Thursday, August 27, 2020

Back to Rain

After a remarkable spell of warm and dry weather to close out summer, rain returned to Fairbanks earlier this week, pushing August's rainfall total to 2.08"; and this means that all three summer months have been wetter than normal.  Perhaps surprisingly, even in the wet regime of recent years, only 2014 achieved that distinction, although 2017 was close (July 2017 just missed the mark).  Other years were 1930, 1932, 1948, 1955, 1962, 1990, and 2008, although we perhaps shouldn't compare earlier decades to the current standard "normal" of 1981-2010.

Before saying a few more words about rainfall trends, I'll just make a note on the recent warm spell: with 11 consecutive days (Aug 13-23) having a high temperature above 70°F in Fairbanks, this is actually the longest such spell since at least 1930 for so late in the season.  However, it's a long way from being the greatest late summer heat wave, as other years reached into the 80s with warm spells of similar duration.  For instance, 1950 saw highs in the 80s on 4 of 5 days from August 19-23, with most days above 70° for two and a half weeks.  August 2004 had 10 days of 80°F or above (the last on the 20th), and August 1977 saw an amazing 87°F and 86°F on the 21st and 22nd respectively.

Returning to rainfall, I got to wondering whether the persistently high rainfall of recent summers has a clear signature in the duration of rain: is it raining more of the time, or is the rain just heavier when it occurs?  Here's a quick analysis to address the question: the chart below (click to enlarge) shows the fraction of Fairbanks hourly (top of the hour) observations reporting rain since 1973.  I'm showing June-September here, because these are the months with by far the greatest increase in precipitation since 2014 (see this recent post); and here we're looking at observations of rain, not snow.  There's a clear signal for more frequent rain in each of the last 7 years, so we can say that yes, it has been raining more of the time in summer and early autumn since 2014.


Interestingly, however, the number of days with measurable precipitation has not increased as obviously; it's up this year, but 2018 and 2019 did not see an unusual number of wet days, and nor did 2014.


Here's the total June-September precipitation, illustrating again the sustained nature of the new wet regime.


So what do we have in summary?  Since 2014, it has been raining more of the time in summer and early autumn, and the frequency of heavy rain events has gone up considerably (as shown in the earlier post), but the number of days with rain hasn't changed much.  So when it rains, it rains for longer and comes down harder, but the frequency of dry versus wet days is not substantially different.

Wednesday, August 19, 2020

Warmth for Summer's End

An interesting feature of the interior Alaska climate is that early autumn tends to bring drier and more settled, sunny weather after the typical rains of August, but this year the improving trend seems to have arrived several weeks early.  It's warm, too, with today being the 7th straight day that the temperature has exceeded 70°F in Fairbanks; that hasn't happened this late in the year since 2009 (and that was in early September).  Prior to that, we go back to August 2004, which was very hot and led to a highly unusual late-season extension of that record-breaking fire season.

Of course most places at lower latitudes are still in the throes of summer heat; Death Valley roasted at 130°F just the other day.  The more rapid drop-off in temperatures up north in most years is directly related to the quick decline in solar insolation at high latitudes.  Compare the two figures below: the first showing the annual cycle in solar radiation near Fairbanks, and the second showing the same for my neck of the woods far to the south.  The quick decrease in Fairbanks is partly related to increased cloudiness as summer advances, but it's also a result of the solar geometry.

The summer maximum is even more fleeting at Alaska's northernmost city (see below); but notice how high the peak is - almost as high as the summer peak in Georgia, and that's with much more cloudiness at Utqiaġvik.  (Another interesting feature is that even the cloudiest days in Utqiaġvik still receive a large amount of solar radiation in early summer, owing to 24-hour daylight.)

The late-summer decline in solar energy in Fairbanks means that it's increasingly uncommon for afternoon temperatures to achieve their full "potential" based on temperatures aloft.  Increasing cloudiness of course bears responsibility for this, but even if it's sunny with warm air aloft, the weakening sun in August is increasingly unable to produce a strong surplus of heating at the surface.

For example, here's yesterday's 4pm balloon sounding from Fairbanks; there's certainly a nice well-mixed boundary layer, and there's a small excess of temperature (a super-adiabatic layer) at the surface from the solar heating.  The near-surface temperature of 72°F at the balloon release site is 24°F higher than the 850mb temperature of 48°F; this equates to a lapse rate of 10.0°C/km, which is just slightly above the dry adiabatic (well-mixed) rate of 9.8°C/km.


Back on a sunny July 4th, the additional solar heating at that time allowed the 4pm surface temperature to be 27°F higher than at 850mb, for a lapse rate of 10.5°C/km; the sunshine eked out another 2-3°F compared to what it might have done with the same air mass six weeks later.


The seasonal changes are more noticeable when we compare the distribution of surface-to-850mb temperature differences by month - see below.  For simplicity, I've used the daily maximum temperature at the surface, even though this does not always occur in the afternoon.


As summer advances, the highest lapse rates (like that on July 4) become much less common, and surface temperatures tend more and more to underachieve relative to conditions aloft.  Of course this only means one thing: we're on the long road back to the semi-permanent temperature inversion of the winter months.


Thursday, August 13, 2020

Reduced Temperature Variance

If residents of central Alaska have a sense that this summer has seen unusually steady temperatures, without extremes of either cold or heat, they are correct.  In fact, according to daily Fairbanks data, the standard deviation of daily mean temperatures since June 1st is the lowest on record.  Since at least 1930, there's never been a summer with less variability in daily temperatures through August 12.

The slight downward trend over the 90 years of data is not statistically significant, but the trend since 1990 actually is marginally significant at a 95% confidence level.

This summer's daily temperature variance is also the lowest on record - and by a larger margin - at McGrath, and it's the second lowest on record at Bettles.

Here are daily temperatures since May 1 in comparison to normal for the time of year in Fairbanks.  Lots of small deviations from normal.

What explanation might we propose for this summer's unusually steady conditions?  It would be interesting to look at seasonal climate drivers that tend to impart low variance, but for now the answer is rather simplistic: the general weather pattern has precluded the strong ridge/trough waves in the atmosphere that create larger swings in temperature from day to day and week to week.  In particular, it's the ridges that have been missing, as 500mb heights have been lower than normal over eastern and/or southern Alaska.  This seems to have been a relatively stable configuration, with few significant perturbations to bring much warmer air to Alaska's interior.

Here are 500mb anomaly maps from June, July, and August so far:


As for the downward trend in the past 30 years, it may well be the reduced frequency on the cold side of the climate distribution that explains the reduction in summer temperature variability.  In other words, as the Arctic has warmed far faster than the rest of the Northern Hemisphere in recent decades, air from the north will have warmed much more than air from the south.  It would be useful to look at this in more detail when time permits.

Update August 14:

It turns out there's no meaningful correlation between the mean and the variance of summer daily temperatures (after removing the long-term trend from the mean), so it seems we can't say that cool summer weather patterns (for Fairbanks) are inherently more stable.


Sunday, August 9, 2020

Historical Rainfall Extremes

In the wake of last weekend's epic rains in central Alaska, I've been poking around in historical rainfall data from the Alaska RAWS network (data courtesy of Rick Thoman) and a few other sources.  The question is whether any previous events can be identified in interior or northern Alaska that approach or even exceed the 5.35" peak 24-hour total that occurred at the McKinley River site last weekend.

According to Rick's earlier research, the previous record for rainfall in 24 hours is 4.58" at Clear, about 20 miles south of Nenana; this occurred in the same event as the Fairbanks flood of August 1967.  The observation was a standard once-per-day measurement, so it's almost inevitable that a higher peak 24-hour total occurred.

Does the RAWS data reveal anything more extreme than this in recent decades?  The short answer is no - if we're talking about interior Alaska, i.e. north of the Alaska Range.  Epic rains occur along Alaska's southern and southeastern coastline, of course, and I found an event with over 6" of rain in 24 hours at the Ruth Glacier RAWS on the south side of Denali NP (at 3300' elevation).  But it's obviously a very different climate on the south side of the divide.

In terms of a new finding, the highest I uncovered for the interior is 4.20" in 24 hours at the Salmon Trout RAWS, which is at 2200' elevation above the Porcupine River, not far from the Canadian border (and 66.8°N latitude).  This occurred on August 7, 2010, and the peak 12-hour rainfall was 3.05".

Another high-latitude one that surprised me - although it's not quite in the same league - is 3.01" in 24 hours at the Red Dog CRN in far northwest Alaska (68.0°N, elevation 940').  That was in mid-August 2012.

I would certainly be remiss not to mention also the remarkable 4.16" last summer (August again) at Denali NP's Eielson Visitor Center (3760' elevation on the north side of the park).

For context, the record 24-hour rainfall in Fairbanks is 3.44", which was of course in August 1967.  Nothing close to this has occurred at valley-level in Fairbanks since, but up in the hills there have been some 3" rains in 24 hours; for instance, the Fairbanks 11NE CRN reported 3.07" in 24 hours in July 2014, and interestingly the Munson Ridge SNOTEL has reported over 3" in three recent years (3.76" in August 2014, 3.23" in July 2016, 3.09" in August 2019).  Given the frequency with which Munson Ridge sees big rains, it may not be many years before they see 4-5" in a day; we can only imagine what occurred up in the hills in 1967 (e.g. the Gilmore Creek co-op north of town recorded 6.18" in two days).

On a 6-hour time scale, the 3.32" at McKinley River last weekend easily beats anything else I can find for the interior.  The closest is 2.48" in 6 hours on July 6, 2007, at the Preacher Creek RAWS to the west of Circle.  There are just a handful of other events exceeding 2" in 6 hours.

If anyone can suggest other notable historical rain events that might be contenders, I'd be glad to dig into the data a bit more; the RAWS data obviously only goes back a few decades at best.

Monday, August 3, 2020

Extreme Rain

Here's a look at observed 48-hour rainfall totals from the weekend's storm; click to enlarge the map below.  As usual the data is sparse, but it suggests that over 3 inches fell across a large area on the northwest side of Denali NP, as well as between Fairbanks and Livengood.

The highest totals on the map are as follows:

5.76"  McKinley River RAWS
4.17"  Wien Lake RAWS
3.88"  Wonder Lake RAWS
3.61"  Wickersham Dome HADS
3.16"  Chatanika RAWS

The high-quality Denali 27N CRN sits in close proximity to the Wonder Lake RAWS and reported 4.10", so that provides strong support for the highest amounts.

Rainfall totals in the middle Tanana Valley were somewhat lower than expected, although Nenana received a hefty 1.71" and Fairbanks airport reported a respectable 1.22".

Part of the reason for the lower totals east of Nenana is that the initial disturbance from the east arrived much later than expected on Saturday evening and did not produce the widespread strong afternoon thunderstorms that were advertised by the NWS.  It wasn't until the front moved off to the west in the early hours of Sunday that it started generating prolific rainfall, as illustrated by the rainfall accumulation chart from the McKinley River RAWS:

In this chart the green dots and the blue line show the same thing, but they pertain to the left and right vertical axes respectively.  Incredibly, the instrument recorded 2.01" of rain in 2 hours and 3.0" in 4 hours.

The NOAA precipitation atlas does not have return interval estimates for this particular site, but for Wonder Lake the 1000-year return interval for a 2-hour total happens to be 2.01".  It's almost the same at Minchumina, and the McKinley River RAWS is half-way between the two sites, so it seems this was indeed about a 1-in-1000 year event for the specific location.

And according to Rick Thoman:

All in all, I'd argue the forecasts were quite good, and the strong wording of the NWS messages was justified even though rainfall underperformed a bit in Fairbanks.  In particular, the forecasters' assessment was highly prescient when it spoke of "very heavy rain... 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".  It's not easy to predict extreme and historic weather events, because history provides no guide, so this was quite a triumph of modern technology.