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.


1 comment:

  1. Good explanation. Not sure I need a reminder that summer is winding down, though ;-). As it happens, I am in Fairbanks getting a reprieve from the even-wetter-than-usual panhandle weather, I can report that moderate warmth has continued since you wrote this, and has destablized the air enough to result in some very beautiful afternoon CU/TCU/CB buildups, and brief thunder a couple nights ago.