Thursday, February 27, 2020

Solar Power in Alaska

Last week an article about solar farms in Alaska caught my eye:

The discussion indicates that despite the obvious shortcomings of solar power generation in a place that receives so little sunshine during the season of peak energy demand, the cost of solar panels has come down enough to make it worthwhile to install solar farms anyway.

The BBC article also states that, "perhaps surprisingly, Alaska is a sunny place", and cites a 2015 piece by Brian Brettschneider to back up this claim.  However, the BBC author unfortunately made an unwarranted jump from daylight to sunshine; while it's true that year-round daylight totals are actually greater in Alaska than points farther south, the total amount of solar energy is nowhere near as great.

This distinction raises an interesting question, however: do the long daylight hours and relatively clear skies of early summer produce enough solar energy to be competitive with locations in the Lower 48?  To take a quick look at this, I used data from the CRN sites that have been running since 2002 near Fairbanks and since 2010 on the Kenai Peninsula, and I compared total solar energy in May through July to 3 sites in the central and eastern U.S.

First, here's the average rate of solar energy input available at the surface, based on the full period of record at each site (15-17 years except for 9 years at Kenai).  The two Alaska sites receive less energy despite having much longer daylight hours, but the difference is not huge, and this reflects the point of the BBC article - that there's plenty of solar energy to be harnessed in Alaska for part of the year.  Click to enlarge:

The shortfall in total energy despite longer daylight hours is a function of both cloudiness and solar elevation angle.  The chart below illustrates the cloudiness aspect by showing how much of the clear-sky maximum is received at each location.  On the Kenai Peninsula, most summer days are fairly cloudy, and even in the relatively sunny interior, 6 out of 10 days have enough cloud to keep solar radiation at less than 70% of maximum.  In contrast, 6 out of 10 days have more than 70% of maximum in my neck of the woods (Georgia).

So while solar power does have fairly good seasonal potential in Alaska, even the long daylight hours of summer are not enough to make it fully competitive with the rest of the U.S.

Friday, February 21, 2020

Yukon Frozen at Dawson

Long-time readers will recall that in the past few winters I've drawn attention to the interesting failure of the Yukon River to freeze over properly at Dawson in the Yukon Territory.  For the past three years the Yukon government has been unable to construct an ice bridge across the river to West Dawson, and local residents have resorted to alternative routes to make the crossing.  Here are some posts from previous years:

Happily, this winter is a return to normal, as the government-sanctioned ice bridge was open by Christmas and was available for heavy traffic a month ago.

One could be forgiven for thinking that colder weather this winter is the reason for the more normal freeze-up, but in fact it wasn't particularly cold at all during the freeze-up period.  The average temperature in November and December was -2°F, compared to -5°F and -7°F in 2016 and 2017, respectively; the 1995-2015 average was -5.1°F for these months.  Nor was there a pronounced cold spell; only 6 days before the turn of the year dropped to -30°F, compared to a 1995-2015 median of 10 such days.

The accumulation of freezing degree days shows the same thing: this winter was apparently no more favorable for freeze-up in terms of thermal conditions, although the January cold provided a good boost after freeze-up.  Click to enlarge the chart below.

This simply confirms what we noted in prior years: unusual warmth did not explain the persistent lack of freeze-up in the last 3 years, and unusual cold can't explain the river's return to normal behavior this winter.

As noted before, there are many possibilities for potential causes of the abnormality in recent years, but one new clue comes from a look at warm-season precipitation in the upstream Yukon drainage.  The chart below shows May-September precipitation at four sites for the past 5 years, and clearly last summer was drier than any of the preceding 4 years.

The 1981-2010 normal for Mayo and Dawson is about 195mm for May-September precipitation, so last summer's deficit was fairly substantial.  (Whitehorse was close to its normal of 155mm, and I don't have a normal value for Carmacks.)  This is a tiny sample across a very large area, of course, but the river level data from Dawson support the idea of reduced flow late last year.  Here are the September mean level values from the past few years (data obtained here):

2015   2.83m
2016   2.02m
2017   1.56m
2018   1.85m
2019   1.03m

To my mind it makes sense that lower flows would freeze over more easily, but I don't think this is all there is to it; I believe there was an ice bridge in 2015-16, but that year apparently had a very high flow rate going into autumn.  At any rate, it's nice to see something more normal this winter.  Here's the webcam view from a couple of weeks ago.

Saturday, February 15, 2020

Alaska Climate Divisions

Last week a UAF news article highlighted the value of the Alaska climate division analysis that was developed a few years ago by Peter Bieniek and others from UAF and other universities, along with NOAA collaborators such as Rick Thoman.  NOAA has long used so-called climate divisions in the lower 48 to keep track of climate variations in climatically similar regions, but nothing comparable was available for Alaska until this work by Peter et al.

The journal article describing the new climate division work was published way back in 2012, but as the article explains, it took a few years for NOAA to adopt the divisions for "official" monitoring.

I'm a big fan of the Alaska climate divisions, but one of the potential shortcomings is the relative scarcity of ground-truth station data; only 42 sites (including some Canadian) were used to determine 13 climatically similar regions, and some divisions had far more sites than others.  The Northeast Interior division, for example, contains only one station (Fort Yukon), and the North Slope division has only one non-coastal site (Umiat).  Such is the world of historical Alaska climate analysis.

For reference, here are the Alaska climate divisions:

After reading the UAF news piece, I started wondering if modern reanalysis data would produce similar climate divisions to the Bieniek results.  To address this, I used monthly mean temperature data from the ERA5-Land reanalysis, now available from 1981 through most of 2019.  ERA5-Land is a higher-resolution version of ERA5 (9km vs 31km grid spacing) that models only surface variables such as 2m temperature, 10m wind, humidity, snow cover, and so on; it does not deal with oceans or the atmosphere aloft.  I'm hopeful that ERA5-Land may be an improvement over ERA5 for Alaska in winter (see this post from a few weeks ago), although I haven't done any investigation on this yet.

Regardless of the possible deficiencies of ERA5-Land, it's interesting to see what the climate division analysis produces.  I ran cluster analysis on the gridded monthly mean temperature anomalies (standardized) from 1981-2018, and the following maps show the results, ranging from 3 to 10 clusters, based on two alternative methods.  Bieniek et al tested these two methods and a third, but they focused on results from Ward's method (right column below).

K-means methodWard's method

There are a number of interesting aspects to the results.  First, the K-means cluster boundaries tend to jump around somewhat, because the method starts with a random choice each time and iterates to a solution.  For this reason it is also not 100% reproducible, i.e. you can get different results when you run it again.  In contrast, a hierarchical method like Ward's is reproducible, and the boundaries don't move around as the clusters are progressively sub-divided.

Despite the differences in the results, certain features are similar: the North Slope division emerges quickly and remains very well-defined throughout; a Panhandle division emerges at k=6 for both methods; and the clusters are really quite similar for k=5,6,7, and 9.

Perhaps most interesting, in my view, is the absence of some of the distinctions that are found in the Bieniek results.  For example, even if we go all the way up to 15 clusters (see below), there is no sub-division within the Panhandle, whereas Bieniek has three Panhandle divisions and another for the Northeast Gulf.  Similarly, the ERA5-Land clusters give no separation between Aleutians and Northwest Gulf (e.g. Kodiak Island).  As the number of clusters increases, the sub-dividing mostly takes place in the interior and eventually on the North Slope.

On the other hand, the ERA5-Land clusters quickly break apart the West Coast region, rather than keeping it together as Bieniek does.

I mention these differences out of curiosity, not to suggest that the Bieniek divisions are wrong.  It's very likely that ERA5-Land has certain deficiencies that would hamper the assessment of climate similarity - for instance, the reanalysis may be wholly inadequate in the very complex terrain of the Panhandle.  More investigation would be needed to see how well ERA5-Land reproduces climate in the vicinity of the stations used by Bieniek et al.

Lastly, it's not clear to me whether there is an optimal number of clusters based on the ERA5-Land analysis.  Traditionally one looks at the distribution of within-cluster variance and seeks to find a threshold beyond which (i.e. for smaller numbers of clusters) the variance starts to increase more quickly; but the results from ERA5-Land show no obvious stopping point.  Bieniek also found that using gridded data made it impossible to tell where to stop.

Personally I like the look of the K-means solution with 9 divisions, but it's purely a personal preference.  I'd be glad to hear any comments from readers.

Saturday, February 8, 2020

North Slope Cold

Just a quick update this evening to note an episode of extremely low wind chill on the North Slope owing to frigid temperatures combined with breezy conditions.  This is a combination that would be almost unheard of in Alaska's interior, where severe cold (say -50°F or below) develops only when winds are calm.

The lowest wind chill value I've seen was at Nuiqsut, just south of the Colville River delta: yesterday evening it was -52°F with a 10 knot sustained wind, which is good for a -82°F wind chill index.  This morning was even colder (as low as -55°F) but with a slightly lighter breeze.  The only colder time in the 20-year history of data from Nuiqsut was in early 2012, when the air temperature reached -62°F (Jan 24) and the wind chill touched -86°F (Jan 31).

Deadhorse also saw its second worst wind chill episode last night, reaching -80°F (rounded) with an air temperature of -47°F.  Only late January 2012 was colder  (-51°F, wind chill -85°F).

Other sites with wind chills in the same vicinity were Kuparuk (-80°F) and the notorious (but well inland) cold spot of Umiat (air temperature -55°F, wind chill -79°F).

It's interesting to note that Deadhorse has seen an average wind chill of -47°F so far this year, and this is second only to 1989 (-48°F); mostly complete hourly data extends back to the early 1980s.  Of course January 1989 was an extremely cold month in Alaska, and like this year, 1989 also saw a very strong polar vortex with lower than normal MSLP in much of the Arctic basin (strongly positive AO phase). 

Compare the MSLP maps for January 1989 and 2020 below.  While they're not identical, the north-south pressure gradient across Alaska is very similar.  Of course below-normal MSLP in the high Arctic would tend to favor stronger than normal offshore (and therefore cold) winds along Alaska's north coast, and so it makes sense that wind chills were also severe in 1989.

Thursday, February 6, 2020

North Pacific Blog Post

With the colder weather for northwestern North America this winter, there have been accompanying changes in North Pacific temperature patterns.  I posted a few comments here: