If we look at the total annual snowfall in the cold season (see chart below), we see that recent years have seen snowfall on the low side compared to the 1981-2010 climate normal period; 12 of the last 18 years have been below the 1981-2010 median of 52.5". This by itself could be part of the explanation for the apparently lower snow density, because the fractional compaction of snow will decrease with smaller snow amounts. Consider the bottom 12 inches of the snowpack: it will be compacted more if a greater mass of snow accumulates above; and so lower seasonal snowfall will produce lower snowpack density, on average.
Another interesting trend in the snowfall history is that snowfall has been occurring later in the season in recent years than in the 1981-2010 period overall. The chart below shows the date at which half of the season's snowfall total was reached; in the 1970's and 1980's, nearly every year saw half of the snowfall occurring before the turn of the year, but in recent years the midpoint date has become later by two or three weeks on average. I would suggest that this could also affect the late winter snow density, because if snowfall is occurring later, then it has less time to settle and undergo compaction; so again this might contribute to a "fluffier" snowpack.
One final thought about the snow depth trend is that measurement procedures could perhaps be subtly changing, leading to a bias in the recent years' observations compared to former years. I don't think this is particularly likely in recent years, but interestingly the data suggest that something may have changed in or around 1983: consider the chart below, which shows the accumulated reduction in snow depth as a fraction of the total snowfall, through February 28. I calculated this by adding up all the snow depth losses from day to day each season through February 28; this could be from compaction, sublimation, melting, or even wind redistribution. Then I divided by the snowfall total, because of course total snow depth loss (mostly compaction) is closely related to the total snowfall. The chart shows that since about 1983, on average 30 % or less of the snowfall has "disappeared" through these various processes, whereas prior to 1983, the loss was often above 40 % or even 50 %. It's an intriguing result, perhaps worthy of more investigation; but the simplest explanation may be the most likely, which would be that measurement practices or site exposure have somehow changed.
Excellent learning source for we readers. Thank you Richard.ReplyDelete
Not sure of the source but Rick has either written here, presented info in formal seminars, or written personal notes regarding the moving of observation locations over time. I recall reading about the changes. He may offer a clue re: Any mid-'80's change and its potential effect on snow data.
In the meantime, here's some source material that mentions potential changes in location:
I'd first be looking at Methods and Means for determining snowfall and depth in Fairbanks. If it's been documented and consistent then perhaps other factors have come to play a role. Where and how did they do the measurements over time?Delete
The Fairbanks airport is subject to human activity...aircraft generated winds and snow being redistributed via huge blowers off active surfaces.
Good points, thanks. It seems the official observations have been made at the airport since 1951, but I don't yet know if there have been any changes in the precise location or methods in that time.
In looking around, I found the following link which contains a nice history of all NWS office locations past and present for all 50 states:
Thank you looking into this some more Richard. In a week or so I'll have more time to look into this but the GHCN database does have snow water equivalent data for 1952 through 2000. I took the liberty of adding a single chart to the end of your post without any additional text. It shows the snow depth, snow water equivalent (SWE), and the snow density (depth divided by SWE) for all February's between 1952 and 2000. Interestingly, the trend is toward greater density (more compacted) snow pack particularly since the 1980s. Perhaps that trend has reversed and the relationship between snowfall and snow depth has returned to the pre- 1981-2010 climate normal period level. Alternatively, I would hypothesize that perhaps more of the snow is falling in the colder time of year now (as a proportion), it is more likely to form in the atmospheric dentritic growth zone and is therefore fluffier.ReplyDelete
Thanks Brian, that's a useful chart. So density does follow quite closely with total snow, as we expect from compaction; the 90's was a time of high snowfall and high-density snowpack. The trend does appear to have reversed.Delete
Perhaps there is too much variability but you could do a scatter plot of average temp vs any of the snow properties over the various periods of time. Any patterns might give insight into the temp's role.ReplyDelete
Also, the snow depth graph reminded me of the century temp graph. How does the PDO factor?
Good suggestions, Eric - I'll see if I can dig up anything. Initial query of the data shows slightly higher snow and snow depth in negative PDO years, but similar ratio of the two.Delete
I may as well throw this into the pot. I've observed these rain and melt events impacting subsequent snow levels, rates of sublimation, and snow texture.ReplyDelete
Once the existing snow cover is "sealed" with a surface layer of ice, it can remain in essentially a fixed state and level until Spring. More snow and wind may cover but not disturb as easily, and the snow under the cap can change states to a more granular nature like small ball bearings (corn snow).
In March and April the surface covering the ice layer will melt or sublimate normally , but the sealed layer takes some longer time to disappear. It's really apparent on lakes with ice covered drifts that linger longer than normal snow would.