Friday, June 26, 2020

June Rains

Remarkably wet weather has occurred over the eastern interior in the past week, leading to river flooding and unusually cool temperatures.  Fairbanks recorded 2.41" of rain for the week ending Wednesday, which equals the wettest June week on record (back in 1977).  Only about 10% of Junes see this much rain in total for the whole month in Fairbanks; this isn't supposed to be rainy season in interior Alaska.

Here's the Tanana River gauge at Fairbanks.

As for temperatures, the high temperature was only 53°F on Monday, and we have to go back to 2009 to find such a chilly day in the second half of June.  Besides the rain itself, the lack of sunshine was obviously a major reason for the chill; the chart below shows daily solar radiation at the CRN site near Fairbanks.

Remarkably, the 3-day solar radiation total for June 21-23 was the lowest on record (2002-present) between late March and early August.  In other words, in the last 18 years no 3-day period in April, May, June, or July has been as dull and cloudy as Sunday-Tuesday of this week; and this is all the more remarkable as it occurred right at the solstice.

Up in the hills, the Munson Ridge SNOTEL site (3100') reported 5.0" of rain in the past week, taking the year-to-date total to a remarkable 16.5".  This is far and away the highest on record (1982-present) for the year-to-date, with the previous wettest being 11.6" in 1994.  (However, it's not uncommon for big rains to arrive in July at Munson Ridge, with several years reaching about 15" year-to-date by mid-July.  In 2016, an incredible 14.9" fell in July alone.)

The 500mb analysis from Monday afternoon gives a sense of the interesting and unusual pattern that gave rise to the persistent cloud and rain: a trough extending from the Gulf of Alaska to NW Canada, and a strong upper-level low to the north of Alaska.  I haven't looked into the details, but it seems this set-up produced a persistent frontal zone with low-level convergence and large-scale ascent of air over eastern Alaska.  More typically Fairbanks sees rainy weather in summer when there's a strong westerly flow and a ridge to the south; this was a very different situation.

Here's a map sequence of estimated precipitation totals day by day.

Tuesday, June 23, 2020

Winter Cold - Part 4

Back in January through March I penned three posts about the surprising cold in Alaska last winter, trying to identify a key cause for the dramatic change from the warmth of recent years (see here, here, and here).  In my view the discussion wasn't very satisfying in the end, as it failed to pinpoint an obvious driver for the winter pattern.  However, I finished up the last post with a promise to explore in more detail the close similarity with the winter of 1989-1990, so that's what I'll try to do here.

First let's remind ourselves of the broad pattern similarity between the two winters.  Here are the 500mb height and temperature anomaly maps for the two winters (December-February); notice the strong west-east ridge across the central North Pacific as well as the cold trough over Alaska and the strong similarity in the northeastern Pacific region (e.g. warm in British Columbia).  Click to enlarge the maps.

I also mentioned last time that the similarity extended to the temporal evolution of the high-latitude circulation pattern, with the Arctic Oscillation becoming increasingly positive in both winters.  This is illustrated in the chart below, which I find quite remarkable: the AO index evolved in strikingly similar ways at times through the two winters.  For instance, both winters saw increasingly positive peaks of the AO, and even with similar timing: around December 1 and early January, and then a pair of enormous peaks during February.

A daily AO index above +6 is a rare extreme, and outside these two winters only one other date since 1950 has achieved this feat: January 14, 1989, which was right at the onset of the infamous Alaska cold snap.  Coincidence?  Not a chance.

An obvious question therefore is, was there a common mechanism that was driving the AO towards a more amplified positive state as these winters progressed?  In considering this possibility, the first place to look is the tropics, where we might hypothesize that a persistent pattern of rainfall anomalies provided a favorable configuration to strengthen the circumpolar flow in the high latitudes.  This could occur either by a pathway that directly strengthens the westerly momentum at higher latitudes, or by an unusual reduction in disturbances to the normal wintertime polar vortex.  In atmospheric dynamics, the tropics tend to drive the extratropics more than the other way around - for example, the dominant El Niño/La Niña oscillation has very predictable effects on higher latitude circulation patterns.

In Part 2 of the discussion I looked at the tropical rainfall patterns from last winter and failed to find a similarity to historical patterns associated with cold in Alaska.  However, that analysis was a broad overview, and 1989-1990 was mentioned as a relatively close match, so let's take a closer look at the tropical features of the two winters.

First, the sea surface temperature maps (see below) show a strong similarity with respect to a region of anomalous warmth centered near 0°N 180°E.  (Note that I've plotted both maps in terms of the departure from the immediately preceding 30-year mean to put them on equal footing in regard to climate trends.)  Equatorial warmth near the Date Line corresponds approximately to the Niño4 region, which is the westernmost of the boxes that are used to measure ENSO SST anomalies (see here for explanation).  We can therefore describe the warmth as a kind of El Niño warming, but it's a "Central Pacific" or "Modoki" El Niño anomaly.  A traditional El Niño has its equatorial warmth focused much farther east, closer to South America.

Looking at rainfall estimates from the state-of-the-art ERA5 reanalysis gives an indication of how the tropical circulation was displaced relative to normal.

Both winters saw enhanced rainfall near the equator at about 170°E (on the west side of the warm SST region), and both years also had dry conditions to the east of the Date Line around 0-5°N, and wet conditions north of this and extending northward roughly to Hawaii.  Another key similarity is the zone of dryness north of the equator from about 80°E-140°E, reflecting a large region of unusual subsidence (sinking air and dry weather) from Sri Lanka to the Philippines.  (These maps are showing absolute rainfall anomalies rather than the more usual "percent of normal", because it's the absolute magnitude of the tropical heating anomalies - caused by excess condensation in deep rain clouds - that we're interested in.)

Finally, the lower atmospheric wind patterns tie together the SST and rainfall anomalies, and really illustrate again how similar the two winters were in the Pacific basin.  The maps below show the average 850mb winds (speed and direction) at left, and the maps on the right show the departure from normal of the westerly wind speed.

The pattern of west-east wind anomalies is remarkably similar from the tropical region spanning the eastern Indian Ocean to the western Pacific Ocean, and all the way up to the central and northern North Pacific.  Focusing on the equator, notice the change from easterly wind anomalies over Indonesia to westerly anomalies over the western Pacific; this implies a very similar focus of subsidence and lack of rainfall near 130-140°E.  Farther east, the westerly wind anomaly extending to near the Date Line is consistent with high SSTs, because the usual easterly trade winds (a cooling influence) were weakened or even reversed in this area.

Putting it all together at last, my interpretation is that the pronounced realignment of tropical rainfall patterns was closely linked to the intensification of the mid-latitude ridge across the North Pacific and, therefore, the positive AO phase.  A first and simple explanation is that enhanced rainfall near the Date Line strengthened the Hadley circulation over the central Pacific, with significantly more mass being displaced poleward at high elevation and then descending to reinforce the subtropical ridge to the north (roughly north of Hawaii).  A stronger subtropical ridge is a key aspect of the positive AO phase, as it implies a stronger pressure gradient northward to the Arctic, and therefore a stronger mid-latitude jet stream.

A second and slightly less straightforward aspect involves the fact that rain clouds over Indonesia and the far western Pacific are usually a strong source of wave disturbances that travel north (and south) into the extratropics and affect the high-latitude circulation.  When these disturbances are frequent and strong, they tend to weaken the polar vortex and produce a less positive AO phase; but last winter these wave disturbances were weaker than usual, and this contributed to the polar vortex becoming very strong and the AO becoming increasingly positive.  And so the reduction in rainfall near the Maritime Continent seems to be a key piece of the puzzle, just as much as the enhanced tropical rainfall farther east.

If you've made it this far, congratulations - this turned into a slightly technical discussion.  Understanding the remote influences and connections in global climate is an enormous challenge, but the remarkable convergence of events across the North Pacific and Arctic in the two winters of 1989-1990 and 2019-2020 provides a clear illustration, I think, that these connections are real and significant.

Tuesday, June 16, 2020

Strong Storms

After commenting on Friday that lightning activity has been relatively subdued from Fairbanks east so far this season, strong thunderstorms have been popping up in the hills around Fairbanks since the weekend.  The NWS in Fairbanks has issued special statements each day since Saturday to advertise the risk of heavy rain, small hail, and frequent lightning, and as Rick Thoman noted, Fairbanks airport has already reported thunder on 7 days so far this season.  This is tied with 2018 for most on record for so early in the season; the record for an entire season appears to be 1992, with 17 days of thunder.

The most notable storm for some local residents occurred on Saturday afternoon, when intense convective cells moved westward across Eielson AFB just before 5pm.  Here's the radar appearance of the storms at 15-minute intervals on approach to Eielson.

The Eielson ASOS reported 0.78" of rain, including 0.58" in 14 minutes, which is a healthy 2.5"/hour rainfall rate.  The return interval for experiencing such a high 15-minute rainfall amount at Eielson is greater than 50 years, so this was a storm of rare intensity in terms of rain rate.  However, other aspects were not too remarkable: the peak wind gust was 43 mph from the northeast, and the temperature dropped from 75°F to 54°F.  It appears no hail was reported at the airport, which is a bit surprising in view of the intense radar
signature of the storm as it passed over the base.

Fairbanks NWS did issue a severe thunderstorm warning for the storm, and this marks only the third severe thunderstorm warning for the Eielson ASOS location since 2005.  The first was on June 18, 2007, and the second was on June 19, 2017; it's interesting to see these dates falling so close together on the calendar.

Here's an animation showing the evolution of the storm system over a longer period.

Another example of a strong storm occurred the previous day (Friday); this one formed just to the northeast of Fairbanks and dropped south across Chena Hot Springs Road at about 3pm.

It's fascinating (to me) to reflect on the extreme opposites of atmospheric stability that occur in Fairbanks-land at opposite ends of the year.  Six months ago, with effectively zero solar heating, the valley-level atmosphere was profoundly stable, with a near-permanent surface-based temperature inversion.  Now, with daily solar radiation often rivaling locations much, much farther to the south, episodes of intense instability allow these strong thunderstorms to develop.  The atmospheric physics and the sensible weather could not be more different.

Friday, June 12, 2020

Southwest Lightning

Last week I mentioned the beginning of thunderstorm season for interior Alaska, and I noted the unusual concentration of lightning activity over southwestern areas in the first few days of widespread activity.  Rick Thoman put out the following comment today on Twitter:

Lightning data confirms that the situation is very unusual; through yesterday, the Alaska Lightning Detection Network recorded 6942 lightning strikes to the west of 160°W, which is about the longitude of the Yukon River near Grayling and Anvik.  For this area, this is twice as much lightning as in any other year through the same date (with comparable data back to 2012), and only 2014 had more lightning west of 160°W in the entire season.

The maps below compare this year's lightning distribution with that of 2012-2018.  Click to enlarge.  There has been relatively little lightning to the east of Fairbanks so far in 2020: only 4357 strikes to the east of 148°W, which is much below normal.

What's the explanation?  Nearly all of the lightning in the far west occurred on three days: May 31, and June 2-3.  This was related to a very unusual episode of easterly flow that transported warm and unstable air to the west coast, allowing the far west to get a taste of typical interior summer weather.

Here's a sequence of 500mb maps that shows the prolonged easterly flow that developed between low pressure to the south and a strong ridge to the north.  Bethel reached 76°F on May 30, tying the record high for the date.

3pm AKST May 29

3pm AKST May 30

3pm AKST May 31

3pm AKST June 1

3pm AKST June 2

Sunday, June 7, 2020

Summer Frost

Despite continuous daylight, temperatures have dropped to freezing or below in some of the sheltered cold spots of the eastern interior in the past few days.  Examples include the Salcha RAWS site (29°F this morning), the Eagle airport (28-32°F the last three nights), and Chicken (27°F on Friday and Saturday mornings).

This isn't too unusual during the first half of June, and indeed more severe freezes have occurred even in recent years.  In 2017, Salcha saw 23°F on June 14; in 2006, Chicken dropped to a remarkable 15°F on June 5 (and Eagle hit 20°F).

The graphics below show freeze events in May through September for Eagle airport and Chicken.  Note that the Eagle airport reliably sees much colder minimum temperatures than the co-op site in town at this time of year; the co-op site only reached 36°F on Friday.  Click to enlarge the figures.

It's interesting to see the absence of any hard freezes (28°F or lower) in Eagle from mid-June to mid-August in the past 10 years, and similarly Chicken has not seen a hard freeze in the height of summer for 5 years.  This is consistent with the background warming trend and the amplified warm anomalies that have mostly prevailed in the past several years.

Higher humidity has also probably contributed to the lack of frost, because water vapor in the atmosphere is extremely efficient at reducing net radiation loss from the surface when the sun is near or below the horizon.  All else being equal, daily minimum temperatures will be higher when humidity aloft is higher, and this is a strong effect.

Nevertheless, Chicken has still managed to see a few marginal freezes in the height of summer in recent years, so it's not quite time to declare the existence of a safe growing season in this notorious cold spot.

Wednesday, June 3, 2020

Lightning Season

Thunderstorm activity has sprung to life in parts of Alaska in the past several days, but somewhat unusually the lightning has been focused in the southwestern interior and near (and over) the Norton Sound.  Historically only about a quarter of Alaska's lightning occurs west of Tanana.  Here are maps of lightning strikes from the past few days (click to enlarge):

Saturday May 30

Sunday May 31

 Monday June 1

Tuesday June 2

The onset of lightning season is right on schedule, as the historical data show a very rapid increase in Alaska's lightning activity at the beginning of June.  See this 2016 post for previous comments on this:

The outbreak of thunderstorms also signals that humidity has risen to a level high enough to support widespread deep convection (atmospheric thermal overturning); moisture is a key ingredient for thunderstorms in general.  To explore this idea in more detail, I looked at the historical data to see if there is a close link between rising humidity levels and the first widespread lightning activity in Alaska.

To measure humidity, I used the average of daily mean dewpoint at McGrath, Fairbanks, and Eagle, and I smoothed the daily values over 3 days to remove some of the daily noise.  Using data from 2000-2012, I then compared the peak year-to-date dewpoint with the peak year-to-date daily number of lightning strikes; the hypothesis is that there might be a threshold value for dewpoint that, once reached, allows thunderstorms to suddenly become widespread.

Results are shown below.  If the chart is a little confusing, consider the black line (the year 2000): the data show that there was essentially no lightning until the 3-station dewpoint index reached 40°F, but then 6600 lightning strikes were observed in one day when the dewpoint index first exceeded 42°F.  Of course there's no time dimension in the chart, and it doesn't deal with multi-day lightning totals, but I think it's quite illuminating nonetheless.

The results indicate that widespread lightning (more than 2000 strikes per day) tends to be uncommon until the dewpoint exceeds 40°F, but it's also rare to avoid widespread lightning when the dewpoint index reaches 45°F.  This suggests there is a fairly narrow range of humidity that produces the first outbreak of widespread thunderstorms in Alaska; but I'll admit this could be partly a case of correlation without causation, as it may be that humidity and thunderstorms increase simultaneously without one (solely) causing the other.

As for this year, we're right on track once again; here is the 3-station dewpoint index from the past several days.

May 29   38.6°F
May 30   37.9°F
May 31   39.1°F
June 1   42.0°F
June 2   44.4°F