The observations from Bethel airport recorded the remarkable persistence of rain at sub-freezing temperatures; rain began at 1am on Tuesday with a temperature of 21°F, and it continued uninterrupted until 4am today, by which time the temperature had risen to 31°F. The midnight-to-midnight precipitation total was 0.52", which was all rain.
Yesterday's 3pm surface analysis from Environment Canada shows the situation: a low pressure system to the south, strong high pressure (with associated cold surface air) draped over northern Alaska, and a frontal zone just to the south of Bethel.
Surface temperatures showed a sharp gradient between the Y-K delta and the Alaska Peninsula - see the red numbers in the 7am plot below.
Looking at the lowest levels of yesterday's 3pm Bethel sounding (below), the cold surface flow is from the northeast, but not far above the surface the flow has a southerly component, and a substantial above-freezing layer is evident from about 980mb up to 880mb. This is a freezing rain temperature profile: snow formed at higher levels melts as it falls through the warm layer, but then it is super-cooled below freezing in the short journey through the surface cold layer - and then it freezes on impact with any surface object.
Farther up the valley in McGrath, freezing rain developed around noon (at a temperature of 15°F!) and persisted for about 12 hours, although it was mixed with ice pellets or other solid precipitation at times owing to the deeper, colder surface layer. Additional light amounts have been occurring on and off today.
It would be worth taking a look at historical data from Bethel and McGrath to see how often freezing rain events of this magnitude have occurred in the past. Fairbanks has seen a considerable number of freezing rain events in recent years, but the phenomenon is by no means just a recent one in interior Alaska (see e.g. here https://ak-wx.blogspot.com/2018/01/freezing-rain.html). A comprehensive study of whether and where freezing rain is becoming more common, and whether it's an expected consequence of a warming climate in the far north, would be interesting and useful.
Update Nov 8: here's a paper about rain-on-snow events in Alaska, suggesting that frequency is likely to increase in (interestingly) southwestern and interior regions.
https://iopscience.iop.org/article/10.1088/1748-9326/aac9d3/pdf
And here's another of my posts from some years ago:
http://ak-wx.blogspot.com/2013/11/winter-rain-climatology.html
Wow.
ReplyDeleteIt seems to me there have been very few freezing rain events of this magnitude in Alaska in a long time. And I think the fact that it happened on the Kusko delta is perhaps especially unusual due to the lack of topography to trap a pool of cold air in place. Quite a balance of flows to keep it going for so long.
It's conceivable that a warming climate might increase this but very hard to know for sure as there are so many factors, including the needed cold air holding in place like this. And changes in how things like this are reported and covered has changed so much (ie change to automated obs, twitter etc). Looks like the Kipnuk automated station maybe had trouble with this event as it had no actual reports of freezing precip (some unknown, also temp missing for some hours) yet the NWS facebook post said they had more than twice as much glaze as Bethel.
Yes, one would think the flat terrain of the delta would not be conducive to maintaining a surface cold pool, but at the same time the ready access to fresh cold from the north is a positive factor with suitable flow alignment.
ReplyDeleteAs far as historical estimates are concerned, the best approach might be to use the modern generation of reanalysis data to come up with a long-term climatology of freezing rain profiles/events. A good thesis project!