Thursday, June 21, 2018

Update on Lightning and Fire

Prompted by some comments on my post about Alaska fire acreage a couple of weeks ago, I acquired the most recent data from the Alaska Lightning Detection Network and pulled up a comparison to recent years - see below.  Earlier this month the cumulative number of lightning strikes recorded by the network was the highest for the time of year in the modern data set, but ever since last week's cold blast there has been almost no activity (at least until today).  (Note that the lightning sensors were changed in 2012, so it's not possible to do a direct comparison with earlier years.)

Fire acreage statewide currently stands at about 210,000 acres, which is also above most recent years, although 2013 and 2015 really took off in the latter part of June.  Today's cumulative acreage is about 10 days ahead of the long-term median in terms of the rate of burning statewide.

The year-to-year variability in fire acreage is obviously much higher than that of lightning, as there are other critical factors that control fire growth.  For example, both lightning and acreage were very high in 2015, but in 2013 acreage was high while lightning was relatively sparse; of course 2013 was very hot and dry, so fuel conditions were very conducive to the spread of fire.

Unsurprisingly, the ratio of acreage to lightning strikes is wildly variable - see below (calculated here whenever the cumulative number of strikes exceeds 1000).  In 2015, over 30 acres burned for every lightning strike detected, on average.  It would be interesting to compare this number to fire behavior in the lower 48.

While fiddling with the lightning data, I also determined the days on which the most lightning strikes were detected within 100 miles of a few different sites.  This allows us to look at the typical weather patterns associated with particularly intense lightning activity in different parts of the state.  For example, here's the average 500mb height pattern (the departure from normal) for strong lightning activity near Fairbanks: unusually high pressure aloft is centered to the northeast, and Fairbanks lies just to the south of the anomalous ridge axis.

When lightning activity is intense within 100 miles of McGrath, the ridge axis tends to be located much farther west, and a trough is evident over the Gulf of Alaska.

Below are the maps for strong lightning activity within 100 miles of Ambler and Eagle, respectively.

1 comment:

  1. There's lots of variables that need to be included but often are unavailable when considering total acreage vs lightning strikes. Type and abundance of combustible fuels on the ground; local humidity during and after the lighting strike, electrical polarity of strike -+ or -; nearby terrain - flat, rising, or downhill; nearby water courses; reaction by fire suppression agencies to the fire and protection requirements per their fire plan for the area; and likely more.

    I suspect during a hot year with frequent weather sources that promote thunderstorms the resulting fires overwhelm suppression. Strikes may not correlate to resulting acreage as all it takes is one discarded match according to Smokey the Bear.