More on Snow Ratio

A few weeks ago I commented on the decrease in "snow ratio" of the last decade or so in Fairbanks.  Although winter precipitation obviously still falls as snow the vast majority of the time, the recurrent warmth of recent years has produced a lower ratio of snow depth to liquid-equivalent precipitation, and the occasional winter rain events (a few of them very large) have contributed significantly to this trend.

One would expect similar or larger trends to be found at western Alaska sites, where the proximity of the Bering Sea has produced excessive warmth in recent years: other than the North Slope, the west coast and Aleutians have seen the most significant winter warming of anywhere in Alaska in the past few decades.

But interestingly the results are mixed when looking at Bethel and Nome.  [Note that "snow depth" in the charts and discussion refers to the accumulation of daily snow amounts - the "total snowfall" - and not the actual depth of snow on the ground at any one time.  The latter is affected by compaction, evaporation, melting, blowing away, etc.]

Sadly snow measurements ended at most Alaska climate sites a few years ago, so the data isn't up-to-date, but I'm not sure there is much to glean from these two sites.  Bethel had several low-ratio winters after 2010, but the historical record looks a bit unrealistic, with clusters of high and low ratio winters that do not look as randomly distributed as we would expect.

As for Nome, the upward trend is quite striking, but I'm skeptical, given what we know about the dramatic warming in recent years.  It seems possible that snow depth may have been systematically lower in early decades, perhaps because of less shelter from buildings near the observing site, or perhaps because of different measuring practices.

We can look at the ERA5 reanalysis data for an alternative view of changes in the proportion of snow versus rain.  My last post showed that the model shows a change in Fairbanks since 2010-11, which agrees with the ground-truth observations.  On a statewide basis, the fraction of winter precipitation falling as snow has also dropped, with an increasing number of years with percentages in the low 90s, and fewer years (but still some) with nearly all precipitation falling as snow.

Note that the data prior to 1950 is a newly-released component of ERA5, and I included it "just because I can".  It's highly uncertain and probably not worth much at all.

On an annual basis the fraction of precipitation falling as snow has dropped off quite a bit since the 1970s, according to ERA5, with 1992 being the last year with more than half falling as snow (and that certainly was a very unusual year).

Here's a map of the total fraction of November-March precipitation falling as rain in the last 12 years (this makes for a more pleasing map than the inverse fraction):

The change from the prior 3 decades is heavily concentrated over the Bering Sea, where the increase in the percentage has been as much as 10% (e.g. a change from 50% to 60%).

The increase over land is much smaller in absolute terms, but of course it is large in relative terms.  Here's the percentage change, but only for locations with at least 0.1% rain fraction in the 1981-2010 period.

Looking farther afield, it's interesting to see that the largest changes, in absolute terms, have been across eastern Europe, the Norwegian Sea, and eastern mid-latitude China.  These are places where both rain and snow are common in winter, and the relative fraction varies a lot from year to year and decade to decade.  The tendency for a positive Arctic Oscillation in recent years has certainly produced warm winters in much of Europe and Asia.

The charts below illustrate the year-to-year changes for all land area north of 60°N.  It's interesting to see the annual fraction of snow apparently peak in the 1970s, consistent with a cool phase of the Atlantic Ocean.