explained by the effect of higher currents in oceanic
thunderstorms, as well as the influence of electrified shower clouds
[Mach et al. , 2009; 2010; 2011].