Goodman, Steve

Virts, Katrina

English

NASA Technical Reports Server

Lightning Density and Thunderstorm Initiation in the Lake Victoria Region in East Africa
Katrina Virts (University of Alabama in Huntsville, katrina.virts@uah.edu) and Steven Goodman (Thunderbolt Global Analytics)
1. Motivation
The Lake Victoria basin in eastern Africa is 
home to over 30 million people, over 200,000 
of whom work in the fishing or transportation 
industries on the lake.
The Red Cross estimates that 3,000 to 5,000 
people are killed every year on the lake 
because of thunderstorms.
We want to understand:
• How often and where do thunderstorms 
occur?
• Where do the strongest thunderstorms 
initiate, and under what conditions?
Figure 1: Elevation (m) of Lake Victoria basin.
4. Conclusions
We have examined climatological lightning occurrence and the initiation of high-impact, prolific thunderstorms in 
the Lake Victoria basin using new, continuous observations of total lightning from ENGLN.  We find that:
• Lightning is an almost daily occurrence over Lake Victoria.
• Strong seasonal and diurnal cycles, with most frequent lightning over the lake at night during equinox seasons.
• Afternoon, terrain-related thunderstorm initiation hotspots northeast of the lake.
• The vast majority (85%) of storms producing over 1,000 flashes over the lake initiate in situ during the 
night.  However, the more prolific the storm, the more likely that it initiated over land and then 
propagated over the lake.  This trend is especially prevalent during the equinox seasons.
• The initiation times of prolific LV storms have a bimodal seasonal cycle:
• Solstice seasons:  storms initiate in early morning (05-08 LT), ~12 hours after the peak afternoon lightning.
• Equinox seasons:  thunderstorms initiate from late evening through the night (22-04 LT), or about 5-6 hours 
earlier than during solstice season.  These storms are more likely than solstice-season storms to 
initiate over land and then propagate over the lake.
• Our results suggest that during equinox seasons, afternoon thunderstorms over land have more direct and 
indirect influence on the intense nocturnal storms over Lake Victoria, compared to solstice seasons.
• The most extreme Lake Victoria clusters are associated with enhanced instability and low-level moisture, and 
converging low-level winds over the lake.  Mid-level wind and deep layer shear anomalies are small.
Acknowledgments: The authors thank ENGLN for providing the lightning data, the Met Office for collaboration in 
the HIGHWAY project, and the NASA Postdoctoral Program for support.
3. Initiation of Prolific Thunderstorms over Lake Victoria Basin
• Starting in 2014, Earth Networks Global Lightning Network (ENGLN) placed 15+ low frequency-high frequency sensors 
around Lake Victoria → first continuous total lightning observations in this region.
• We studied lightning occurrence from September 2014 – August 2018.
Figure 2: Annual-mean 
lightning density (flashes 
km-2 yr-1) measured by (a) 
ENGLN and (b) TRMM-
LIS.  Note the different 
color scales.
Season Mean lightning density over LV
DJF 164 flashes km-2 yr-1
MAM 212 flashes km-2 yr-1
JJA 126 flashes km-2 yr-1
SON 215 flashes km-2 yr-1
2. Lightning Climatology of Lake Victoria Basin
• Solar heating, lake breezes, and valley breezes produce
afternoon lightning maxima over land, especially north and east of Lake Victoria.
• At night, land cools more rapidly than water, and land breezes and mountain breezes lead to convergence over the lake 
that may be enhanced by outflow boundaries from daytime convection over land.
• A nocturnal lightning maximum propagates from northeast to southwest over the lake.
• Thunderstorms occur on an almost daily basis over Lake Victoria: >100 flashes over the lake on 89% of days.
• ENGLN and the Lightning Imaging Sensor (LIS) aboard the TRMM satellite report similar climatological lightning patterns.
• Lightning is most frequent over the lake, over the plains to the north, and between the lake and highlands to the east.
• Lake Victoria is on the equator → lightning occurrence shifts latitudinally with the equatorial trough during the year.
• Lightning is most frequent during the equinox seasons
(March-May, September-November) and least frequent
during the solstice seasons (December-February, June-August).
Figure 3: ENGLN lightning density (flashes km-2 yr-1) during (a) daytime, (b) evening and early night, and (c) night and morning.
In order to examine the initiation of individual thunderstorms, ENGLN flashes were clustered using a weighted Euclidean 
distance method: WED2 = (D/10)2 + (T/600)2, where D is the distance between two flashes in kilometers and T is the time 
between them in seconds.  Two flashes are assigned to the same thunderstorm cluster if WED < 1.
Figure 4: Location of first flash of lightning clusters producing (a) more than 1,000 flashes during their lifetime and (b) more than 1,000 
flashes over Lake Victoria (i.e., within the red polygon in Fig. 1).
• The vast majority (~85%) of prolific Lake Victoria clusters initiate in situ over the lake.
• Terrain-related maxima over the steep western slopes of Mt. Elgon and other topographical features in Fig. 4a are absent 
in Fig. 4b → while they often propagate toward the lake, these clusters generally dissipate before reaching it.
Figure 5: Histograms of the month (abscissa) and local time (ordinate) of the first flash of lightning clusters producing (a) more than 
1,000 flashes during their lifetime and (b) more than 1,000 flashes over Lake Victoria. The diurnal cycle is repeated for clarity.
• Clusters producing >1,000 flashes initiate around 11-14 LT, just before the lake and valley breezes reach their maximum 
amplitude, and 1-2 hours earlier than the average cluster.
• The initiation times of clusters producing >1,000 flashes over Lake Victoria exhibit a bimodal seasonal cycle:
• During solstice seasons, clusters primarily initiate between midnight and noon, peaking at 05-08 LT, about 12 hours 
after the afternoon convective peak.
• During equinox seasons when lightning is most frequent over the lake, initiation times range from early evening through 
the night and morning.  The hour of peak initiation ranges from 22-04 LT.
• Earlier initiation times during equinox seasons suggest that clusters may be triggered by outflow boundaries or 
gravity waves propagating away from decaying convection over land, or that land-originating convection 
undergoing a lull in lightning production may be reinvigorated by the warm lake waters and land breezes.
Figure 6: (a) Monthly percentage of flashes over Lake Victoria that were produced by clusters initiating over the lake.  (b) For bins based 
on the number of flashes that clusters produced over Lake Victoria (abscissa), the percentage of clusters in each bin that initiated over 
Lake Victoria (ordinate).
• During solstice seasons, 80-95% of flashes over Lake Victoria were produced by 
clusters initiating in situ over the lake.
• During February-April fewer than 70% of flashes over the lake were produced by 
clusters initiating there.  During March, over half of Lake Victoria lightning was 
produced by thunderstorms initiating over land.
• Clusters producing <10 flashes over the lake initiate almost exclusively in situ.  
As flash count increases, the likelihood of initiation over land increases.
This trend occurs in all seasons but is more marked during equinox months.
• Of the 100 most extreme Lake Victoria clusters (>85,000 flashes over the lake), 
64 initiated over the lake and 36 over land, primarily north and east of the lake.
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7 Figure 7: (left) Composite and (right) anomalous ERA5 (top) CAPE, (middle) 850-hPa geopotential height and winds, and (bottom) 850-hPa specific humidity 3h prior to 
initiation of the 100 most extreme Lake Victoria clusters.  Anomalies were obtained by 
subtracting the climatological monthly-mean for that hour.
• Extreme Lake Victoria clusters initiate in an environment of enhanced 
CAPE and low-level moisture, supporting vigorous convection.  A broad 
trough extends from the lake northward, with low-level convergence 
over the lake.  Low-level winds over the lake are anomalously westerly.
• Mid-level winds are easterly over the basin and do not statistically differ from 
the climatological-mean, and deep layer shear anomalies over the lake are 
small (not shown).
https://ntrs.nasa.gov/search.jsp?R=20200000453 2020-03-11T13:34:54+00:00Z


Lightning Density and Thunderstorm Initiation in the Lake Victoria Region in East Africa

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