Friday, July 31, 2020

Copious Rain and A Permafrost Study

I wasn't expecting to return to the topic of summer rain quite so soon, but for two reasons it makes sense to do so.  First, the current weather forecast from NWS Fairbanks is worthy of comment, with a flood watch in effect over the weekend for the central interior, middle Tanana valley, and Denali region.  The rainfall forecast for the next 3 days is striking to say the least; any time you have 15% or more of your normal annual precipitation predicted for a weekend, it's worth paying attention.


The special statement issued by the NWS includes quite a collection of hazards:

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE FAIRBANKS AK
508 AM AKDT FRI JUL 31 2020

AKZ220>226-011415-
YUKON FLATS AND SURROUNDING UPLANDS-CENTRAL INTERIOR-
MIDDLE TANANA VALLEY-DELTANA AND TANANA FLATS-
UPPER TANANA VALLEY AND THE FORTYMILE COUNTRY-DENALI-
EASTERN ALASKA RANGE-
INCLUDING FORT YUKON, VENETIE, CENTRAL, CIRCLE, STEVENS VILLAGE, 
BEAVER, CHALKYITSIK, BIRCH CREEK, CIRCLE HOT SPRINGS, 
EAGLE SUMMIT, TWELVEMILE SUMMIT, NENANA, ANDERSON, TANANA, MINTO, 
MANLEY HOT SPRINGS, RAMPART, LAKE MINCHUMINA, LIVENGOOD, 
FAIRBANKS, FORT WAINWRIGHT, EIELSON AFB, ESTER, NORTH POLE, 
MOOSE CREEK, TWO RIVERS, FOX, CHATANIKA, CHENA HOT SPRINGS, 
SOURDOUGH CAMP, SALCHA, DELTA JUNCTION, FORT GREELY, 
HARDING/BIRCH LAKE, DRY CREEK, DOT LAKE, HEALY LAKE, TOK, 
TANACROSS, EAGLE, TETLIN, NORTHWAY, ALCAN, CHICKEN, BOUNDARY, 
HEALY, DENALI NATIONAL PARK, CARLO CREEK, KANTISHNA, 
MENTASTA LAKE, BLACK RAPIDS, DONNELLY DOME, TRIMS DOT CAMP, 
EAGLE TRAIL, AND MINERAL LAKE
508 AM AKDT FRI JUL 31 2020

...COPIOUS RAINFALL LIKELY OVER MOST OF THE EASTERN/CENTRAL 
INTERIOR AND ALASKA RANGE SATURDAY EVENING...

...INCREASING LIKELIHOOD FOR A SIGNIFICANT OUTBREAK OF STRONG 
THUNDERSTORMS OVER THE EASTERN INTERIOR...

STRONG THUNDERSTORMS ARE EXPECTED BEGINNING LATE SATURDAY 
AFTERNOON OVER THE FORTYMILE UPLANDS AND WHITE MOUNTAINS. THESE STORMS
WILL LIKELY PRODUCE DANGEROUS LIGHTNING, LOCALLY TORRENTIAL 
RAINFALL RATES, HAIL, AND STRONG WINDS. EXPECT CONDITIONS TO 
DETERIORATE RAPIDLY FROM THE HOT AND MOSTLY SUNNY AFTERNOON 
CONDITIONS. OUTDOOR RECREATORS, IN PARTICULAR, SHOULD EXERCISE 
SIGNIFICANT CAUTION AND BE WEATHER-READY. 

EXPECT TORRENTIAL RAINFALL RATES WITH UP TO AN INCH PER HOUR OVER
LOCALIZED AREAS. TOTAL RAINFALL WILL RANGE FROM 0.5-2" TOTAL OVER
THE EASTERN INTERIOR UP TO 2-4" OVER THE CENTRAL INTERIOR, WITH 
THE HEAVIEST TOTALS CURRENTLY EXPECTED FROM TANANA TO MINCHUMINA 
TO THE MINTO FLATS. HEAVY RAIN WILL TRANSITION TO LIGHTER RAIN, 
BUT CONTINUE THROUGH THE WEEKEND OVER THE CENTRAL INTERIOR. 

RIVER LEVELS REMAIN ELEVATED WITH MUCH OF THE SURROUNDING TERRAIN
STILL SATURATED FROM WEEKS OF CONTINUOUS WET WEATHER. THIS MEANS 
THERE IS POTENTIAL FOR SHARP RISES ON RIVERS INCLUDING THE 
CHATANIKA, CHENA, LITTLE CHENA, SALCHA, AND GOODPASTER, AS WELL AS
SMALLER CREEKS AND TRIBUTARIES ACROSS THE EASTERN INTERIOR. 

LARGER RIVERS INCLUDING THE TANANA RIVER WILL BEGIN RISING EARLY 
NEXT WEEK AS WATER MOVES THROUGH THE SYSTEM. 

MUDSLIDES AND QUICK RISING STREAMS IN THE ALASKA RANGE, ESPECIALLY
NEAR DENALI, WILL ALSO BE A SIGNIFICANT CONCERN. 

FOR THE LATEST RIVER INFORMATION GO TO WWW.WEATHER.GOV/APRFC. 

FOR THE LATEST UPDATES VISIT WEATHER.GOV/FAIRBANKS


The weather pattern responsible for all this involves a strong upper-level trough over the Gulf of Alaska, with warm and moist flow reaching the interior from the southeast.  This bears some resemblance to the rainy weather setup in late June.  The precise origin of the forcing for this weekend's expected deluge is not quite clear at first glance, but there's no doubt about the magnitude of the rainfall signal in the model forecast - see below.




This latest onslaught of moisture will add to the year's precipitation surplus and will favor increased permafrost thawing this summer.  That's according to the conclusions of a new paper published by Tom Douglas of the Army Cold Regions Lab in Fairbanks:


This work was highlighted by Ned Rozell back in June:


The paper shows that permafrost active layer (seasonal thaw layer) depth increased significantly across a diverse set of locations near Fairbanks from 2013-2017, and there seems to be a correlation between the annual thaw depth and summer rainfall totals, with wetter summers tending to produce more thaw.  This makes sense, as surface water above 10°C carries a substantial amount of sensible heat as it percolates down to the 0°C top of the permafrost; and the effect may be multiplied in areas where thawing permafrost is subsiding, leading to greater liquid water influx from surrounding areas.

One slight surprise with the results (as discussed in the paper) is that the dramatic thaw in the record-wet summer 2014 was not reversed in 2015 or 2017, despite those summers being less wet.  But curiously, the rainfall data in the paper seems to be slightly off: the stated rainfall totals for 2015 and 2017 are quite a bit lower than what actually occurred in Fairbanks, so the "problem" is not as great as the discussion suggests.  As a recent post here showed, every year since 2014 has been considerably wetter than normal in Fairbanks (with the vast majority of the increase coming as rain), so it's no surprise that permafrost hasn't had a chance to recover.  And from the point of view of rainfall, it certainly isn't doing any better this year.

Update Saturday 5pm:

Here's a satellite loop showing the frontal zone developing over the southeastern interior.  Progress is a bit delayed, but big rain is coming.


Today's GFS forecast:



This afternoon's NWS forecast discussion is worth saving for future reference:
Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
222 PM AKDT Sat Aug 1 2020

.SYNOPSIS...
A significant storm system is expected to impact the Southern
Interior and Alaska Range beginning late this afternoon and
continue through late Sunday or Monday morning. A strong front is
expected to stall along a Fairbanks to Denali National Park line
and will produce very heavy rain, perhaps record setting rainfall
in the Alaska Range and Southern Interior. The front has slowed
down and will not arrive during peak heating this afternoon so
the chances for strong thunderstorms in the Central Interior have
diminished some, however chances for strong thunderstorms remains
high in the Eastern Interior this afternoon and evening. Very
heavy rain is expected through monday Morning including the Tanana
Valley where 1 to 3 inches of rain is expected through Monday
morning and in the Alaska Range especially near Denali National
park where 2 to 4 inches of rain is expected with local amounts
pushing nearly 6 inches. The potential for local flash flooding is
a very high across much of the Alaska Range, especially around
the Denali area. Elsewhere, cooler and showery conditions will
persist over the West Coast and Western Interior with continued
fog and stratus over the North Slope.

&&

.DISCUSSION...

Upper Levels and Analysis...
A strong ridge over the Central and Eastern Interior will weaken
and retreat to the north as strong upper level low over the
panhandle pushes northwest into the southeast interior this
evening and continues to push northwest into the Central interior
near Fairbanks tonight. 850 mb temperatures currently around 14
above will drop rapidly to around 6 above by Sunday afternoon.
Rapid cooling aloft combined with diluent flow aloft will produce
a closed low centered near Fairbanks late tonight and Sunday.
This closed low will produce northerly flow at the surface and
aloft over the Central Alaska Range and will enhance rainfall with
moderate upslope flow. The low stretches along an upper level
boundary along a line from Fort Yukon to Fairbanks to Denali
National Park. Very heavy rainfall is expected to fall along this
stalled front tonight through Monday morning. Over the North
Slope, southwest flow and a stationary front remain in place,
keeping showers and clouds and cooler weather there, as well.

Model Discussion...
The models continue to struggle badly on the timing and placement
and amount of heavy rainfall over the Central Interior and
Central Alaska Range even at less than 6 to 12 hours before the
start of the event. We utilized a blend of models again with a
heavier weighting of the ECMWF due to its consistency over the
last several runs on the timing, placement and amount of heavy
rainfall and retained reasonable continuity with previous forecast packages and latest model updates. The heavier weighted ECMWF forecast favors widespread heavy rainfall from the Alaska Range to the Tanana Valley, thus ongoing flood watches will remain in place. Very small differences in feature location will result in significant differences in timing, placement and amounts of heavy rainfall especially with the upslope precip enhancing and downslope precip limiting terrain of the Alaska Range with very subtle changes in wind direction. Central and Eastern Interior... A very nice afternoon with sunny and warm conditions ongoing over the Central and Eastern Interior as the upper level ridging continues to build and temps rise into the 80s. We will likely make a run at the warmest day of the summer this afternoon before a strong front pushes in from the east. The sunny, nice weather will deteriorate rapidly beginning late afternoon over the Eastern Interior and this evening over the Central Interior as a strong front pushes into the area from the southeast. Strong thunderstorms are expected to develop over the Fortymile Uplands, Eastern Alaska Range and White Mountains then move westward late this afternoon and evening. Instability will be very high as CAPE values will exceed 1500-1800 j/kg, a high value in Alaska for instability. Storms will produce may produce torrential rainfall in excess of an inch per hour in some areas, along with hail, deadly and frequent lightning, and strong winds. By late evening and overnight, the storms will merge into a complex and shift west over the Tanana Valley and Western Alaska Range northward into the Central Interior. There remains some uncertainty on the exact timing, location and amounts of the heavy
rainfall but at this point we expect heavy rain rain over the entirety of the Alaska Range with moderate to heavy rainfall over the Tanana Valley by late tonight and Sunday morning. Expect rainfall to continue through Monday morning, at least, with the potential for 1-3" of rain in the Tanana Valley including Fairbanks and 2-4" across the Alaska Range, highest values near Denali possibly reaching 6 inches through Monday morning. Thunderstorms and moderate rainfall will also impact areas north including the southeast Brooks Range and Yukon Flats, although flooding rainfall is not expected. West Coast and Western Interior... Quiet weather will persist out west, especially along the coast as a northerly wind gradient at the surface increases. Some fog and
stratus will remain a problem over the immediate coasts, however, Interior will gradually warm up with threats for isolated thunderstorms each afternoon through Monday. North Slope and Brooks Range... A stalled front continues to sit over the North Slope, bringing light rain and continued clouds. This will weaken, and southerly flow will increase over the eastern half of the Brooks Range and North Slope tonight and Sunday, bringing rain and possibly an isolated thunderstorm over the southern slopes of the Brooks Range. This will last through Monday while the remainder of the North Slope will continue to see cooler and stratus/fog conditions given the light onshore northeast flow. Coastal Hazard Potential Days 3 and 4...None. && .FIRE WEATHER... One final hot day is in store for the Central/Eastern Interior this afternoon before a powerful storm rolls through this evening and overnight. There will likely be a lightning outbreak over the Eastern Interior with widespread lightning, but storms will also bring copious rainfall. Extensive thunderstorms will also impact the drier areas of the Interior including the Yukon Flats. Expect heavy rainfall from the Alaska Range to the Central Interior and Tanana Valley through Monday. Cool abut mostly dry with isolated
thunderstorms will be the main story over the Western Interior. The West Coast will remain cool and dry. && .HYDROLOGY... The focus continues to be an incoming heavy near record setting rain event for parts of the Central and Eastern Interior beginning late this afternoon and into Monday. Models continue to struggle with the placement of the heaviest precipitation in the Interior (see model discussion above), but the emerging concensus is a broad area from Delta Junction to Fairbanks to Lake Minchumina southward into the Alaska Range with 1 to 3 inches of rainfall; the highest bullseye of rainfall of 2 to 4 inches with 6 inches locally remains the Alaska Range near Denali National Park through Sunday. Flood Watches remain in effect for these areas. Expect sharp rises on smaller rivers including the Chena, Little Chena, Salcha, Nenana, Delta, and Goodpaster Rivers with continued rises along the Tanana from Fairbanks west. Rock and mud slides are possible in the steep terrain of the Alaska Range along with the possibility of local flash flooding in torrential rainfall.

Tuesday, July 28, 2020

Sea Ice Update

We're at the time of year when many eyes start looking north to the Arctic Ocean to see how sea ice is faring as melt season passes through its summer peak; the perennial question is whether we might see a new record low for ice extent by September.  The current record was set in 2012, and before that 2007 was the big record-breaking year for ice loss.

So far this season, melt-out has been very rapid, and currently we're sitting at record low ice extent for the date.  Here's a nice figure, courtesy of Thomas Lavergne on Twitter.


The particularly rapid loss this month has been closely related to extreme warmth over Arctic Russia that has persisted more or less continuously since winter, and as a result, current sea ice deficits are strongly concentrated on the Russian side of the Arctic.  The latest update from NSIDC is worth a read:

http://nsidc.org/arcticseaicenews/2020/07/siberian-downward-slide/

Here are charts showing the record pace of melt-out this year in the Laptev Sea and the East Siberian Sea.  Interestingly, 2012 was not the record low ice year for either basin; in the Laptev Sea the record low occurred in 2014, and for the East Siberian Sea the fastest and most overwhelming melt-out occurred in 2007.




In contrast, the sea ice situation is quite different in the Western Hemisphere, with Beaufort Sea ice in particular holding up well so far compared to many recent years.  Both here and in the Chukchi Sea, 2012 was the record low ice year.





An interesting coincidence in the current weather pattern is that a very strong storm has developed over the Chukchi Sea in the past couple of days, just as in early August 2012.  The similarity is striking, although the 2012 storm was slightly stronger; compare the Canadian surface analysis maps below:

3pm AKST July 27, 2020

9pm AKST August 5, 2012

The 2012 storm was the strongest summer cyclone of the modern era in the Arctic, and it probably contributed to the record low ice extent that year by breaking up already-vulnerable ice and increasing ice-water mixing.  It will be very interesting to see if there are any discernible effects on sea ice from this year's storm.

Here's a reference on the 2012 storm, for further reading:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL054259

Wednesday, July 22, 2020

Still More Rain

In keeping with recent posts on increased warm-season precipitation, rainy weather has become persistent in Fairbanks-land this month.  Each of the last 7 days, and 9 of the last 10, have seen measurable rainfall in Fairbanks, and although total amounts have not been excessive, the year-to-date precipitation now stands at 9.47", which is not far off the pace of 2014.  At the CRN site to the northeast of Fairbanks, 2020 precipitation is just about tied with 2014.  Here's a chart of the accumulation relative to the 2002-2019 normal for the CRN site.


After a notable cool spell a few weeks ago, daily average temperatures have been hovering very close to normal, although daily highs have remained on the cool side because of cloud and rain.




More interesting than the overall average temperature has been the lack of significantly unusual warmth this summer; the temperature has not exceeded 80°F in Fairbanks since May 10.

Given the date on the calendar, the 82°F on May 10 is now somewhat likely to go down as the highest temperature of 2020, and this would be the earliest date on record for setting the year's high temperature.  The current record for that climate statistic is the amazing 88°F on May 11, 1995, and interestingly 1995 was also a year with a developing La Niña during summer, a very warm North Atlantic, and a very active Atlantic hurricane season.  1995 also saw extreme warmth in September in Fairbanks, with an incredible 78°F on the 21st, and I've always found it fascinating that the same year holds the records (by some margin) for most extreme warm spells on either side of summer.  I wonder if something similar could happen this year...

Thursday, July 16, 2020

Rainfall Trends

As a follow-up to last week's post on increased rainfall in Fairbanks, I did a broader analysis with a couple of gridded data sets to examine the spatial extent of the changes.  This provides a quicker view of broad trends than digging through data from individual observation sites, which often suffer from missing days or other quality issues.

Looking first at a gridded analysis of observed precipitation (courtesy of Germany's Global Precipitation Climatology Centre), I calculated the percentile of the June-September precipitation in each of the six years from 2014-2019, relative to the distribution of June-September precipitation from 1979-2013.  In other words, if (for example) June-September 2014 precipitation was higher than any year from 1979-2013, the percentile would be 100% (the top of the distribution), but if it was equal to the median of the 35-year history, the percentile would be 50%.

I then took the average of the percentiles from the six years; see the map below.  The analysis shows an area centered just to the west of Fairbanks where the 6-year average percentile is around 90%, and this is consistent with the Fairbanks data; the last 6 years have all produced June-September precipitation near or above the 90th percentile in Fairbanks.


The gridded analysis shows unusually wet conditions extending northwestward from the central interior all the way up to Utqiaġvik, and there's another area of sustained high precipitation near the north end of the Alaska Peninsula.  We should regard these results with caution, however; the number of Alaska sites with consistent long-term precipitation data is small, and the gridded analysis is only as good as the data that goes into it.

For an independent view, the map below shows the same calculation from the ERA5 reanalysis, which is simply a long-term sequence of short-range model forecasts; actual precipitation data is not included as an input.  ERA5 confirms that Fairbanks is not the only place with very unusual precipitation during June-September in recent years, and the wet signal is particularly striking along the eastern North Slope.


Out of curiosity, I also looked at the average percentile of June-September shortwave radiation, which of course is mostly tied to cloudiness in June and July.  This shows reduced sunshine in the eastern interior and much more so over the eastern North Slope and the Beaufort Sea.  But over the Bering Sea summer sunshine has been persistently high, according to ERA5.


The percentile of sea-level pressure illustrates the weather pattern that has tended to dominate mid-late summer in recent years: a ridge of high pressure over the Bering Sea.


A global view of average 500mb height anomalies since 2014 at the same time of year reveals that the Bering Sea ridge has been one of the largest flow anomalies worldwide - see below.  (Note that heights are higher almost everywhere because of rising global mean temperatures, but the spatial distribution of the changes is particularly interesting.)


The Bering ridge seems to be part of a pattern of stronger high pressure ridges in the upper mid-latitudes of the Northern Hemisphere (in contrast to smaller height increases in the tropics and the high Arctic).  I believe this is also connected to persistently recurring high ocean temperatures across the North Pacific; I'll write more on this another day.

Sunday, July 12, 2020

Fairbanks Thunderstorms and the rest of 2020

Okay, for old times sake…

The highest annual frequency of thunderstorms in Alaska is over the Tanana-Yukon uplands. Many weather-aware Fairbanksians know that there are thunderstorms, say, out Chena Hot Springs Road than at the airport, but the airport is where there are long terms records. Here's a graphic showing the number of days each year (April-September) when thunder was reported. I have no explanation for the jump in 1988 beyond that it's thundering more frequently nowadays. It's not from automation, since thunder reporting in aviation observations has never been automated in Alaska. When I first noted this some years ago, I asked a former long time, now retired, NWS Fairbanks observer, and he could not recall any changes in procedure or instructions on when to report thunder in the late 1980s, or at any time during his career (coding, of course, changed with the advent of METAR code as the standard in 1996).


So far (through July 11) this summer there have been eleven days with thunder at the airport, already above the recent year average. The typical drop-off in thunderstorm frequency is coming soon, so can we come up with a climatologically based estimate of what will be the total number of days for 2020? To do that, I looked at 1988-2019 (32 years) and asked how many thunder days occurred from July 12 through the end of September. That ranged from zero (three years) to 10 (two years), with an average of 3.5 days. But of course, thunderstorms days only come as whole days (technically, this is an example of "count data"), i.e. there can never be a year with 14.5 thunderstorm days. So to create an outlook for the 2020 seasonal total, I fitted a Poisson distribution (good for count data since it is a discrete probability distribution) to the 32 years of data (the era of more thunderstorms). The primary assumption of a Poisson distribution is that the values are independent. In this case, the number of additional days with thunder this year is not conditioned by the days in 2019, or 1988, etc. That seems like a reasonable assumption in this case. So, putting all the pieces together yields this outlook with is based on nothing but history (no meterology): better than 50-50 chance that there will be 2 to 4 four additional thunder days by the time the snow flies.

Such is the value of Deep Cold for those that are interested in getting "under the hood" of Alaska climate. 😋


Friday, July 10, 2020

10 Years of Deep Cold

I managed to miss the day itself, but yesterday was the 10th anniversary of Rick's first post here on the Deep Cold blog.

http://ak-wx.blogspot.com/2010/07/first-post.html

Rick maintained a high frequency of posts for the first 3 years, and then the authorship transitioned to me (Richard James) in summer 2013.  Brian Brettschneider also contributed a number of posts for a time.  The "official" hand-off was nearly 7 years ago:

http://ak-wx.blogspot.com/2013/07/blog-changes.html

My approach has generally been to write more detailed but infrequent posts, and regrettably the frequency has declined as other priorities have intruded; but I've found it a most enjoyable "hobby" to explore a wide range of topics over the years.  I've often told my wife that I would probably still write the blog even if no-one reads it, because it serves as both a repository of my informal research and a record of a few of Alaska's major weather and climate events.  Even now I find it very helpful to refer back to material from earlier years.

I often think it would be nice to have more daily weather commentary on the blog, along the lines of what Rick used to do, but I do find brevity to be a challenge.  I'm also not sufficiently immersed in day-to-day weather in Alaska, as I don't live in the Great Land.  I'd be delighted if one day that might change, and then I'll report firsthand; but for now, I'll continue with comments from afar.


Thursday, July 9, 2020

Inactive Fire Season So Far

Interesting reading from the Alaska Fire Service today:

https://akfireinfo.com/2020/07/09/alaskas-fire-season-shaping-up-to-be-a-low-year/

"As of Wednesday, there had been 299 wildfires reported this season that had burn an estimated 173,743 acres, according to AICC statistics. Based on historical data, the chances of reaching the 500,000 mark is less than 20%, Strader said. In a typical fire season, Alaska burns about 650,000 acres."

I'll attribute the relative lack of fire to the trend towards La Niña in the tropical Pacific.  See here for my comments back in May.

Wednesday, July 8, 2020

Wet Summers

A few days ago Rick Thoman posted a striking chart on Twitter, showing clearly that Fairbanks has seen unusually high precipitation on a sustained basis since 2014.  Click to enlarge:


Remarkably, the 12-month running total precipitation hasn't dipped below about 12" since the record wet summer of 2014, and such a lengthy wet spell is clearly unprecedented in the roughly 100-year period of record.  When we consider that the long-term median annual precipitation is less than 11", it's quite extraordinary to see six straight years well above that level.




If we assume that each year is statistically independent of the last - which is not true but may not be too far off the mark (at least prior to 2014) - then we can do a crude estimate of the probability that 6 straight years will have such high precipitation by random chance.  The driest of the last 6 years was 2018, with 13.75", and this level was only exceeded 14 times from 1930-2013, i.e. 14 of 84 years, or an annual probability of 16.7%.  The chance that this happens 6 times in a row is then only 0.002%, and (naïvely) the recurrence interval is about 50,000 years.  It seems overwhelmingly likely, therefore, that the climate has changed; the rainfall statistics are not stationary over time.

An interesting question to consider in connection with this change is the seasonal distribution of the increase in precipitation.  June of this year was very wet; did June contribute significantly to the overall increase in 2014-2019?  The answer is yes, but larger increases have occurred in July through September, and in percentage terms the change in September is most striking (see below).  September 2015 and 2014 were the wettest and 3rd wettest on record, respectively, and September 2016 and 2018 were also very wet.


It's inevitable that the largest changes in total precipitation have occurred in the warm season, because that's when most of Fairbanks' liquid-equivalent precipitation falls; and if we imagine a percentage increase in water vapor content (related to, say, warming oceans), the impact in absolute terms would be most noticeable in summer.

The increase in September is interesting, and it's tempting to suggest it has to do with reduced Arctic sea ice; we would expect a large increase in lower atmospheric warmth and moisture at high latitudes as the Arctic Ocean opens up in autumn.  However, I suspect the Arctic warming only provides a background influence on September precipitation, and that other, more immediate causes (such as recent storm tracks and circulation anomalies) can be identified to explain the dramatic difference in the past few years.

Finally, it's fascinating to look at how the distribution of daily precipitation amounts has changed during June through September, the months that contribute most of the total precipitation increase.  The chart below shows that the frequency has increased in all daily precipitation categories above 0.1", and the percentage change in frequency has been very substantial for the wettest days, i.e. those with more than 0.5".



Of course a change in the frequency of the wettest days makes a relatively outsized contribution to the total precipitation increase, and this is illustrated below.  The lion's share of the total increase has arisen from a jump in daily precipitation totals above 0.5".  In 2014-2019, there were 32 of these events in June through September, amounting to an average of 4.62" per year; but from 1930 to 2013, these events occurred at less than half the frequency, and amounted to only 1.83" per year on average.  This difference accounts for about two-thirds of the overall increase in total annual precipitation.



If we consider the rate of observing events of 0.5" or more, we can do another crude estimate of the probability that the past six years could have occurred by random chance.  Using a Poisson distribution with an occurrence rate (1930-2013) of 2.35 events/year, the probability of seeing 32 or more events in 6 years is 0.003%, which is surprisingly close to the 0.002% estimated above from the annual totals.  Either way, the level of statistical significance is very high, and so I think we can say with very high confidence that the past six years represent a different climate regime from earlier decades.  As noted in this 2018 post by Rick on a similar topic, this has important implications for city management and planning.

Thursday, July 2, 2020

July Frost

A very chilly air mass originating over the Arctic Ocean has produced frost in many of the usual cold spots of the central and eastern interior in the past few days.  Here are a few of the readings I noticed:

Fairbanks Area:
UAF Smith Lake   27°F
Ester 5NE Coop   28°F
Goldstream Creek Coop   29°F

Elsewhere:
Tok 70 SE CRN   26°F
Chicken Coop   27°F

Here's the 7-day temperature trace from UAF's Smith Lake 2m sensor:

According to the balloon sounding from Fairbanks airport, the 850mb temperature fell to -0.5°C on Tuesday afternoon, which is the first time since 2009 that the 850mb temperature has been below freezing between mid-June and the end of July.  The last such cold spell in 2009 (June 25) was also the last time that Chicken fell to 27°F in the same high-summer window.

The 26°F at the Tok CRN is the coldest in the 9-year history of the site for this time of year; it's the first instance of 26°F between June 14 and August 21.  Data goes back to autumn 2011.

Here's the reason for the cold: according to a back-trajectory estimate from NOAA, the lower tropospheric air that was over Fairbanks on Tuesday afternoon had traveled from near the North Pole in the previous 10 days.