Thursday, July 18, 2019

Lightning Distribution

As a follow-up to last week's post on extreme lightning activity, I thought it would be interesting to take a look at the spatial distribution of lightning in Alaska, as documented by the Alaska Lightning Detection Network.  As many readers know, the ALDN system underwent a transition in 2012 to a new network with different sensors, so the data are not directly comparable pre- and post-2012.  However, we can look at each period separately and make some qualitative comparisons.

The map below (click to enlarge) shows the density of lightning strikes in the 1986-2012 data, and you can zoom in by viewing the map on the ArcGIS platform at the following link:

The ArcGIS link also includes a second layer that shows the 2012-2018 data; you should be able to toggle the layers by navigating to "Content" on the left side of the interface.  Here's a static version of the 2012-2018 map:

Note that the data values are essentially arbitrary; they are obtained by counting the number of lightning strikes in latitude-longitude grid boxes, so there is a small north-south discrepancy in terms of the land area within each grid box.  Also, as mentioned above the new ALDN sensors record lightning strikes at a different rate than the old network, so we can't do a comparison of total strikes between the two periods.

However, it does seem that we can make one very interesting observation, which is that recent years have seen much more lightning in the southern interior than in the rest of the interior, and this is very different from the spatial distribution in the earlier decades.  The 1986-2012 history shows the highest concentration of lightning in the Yukon-Tanana uplands, and especially in the hills just to the east and northeast of Fairbanks.  This seems reasonable and agrees with earlier results here:

In contrast, the data from the past 7 years shows the highest lightning density along the west and north side of the Alaska Range and in the upper Tanana River valley.  It's a striking difference (pardon the pun).

To confirm the result, I re-calculated the number of strikes within two similar latitude/longitude rectangles: one focused on the 2012-2018 hotspot to the south of McGrath, and the other centered on the hills near Fairbanks.  In the 1986-2012 data, the Fairbanks area saw about twice as much lightning as the McGrath region, but in 2012-2018 the ratio was approximately reversed.

Of course the past 7 years is a short period from which to draw any conclusions, especially when the sum total of lightning activity is so heavily influenced by a relatively small number of very active days.  For example, 20% of all 2012-2018 lightning in my box south of McGrath occurred in a single day: June 23, 2015 (see below).  So the apparent southward shift in lightning density since 2012 might be related to just a few very unusual events, and therefore might not be statistically significant.  But it is quite intriguing nonetheless, and I can't help wondering if the exceptional North Pacific warmth of the past several years might have something to do with it.  Some interesting research could be done here.

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