Friday, April 12, 2019

Snow Depth Conundrum

After last week's early meltout of snow in Fairbanks, I started thinking a bit more about the absence of a long-term trend in the meltout date.  It's a puzzle because late winter temperatures have increased over time; for example, March and April in Fairbanks have been about 3°F warmer since 1980 than they were before, and consequently the accumulated thawing degree days by April 23 (the long-term normal meltout date) have approximately doubled.  The trend towards earlier breakup at Nenana provides independent confirmation of the warming; see this post for the history at Nenana:

As mentioned before, a cursory analysis suggests that higher snow depth at the end of winter could explain the unexpected "resilience" of Fairbanks snow cover compared to earlier decades.  And so we might hypothesize that snowfall has increased over time, but this is not borne out by the data; the chart below shows the March 15 snow depth (purple markers) and the accumulated snowfall (green markers) between the date of establishment of the winter snowpack and March 15.  The long-term trend in snowfall is essentially zero, but there is a rising trend in snow depth (admittedly quite small - about 2.5" over the 90-year history).

What about liquid equivalent precipitation?  If snow density has increased, then precipitation and snowpack water content may have increased despite no change in accumulated snowfall.  Surprisingly, liquid equivalent precipitation also fails to show an increase over time, and in fact there is a slight decreasing trend, although that's mostly because of the incredibly wet winter of 1936-37.

So if precipitation and snowfall haven't increased, then is the snow depth trend just an artifact of changing measurement practices and/or location?  Perhaps, but I'm inclined to believe that snow depth (and presumably snowpack water content) really have increased, because it helps explain the meltout dates.

If we take the ratio of the snow depth to total precipitation, we find a result that suggests there really has been a change in the characteristics of Fairbanks winter precipitation over time; recent decades have produced a notably higher end-of-winter snow depth per inch of precipitation in the previous winter.

Assuming that measurement practices are not to blame, there are only a couple of explanations I can think of here.  One is that cloudiness may have increased, perhaps along with humidity, so that snow evaporation has declined and there is more snow left at the end of winter.  There would be little actual melting of snow prior to March 15, but snowpack can be affected by sunshine, humidity, and wind.  I wouldn't be surprised if cloudiness has increased, but rising temperatures typically dictate rising evaporation rates even if relative humidity increases a bit, so I am not sure how plausible this explanation is.

Another possibility is that Fairbanks used to see more of its winter precipitation as rain, not snow.  Admittedly this seems like an absurd proposition, because freezing (or plain) rain has been a notable winter problem in recent years and seems unlikely to have occurred more often in the colder winters of the past.  However, we do know that a few winters of long ago (e.g. 1936-37) produced some extreme rainfall events, so perhaps we shouldn't dismiss the idea out of hand.  It may be conceivable that the recent climate has produced more of the winter's precipitation as snow, thereby contributing more to the snow pack - but more dense snow, so as not to increase the total snowfall (or else snow depth measuring practices have changed over time).

Can anyone suggest other aspects of the problem that I may have overlooked?  It would be nice to be able to nail down a good explanation for why meltout dates have defied the long-term warming trend.


  1. Hello,

    First, I want to commend you and all of the contributors on a great blog!

    To answer your question: It could be that, with the warming temps over time, the typical snow that falls throughout the season has a lower "liquid equivalent" than it used to (say, 12:1 ratio instead of 15:1). So, the snow on the ground in recent winters has more water in it and is "denser", and therefore takes a bit longer to melt than fluffier snow.

    1. Thanks for the kind words and the comment. Yes, it seems plausible that snow density has increased, but if there's more water in the snow then liquid equivalent should have risen - and apparently it hasn't (judging from total precipitation). But I didn't yet look at snow density explicitly, so that would be interesting extension.

  2. Richard, perhaps you should be looking at heating degree hours instead of days. Case in point: while the mean daily temperature has been above freezing the last month, the hourly temperatures haven't always been. It's often been below freezing for a significant part of the day. And even a couple degrees above freezing is not enough to melt snow unless the sun is strong.

  3. What it seems to me is that the over-night temps. are what drive snowmelt up here. While it certainly has to get well above freezing to initiate snow melt, if it cools off and refreezes at night, there isn't near as much melt as if it stays above freezing over night. That happened this year when it warmed up in mid-March and we got our first hint of breakup, but then it cooled off at month's end and everything froze up due to the hard freezes at night. Now that it's staying above freezing at night again, there's runoff appearing again. Cloudy weather keeps it warmer at night, so might favor early breakup, even tho it doesn't get as warm during the day.
    By the way, the open lead at Nenana is right up against the tripod and it looks like it could go out any time now.

  4. Thanks to all for the comments - much appreciated. I will return to this topic as soon as I have a few spare moments.