Warmest and Wettest in Winter

Today's post is mostly to share some new results pertaining to the Christmas storm, but first a note on recent cold in northwestern Alaska.

Kotzebue reached -40°F or lower on three consecutive days from Tuesday through Thursday last week, with -44°F on the first two nights.  This might not seem too remarkable, but it's the coldest there since February 2012; Kotzebue has had nine straight winters without reaching -40°, and two of them (2013-14 and 2015-16) didn't even reach -30°F.  Prior to 2000, -40° was reached in more than half of all winters.

The recent cold is a brief reprieve from the extraordinary warming that has affected northwestern Alaska in the last decade, particularly in winter.  The change is documented in last year's paper by David Swanson, Pam Sousanes and Ken Hill, as discussed in this post:

https://ak-wx.blogspot.com/2021/03/permafrost-study.html

And now for some more follow-up on the extreme conditions that occurred in southwestern and interior Alaska just after Christmas.  As a direct result of the extraordinarily powerful ridge to the south of Alaska, incredible warmth and moisture were pumped up into the state.  Rick Thoman highlighted some of the extremes that were observed in the balloon sounding data, such as a freezing level of 9600 feet at Bethel and extremely high precipitable water (i.e. total column water vapor) at Fairbanks.  Many records were broken for the time of year; but how widespread was the record warmth and moisture?

To answer this, I used ERA5 reanalysis data back to 1950 and calculated the area over which the December storm brought the highest 1000-500mb thickness on record for deep winter (December-February).  The 1000-500mb thickness measures the average temperature of the bottom half of the atmosphere, and is an excellent indication of unusual warmth or cold over that deep layer.  Here's the result of the calculation for December 26, when record warmth was most widespread:

The darkest red indicates that the maximum 1000-500mb thickness on December 26 exceeded the peak value observed in December through February prior to this event (by "observed", I mean estimated by the reanalysis, which is very reliable for things like this).  The medium red shading indicates the highest thickness since at least 1979, and the light red shows where the thickness was the highest since at least 2000.

By this objective measure, then, the post-Christmas air mass that reached southwestern Alaska was the warmest on record (1950-present) for the deep winter months of December-February.  As a check on this result, I looked at the sounding data from Cold Bay (data since 1946), and indeed the 1000-500mb thickness on December 26 was well above the previous winter record from, coincidentally, Christmas Eve 1989.  Record warmth did not make it far into the interior, however.  For example, the highest Dec-Feb thickness at Fairbanks still stands, from January 26, 2014.

Here's the sequence of maps showing the extent of record high 1000-500mb thickness by day.

How about deep-layer moisture, i.e. precipitable water?  I did the same analysis using ERA5.  Again the most dramatic extent of record conditions was on December 26, and here records certainly were broken in the interior:

Looking again at Fairbanks sounding data, the December-February precipitable water record was broken by nearly 10%, with the previous record being on February 3, 1982.  Here's the daily sequence of record extent:

It would be interesting to take a look at other reanalysis products that extend farther back in time, including for example January 1937, although of course the uncertainty associated with those earlier estimates is very large indeed because of the lack of upper-air observations.

If readers have suggestions for any other ways to slice and dice the ERA5 data to analyze the recent extremes, I'm all ears.

And by the way, it's worth noting that the extreme cold that developed over the eastern interior at the beginning of this month did not set any records for lowest 1000-500mb thickness; it wasn't even the coldest since 2000, according to ERA5.  The cold outbreak did, however, bring the coldest 850mb temperatures since February 1999 in some areas:

Fire, Lightning, and Humidity

Following up on my June 26 post about surprisingly low fire acreage so far this season, I took up reader Gary's suggestion to look at humidity data from Fairbanks.  The idea is that higher humidity this year may have produced "wet" thunderstorms that deliver significant rainfall, rather than "dry" thunderstorms that more efficiently spark fires.

But first, a correction: I realized the chart I showed last time was incorrect, because I had some missing dates; so it turns out that cumulative lightning activity statewide this year has actually been lower than normal.  It was only about 25% below normal on June 20, but since then there has been little lightning despite some notable warmth.  Here's the corrected chart - my apologies:

This greatly reduces the mystery of low fire acreage this year, but nevertheless it's interesting to see that humidity was indeed higher than normal in June when we look above the surface to the 700mb pressure level (about 10,000') in the Fairbanks sounding data.  In the following chart, the red and purple lines show the 700mb relative humidity and dewpoint respectively, and both were well above normal in June.  Higher humidity aloft helps thunderstorms gain strength and produce more rainfall, because it reduces the "entrainment" of dry air that tends to mix away the moist updrafts.

Surface-level humidity, on the other hand, was somewhat below normal in comparison to recent decades.  Interestingly, there's little historical relationship between June fire acreage and humidity at the surface, perhaps because dry conditions mean fewer thunderstorms and fewer fire starts.  However, June fire acreage is clearly related to 700mb humidity; compare the two charts below.

In conclusion, low lightning activity is clearly a major reason for the low fire acreage so far this year (still well under 100,000 acres statewide), but above-normal humidity in the middle atmosphere may also have played a role by enhancing rainfall and minimizing so-called dry lightning.

Summer Humidity

Last week reader Carl asked about humidity this summer in Fairbanks and suggested correctly that dry days were unusually absent.  This is an interesting point, so I created a chart to illustrate my earlier brief response: that the summer's lowest daily-mean humidity was higher than in any other summer in recent decades.  This is true both for relative humidity and dewpoint; there were only 3 days with (midnight-to-midnight) average relative humidity below 50%, and only 3 days with average dewpoint below 40°F.

 

Here's a more focused visualization of how this summer's relative humidity differed from the typical distribution of the past 20 years (click to enlarge).

 

As Carl noted, the lack of very dry days was notable, and correspondingly the number of days with 70-80% relative humidity was higher than normal.  Of course there is nearly always a marked increase in relative humidity as summer advances in Fairbanks, so the driest days are nearly all confined to June and early July, but nevertheless the chart gives a broad sense of the unusual shortfall in dry days this year.

A rather obvious point to make is that low humidity strongly favors growth of wildfires, and so the absence of dry conditions this year is presumably one of the main reasons for the very low fire acreage.  Here's a superb fire acreage chart that Rick Thoman posted the other day.

Finally, while relative humidity was clearly well above normal this summer, dewpoint and atmospheric column-total moisture (precipitable water) were not as high as in some recent years.  Here's an update on a chart I showed back in 2017, when July humidity was extraordinarily high.  The upward linear trends since 1980 are statistically significant (p=0.05) for dewpoint in all three summer months, but interestingly not for precipitable water.

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:

https://ak-wx.blogspot.com/2016/06/thunderstorm-season-approaching.html

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

Warm and Humid on the North Slope

After a slow start to the warm season, much higher temperatures have reached the North Slope in the past couple of weeks.  On Tuesday the airport at Deadhorse saw a high temperature of 80°F, making this the fourth consecutive summer that 80°F has been reached or exceeded there.  Of course the all-time heat record of 85°F was set in 2016, as discussed here.  But prior to 2015, the site went 13 years without getting out of the 70s.

It's also worth noting that the humidity has been very high, relative to normal, in recent days.  According to the ASOS instrument at the Deadhorse airport, the dewpoint reached 61°F on Sunday evening, and the daily mean dewpoint was almost the highest on record - only a couple of days in 2004 were higher.  The top-quality 5-minute data from the nearby CRN site tell the same story, with a peak dewpoint of 62°F on Sunday evening, and Sunday was the most humid (highest dewpoint) of any day in the short history of the CRN site at Deadhorse.

For context, the highest dewpoint so far in Fairbanks this summer is 58°F, and only two of Alaska's 21 CRN sites have seen a higher dewpoint than Deadhorse this summer (i.e. Nowitna NWR and Selawik Refuge).

Extent of Humid July

In the previous post a question arose as to whether the very high humidity observations from Fairbanks airport in July were accurate or if perhaps the sensor might be malfunctioning to some extent.  To shed a bit more light on this, I looked at other sites around the interior and also - thanks to a suggestion from Rick T. - looked at the surface dewpoint reported twice per day on the Fairbanks upper-air sounding.

The sounding data clearly support the record July humidity reported by the ASOS instrument - see the chart below.  Last year saw the most humid July on record at the Fairbanks upper-air site (adjacent to the airport), and this July was even more humid.

Looking at hourly data from Fort Wainwright and Eielson AFB, the same thing is observed; both of these sites also saw a record high monthly mean dewpoint.  This is a record for any calendar month, not just for July; and at all three sites the previous record was in July 2016.

I also pulled out the historical data for several other sites across the interior to see how far afield the record moist airmass extended.  The charts below show the July mean dewpoint for 12 sites divided broadly into eastern and western groupings, with the record high values indicated by markers.  Among the "eastern" sites, Nenana and Northway observed record humidity in July 2017, although the 1973-74 data from Delta Junction and the 1962 record from Fort Yukon look highly suspect, and if we take these out then the record also occurred in 2017 at these sites.

Farther to the west, July 1998 was the most humid July (and calendar month) on record at Galena, Indian Mountain, and Bettles, and July 2004 was the most humid month at McGrath, but a new record was set last month at Minchumina and Tanana.

In conclusion, the data suggest that record humidity occurred last month at least throughout the Tanana River valley, as new calendar month record dewpoints were observed at every site I looked at from Northway down to Tanana (assuming the 1974 Delta Junction data is wrong). 

The July 500mb and MSLP patterns (see below) do not show an amplified pattern over Alaska, but the modest upper-level ridge over northern and western Alaska was persistent and prevented cool, dry air from reaching the interior from the north.  Daily minimum temperatures were almost entirely above normal in Fairbanks as shown in the chart below.

 

It seems to have been the stagnant weather pattern, more than anything, that allowed humidity to pool over the Tanana River valley, although the widespread above-normal sea surface temperatures (and therefore enhanced evaporation) surrounding Alaska presumably played a role.  Here's a map of recent SST anomalies, as shown by Rick in his recent climate briefing.

Summer Humidity

As a follow-up to recent posts on summer temperatures, let's take a quick look at summer humidity trends in Fairbanks.  This topic is timely, as Rick drew attention yesterday to a very remarkable statistic: the number of hours this July with a dewpoint of 55°F or greater in Fairbanks is higher than last year, despite the month not being over yet - and last year the number was apparently far higher than any other year in recent decades.  Here's a chart from Iowa State University.

When I first saw this I really thought there had to be a mistake somewhere, but this is what the hourly observations from the airport have recorded.  There does seem to be a chance that the ASOS sensor is malfunctioning to some extent, as the Eielson ASOS data (see below) show similarly high humidity in a number of earlier years; but there's little doubt that this month has been much more humid than normal.

For a longer term look at each of the summer months, the chart below shows monthly mean surface dewpoint and column precipitable water.  The precipitable water is the total moisture in the atmosphere above a given location and is expressed as the depth of liquid that would result if all the moisture were condensed, i.e. the amount of moisture that is theoretically "precipitable".

The long-term upward linear trends in July dewpoint and precipitable water are highly statistically significant, although for precipitable water there has been little increase since about 1980.  June has also seen increases, but less pronounced, and the long-term changes in August have been quite small.

As noted in the previous post, higher humidity provides an obvious (although perhaps not sufficient) explanation for higher daily minimum temperatures during summer in Fairbanks, because water vapor is a powerful "greenhouse" gas.  See this previous post for more discussion on the topic.

Humidity Trends Aloft

At various times on this blog we've discussed the long-term increase in summer daily minimum temperatures in Fairbanks, for example in these posts:

http://ak-wx.blogspot.com/2013/08/climatology-of-warm-summer-nights.html
http://ak-wx.blogspot.com/2016/08/warm-nights.html

The trend towards later first freeze dates at the airport has also been a repeated topic of discussion (just search for "first freeze").  Spurred by a comment from reader Gary, I thought it would useful to look again at summer humidity trends as a possible part of the explanation.  Back in July I looked at dewpoint measurements at the airport and showed that humidity has increased at the surface in June and July.  But what about aloft?  The chart below shows the monthly linear (least-squares regression) trends for temperature and dewpoint at 850mb, using only the 3am balloon soundings as I'm interested in overnight minimum temperatures.  I began the analysis in 1958 because prior to the International Geophysical Year the soundings were released at 6am, and I wanted to keep the observation time consistent; and also I'm a little less confident of the early humidity sensors.

An interesting aspect of this chart is the absence of warming in November followed by the pronounced warming trend in December, but that's a topic for another day.  The humidity trends reveal an increase in dewpoint above Fairbanks throughout the warm season, and this is consistent with higher minimum temperatures regardless of the temperature trends.  Water vapor is a highly effective absorber and emitter of longwave infrared radiation, so increased humidity leads to greater downward radiation and therefore reduces the rate of net energy loss when the sun is low in the sky or below the horizon.  Anyone who has experienced high humidity in the tropics can attest to this effect: when humidity is very high, the surface temperature may drop only a few degrees on a perfectly clear and calm night over land, because the surface is bathed in infrared radiation from aloft throughout the night.

It's also interesting to see that the dewpoint trend is higher than the temperature trend in June through September, and especially in July and August, and this means that the relative humidity has increased over time; see the chart below.  All else being equal, one would expect higher relative humidity to correspond to greater cloudiness, and of course the presence of clouds is a strong warming influence on daily minimum temperatures.

The chart below shows the contrasting trends in 850mb relative humidity in February and August.  Note that the drying trend in February isn't related to decreasing dewpoint but rather to increasing temperatures, as shown in the first chart above.  Also I'd caution that these trends have been observed over a relatively short period - we know that there were major climate anomalies in the 1930s in Alaska, for instance, so the trends would no doubt look different if we had good upper-air data from earlier years.

Finally it's interesting to take a quick look at the relationship of 850mb dewpoint to the date of first freeze in Fairbanks.  By trial and error I found that the mean dewpoint in the period August 27 - September 8 is most closely related to the first freeze date, with a modest correlation - see the chart below.  Interestingly the correlation is slightly lower for temperature at 850mb; the dewpoint aloft explains a bit more of the variance in freeze dates than the temperature aloft.  As we would expect, when it's dry aloft, frost tends to come early, but when it's humid, the first freeze is delayed.

High Humidity

It has been humid recently in Fairbanks - much more so than usual.  In the first 10 days of the month, the average dewpoint measured at the airport was 55.2°F, which is the highest since at least 1950 for this period of the year.  The dewpoint exceeded 60°F on each of the first 4 days of the month, and it appears Fairbanks has only seen 4 such consecutive days on a handful of occasions in the past (1962, 1990, 1994, 2004), and never earlier than the end of July.  Peak humidity in Fairbanks (in terms of dewpoint) is normally at the end of July.

The humid conditions at the surface are consistent with the presence of a very moist air mass aloft too; the Fairbanks soundings show that the July 1-10 dewpoint was the 3rd highest on record at both 850mb and 700mb.  I think the depth of the moist anomaly suggests that the moisture has been imported to interior Alaska, and the high humidity doesn't just reflect (for example) local evaporation from moist ground.

Is there a long-term trend towards higher humidity in Fairbanks in summer?  Yes, but only slightly.  The charts below show monthly mean temperature and dewpoint since 1950 for June, July, and August.

The 1950-2015 rates of change according to linear trend lines are as follows (although the changes are not linear):

	Temperature	Dewpoint
June	+0.39°F/decade	+0.25°F/decade
July	+0.35°F/decade	+0.13°F/decade
August	+0.17°F/decade	-0.02°F/decade

In each month the temperature has risen more than the dewpoint, which means that the relative humidity has fallen as the climate has warmed.  To maintain the same evapotranspiration rates from vegetation, the dewpoint would have to increase faster than the temperature, so despite the slowly rising dewpoint, moisture demands have become greater for vegetation (at least at the Fairbanks airport site).