Friday, November 29, 2019

ERA5 Winter Precipitation Type

In light of recent posts on winter rain in Bethel and elsewhere, I thought it would be worthwhile to see what the new ERA5 reanalysis data has to say about winter precipitation type in Alaska.  I've mentioned the ERA5 data on previous occasions, for example here:

The ERA5 dataset includes a precipitation type indicator, which makes it a simple task to assess the frequency of each category across the 0.25-degree latitude/longitude grid.  The six categories are: Snow, Wet Snow, Rain/Snow Mix, Rain, Freezing Rain, and Ice Pellets; the determination is based on the modeled surface temperature and the temperature profile aloft.  Here is some more information on the model algorithm:

Based on the hourly ERA5 data for 40 winters since 1979-1980, I simply added up the number of occurrences of each category of precipitation type for the months of November through March.  Below are maps for the four simple categories, i.e. snow, rain, and in-between.  Note that a precipitation type is assigned whenever the model has a non-zero precipitation rate, no matter how small; so for example the model indicates snow falling more than 90% of the time over the Arctic waters, but rates would be extremely low much of the time.  Click to enlarge the maps.

Here are the two other categories, freezing rain and ice pellets, with a different scale to reveal areas of low frequency.  For reference, a frequency of 1% corresponds to 36 hours per winter.

And here are the rain, rain/snow mix, and "wet snow" maps on the same "zoomed in" color scale:

It's interesting to see relatively high frequencies extending around the north side of the Alaska Range to the middle Tanana River valley (e.g. Nenana and Fairbanks); the spatial distribution looks realistic to me, although the absolute values of the frequencies might be biased high or low (more on that in another post).

Finally, here's the sum of frequencies for everything other than dry snow.  I'd be interested to hear readers' perceptions of these results - feel free to leave a comment below.

Monday, November 25, 2019

Chukchi Partial Freeze-Up

Arctic waters in the Chukchi Sea, northwest of Alaska, have finally begun to succumb to the seasonal chill, with Chukchi sea ice extent approximately tripling in just the last 10 days, according to NSIDC daily data.  As of Saturday the basin-wide extent is no longer the lowest on record for the date, as 2019 is now slightly ahead of 2017.  However, with a powerful storm currently moving up the west side of the Bering Sea, and strong southerly winds pushing up into the Arctic ahead of the storm, the situation looks tenuous.

The two maps below show the sea ice analyses from two weeks ago and from today, courtesy of the NWS.  Interestingly the Bering Sea is doing a bit better than in recent years (with some ice along the Alaskan west coast), although it is still behind the 1981-2010 median for the date.  And of course essentially the entire open area to the north of the Bering Strait is anomalous in comparison to that earlier climate normal.

Tuesday, November 19, 2019

Winter Rain History

After a nasty ice storm a couple of weeks ago, Bethel experienced blizzard conditions today, with temperatures below 10°F and 40+mph wind gusts combined with heavy snow this morning.  However, as the warm front approached this afternoon, warm air aloft caused the snow to change over to a brief period of freezing rain again before precipitation ended.

The (nominally) 3pm balloon sounding from Bethel shows the dramatic temperature gradient just above the surface.

At the time the balloon was launched (2pm), the surface air temperature was 16°F, but freezing rain was already occurring as there was enough warm air upstream to melt falling snow; the sounding profile shows the temperature just nudging above freezing at 400m above sea level.  Notice the veering (clockwise directional change) of the wind with height, indicative of the rapid approach of warm air.

As of 6pm this evening, the temperature is up to 36°F, and the wind has gone around to the southeast; it must feel positively tropical compared to the conditions only 8 hours earlier.  Check out the temperature gradient and wind shift across the Y-K delta region in the 6pm surface data (click to enlarge):

Here's the 3am surface analysis, courtesy of Environment Canada; note the strong pressure gradient over southwestern Alaska, with a cold high to the north (-28°F at Galena this morning, -26°F at Kaltag).

In light of the ice storm earlier in the month (see my earlier post), I took a quick look at the historical frequency of liquid precipitation (rain or drizzle) in winter at Bethel and a few other sites.  The chart below shows the annual percentage of winter (November-March) hourly observations that reported rain or drizzle, regardless of temperature, in Bethel.  Note that I've excluded supplementary observations that were not taken at the top of the hour.

I'd be surprised if there are not some systematic biases between the pre-ASOS era (pre-1998) and the modern era, so I think about all we can say is that there's no evidence of an increase in the frequency of winter rain in Bethel.  The frequency has been relatively high in the past decade, but it seems to have been high also in the 1980s and in many previous years.

Here's the same chart for McGrath and Fairbanks, with the same vertical scale to emphasize the drastic difference in climatological frequency of liquid precip in winter.  If the trends are reliable, McGrath seems to have seen a rather steady decline in frequency, whereas Fairbanks has seen an upward trend in the last 20-30 years (but perhaps not exceeding the levels of earlier decades).

For completeness, I include the chart for Anchorage below, which also shows nothing particularly exceptional.  However, the second chart below shows the frequency of liquid precipitation at sub-freezing temperatures, i.e. freezing rain or freezing drizzle, and this does seem to show a decline since the 1980s (the 2013-2014 spike notwithstanding).  It seems possible that the warming trend in the more maritime zone of south-central Alaska has tended to produce more plain rain at the expense of freezing rain in winter.

Tuesday, November 12, 2019

Summer Forecast Verification

This is just a quick follow-up to a post from way back in May, where I showed the Climate Prediction Center's summer forecast and commented on a possible connection between equatorial winds in the stratosphere and summer rainfall in parts of Alaska.  Here's the post:

Let's see how well the CPC forecast did.  First, for temperature: compare the maps below.  The second map shows the estimated summer temperature anomaly (departure from 1981-2010 normal) from the new start-of-the-art ERA5 reanalysis.  The forecast was a strong success, with the very high (>60%) chance of significantly above-normal temperatures in southern and western Alaska being fully justified by the exceptionally warm summer that actually occurred.

Second, for precipitation.  The ERA5 verification map indicates that the wet tilt to the forecast verified as correct for the North Slope, but most of the interior, south, and west was near normal or below normal for summer rainfall.  The expectation for widespread above-normal precipitation was therefore not correct, although the tilt in the probabilities was small and this would be considered only a minor forecast bust.

The ERA5 reanalysis is, of course, only a model, so let's take a second look using gridded precipitation data based solely on surface observations (available only over land).  This shows a wet summer on the Seward Peninsula, which is confirmed by Nome's 10" of summer rain - it was one of the wetter summers in Nome's history.  So this certainly provides some vindication for the CPC forecast.

In my earlier post I suggested that the phase of the equatorial Quasi-Biennial Oscillation (QBO) might produce a drier tendency over northwestern Alaska than the CPC was expecting.  While it did indeed turn out drier in much of the interior, neither of the two locations I cited as showing a QBO influence - Bettles and Kotzebue - had a dry July-August period.  So I won't make any bold claims about this being a success, but it is possible that consideration of the QBO phase might have improved the CPC precip forecast slightly.

Wednesday, November 6, 2019

Kuskokwim Ice Storm

The topic of this post is not ice on the Kuskokwim River, but rather a severe and long-duration freezing rain event ("ice storm") yesterday in the lower Kuskokwim region.  Bethel seems to have been hard-hit, with schools closed yesterday (I'm guessing this doesn't happen often).  The following photo, provided on Twitter by Mark Springer of Bethel, illustrates the kind of difficulties residents ran into (click to enlarge).

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  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.

And here's another of my posts from some years ago: