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:

http://arcg.is/1zvGHm0



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:

https://www.firescience.gov/projects/07-2-1-42/project/07-2-1-42_ams_alaska_ltg_climo_2009.pdf

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.


Friday, July 12, 2019

Lightning Outbreak

Yesterday was an extraordinary day for lightning in Alaska, with the highest number of lightning strikes recorded for any single day in the 2012-present history of the modern Alaska Lightning Detection Network.  The previous daily record from 2016 was broken by about 25%, based on strike counts for 24 hour periods ending at 6 am.

Note that the ALDN database includes plenty of lightning data from northwestern Canada, but I'm excluding anything east of 141°W (the north-south Alaska/Yukon border) so as to focus on Alaska.  (But yesterday also broke the record for the entire domain, and also for days defined as midnight-to-midnight.)





The annual totals and peak daily numbers for each year since 2012 are shown below.  It will be interesting to see if this year sets a record for the annual total; we are already past the climatological peak in lightning activity for Alaska, but a few more big days could push 2019 into first place.


Monday, July 8, 2019

Heat and Smoke

As I'm sure all readers are fully aware, an extraordinarily intense heat wave has been occurring across much of southern Alaska in the past week, and longstanding temperature records have been falling left and right.   Here's a quick summary that Rick Thoman posted yesterday:


 

There's far too much to comment on, but the remarkable July 4 heat in Anchorage may be the most astonishing statistic: the previous all-time record for the city's official climate site was only 85°F.

Dense smoke from wildfires has held temperatures down in some parts of the interior, including Fairbanks, but many locations in western and southern Alaska are well up into the 80s today.  Particularly striking to me is the 87°F currently being reported from Noatak in the northwest - above the Arctic Circle - and 92°F earlier today at the high-quality CRN station to the southeast of Ruby.


Here's an annotated true-color satellite image, courtesy of Rick Thoman and UAF:



Click on the image below for a higher resolution version:


 As bad as the smoke is at the moment in Fairbanks, this summer has a long way to go to equal the awful summer of 2004 in terms of duration and choking density of smoke - see the chart below, showing data from the Fairbanks airport ASOS instrument.  Here's hoping that 2004 remains an outlier.


Tuesday, July 2, 2019

Wildfire Activity

Increasingly unusual warmth in the past couple of weeks has allowed wildfire activity to pick up substantially across Alaska, and as of today the state has seen about double the normal fire acreage for the date.  The typical midpoint for statewide fire acreage is only about a week away, and so 2019's acreage is close to exceeding the median value for the entire season.  The last 3 years were relatively normal for statewide fire activity (at least based on a "modern" normal since 1995), but with another round of heat currently developing across Alaska, there's little doubt that 2019 will end up with significantly above-normal wildfire activity.  Click to enlarge the figure below.


The fire that is drawing the most attention from fire suppression experts is the Shovel Creek fire, which is very close to Fairbanks - just the other side of Murphy Dome.  The fire is not nearly as large as some other fires in the state, but it is expected to grow and there is a high risk to properties near Fairbanks.  Consequently a national-level command (Type 1 IMT) is taking over the suppression efforts tomorrow; I'm not sure how often this occurs in Alaska.

Here are a couple of maps showing how the Shovel Creek fire grew between Friday (3,424 acres) and yesterday (10,008 acres).  Ester Dome (on the west side of Fairbanks) is located in the lower right of the map domain.  Let's hope the trend does not continue.



Last year I commented on the positive correlation between North Pacific sea surface temperatures and Alaska wildfire activity, so perhaps it should be no surprise that this fire season is becoming active.  The Bering Sea and Gulf of Alaska surface temperatures have been far above normal since winter, and Rick Thoman has been documenting (via his Twitter feed) temperature anomalies of several degrees Celsius to the west of Alaska in recent weeks.  From a global perspective, the warmth in the North Pacific is one of the more striking features in the global oceans at present; here's a map of May SST departure from normal in terms of standard deviations.


To reinforce the idea of a link between Alaska fire and North Pacific temperatures, the chart below shows a 25 year history of the North Pacific Mode index in May and subsequent Alaska fire acreage.  Remarkably, all of the very active fire seasons in recent decades have occurred when the NPM index was positive.  Recall that the NPM represents a specific pattern of SST anomalies that extends west-east across the North Pacific to the south of Alaska; not surprisingly, the NPM was positive in May and has become strongly positive in recent weeks (see figures below).




The long-term history of the NPM index does not have a rising trend over time, but this is by design: the NPM definition proposed by Hartmann (2015) removes the linear trend from the SST data before doing the EOF analysis.  In contrast, if we look at area-average SSTs from 40°N to the Bering Strait, there is a very significant warming trend, and the past several years have been persistently very warm.


It stands to reason that higher temperatures would favor increased wildfire activity, all else being equal, because earlier snowmelt allows fuels to dry out earlier in the year, and increased evapotranspiration creates larger moisture deficits if rainfall does not increase.  The long-term increase in temperature has probably therefore produced a background increase in fire activity, or an increase in what is "normal" for a multi-year or decadal period.

However, the year-to-year variability in ocean and atmosphere patterns is still very large and obviously controls the level of fire activity in any given year; and the NPM appears to be closely connected to the key weather factors that influence Alaska fire activity.  A lot more research could be done on this, and it seems likely that useful seasonal fire predictions might be possible.  If anyone is interested in collaboration, leave a comment!