Freezing Drizzle

The Fairbanks scene finally gained a more wintry decoration on Monday, with a couple of inches of snow produced by a weak upper-level disturbance.  But by Wednesday, as strong high pressure developed overhead, a different kind of wintry precipitation developed: freezing drizzle.  According to the airport instrument, drizzle occurred for about 7 hours, with temperatures in the mid 20s.

Here's a previous post on the topic of freezing drizzle.  As noted there, the phenomenon tends to occur more often in early winter than mid-late winter in Fairbanks, and this is consistent with the fact that the saturated lower part of the atmosphere needs to be entirely above about -10°C.  If any part of the cloud is colder, then ice is usually present, and solid hydrometeors always grow preferentially over liquid ones in a mixed phase cloud.

The soundings from Fairbanks at 4am and 4pm Wednesday confirm that the low-level cloud layer was shallow, with temperatures no lower than -10°C, and with dry air above the cloud.

Below are the surface and 500mb analyses from 4pm Wednesday, courtesy of Environment Canada.

With cold air aloft (-34°C at 500mb) and a strong anticyclone in place, one might have expected colder conditions at valley level, but all the moisture kept temperatures relatively high.  This is typical of freezing drizzle situations: in 34 days with freezing drizzle at Fairbanks since 1998, the daily mean temperature was above normal in 32 of 34 cases.

Here's a chart showing the number of hours each winter with freezing drizzle reported by the ASOS instrument at the top of the hour.  This is now the 8th consecutive winter with at least one occurrence.

Freezing Drizzle

Residents of Fairbanks experienced one of the more unpleasant kinds of wintry weather at times last night and this morning: freezing drizzle.  At least, that's what was reported by automated instruments for a period of 7 hours at the international airport and more briefly at Fort Wainwright and Eielson AFB.

Freezing drizzle occurs when cloud droplets grow large enough to produce drizzle at the same time that (a) the cloud and underlying air temperature are below freezing, and (b) the cloud top is no colder than roughly -10°C.  The cloud and drizzle droplets are therefore supercooled, but the cloud is not cold enough to contain much or any ice.  (See this old post for a more detailed discussion.)

Here are the atmospheric profile measurements from Fairbanks airport at 3pm yesterday (top) and 3am today (bottom).  The observations fit the conceptual model for freezing drizzle formation quite nicely, with a saturated layer in the lowest few thousand feet - where the red (temperature) and green (dewpoint) lines are superimposed - but cloud-free air above that.

Here are a couple of shots from the Ester Dome webcam that normally looks out over Fairbanks.  These were taken after the freezing drizzle ended, but they show the low cloud layer below with fair weather above.

Looking at hourly ASOS data since 2000, Fairbanks tends to report freezing drizzle on just one or two days a year, on average, although the distribution is very uneven: a few winters account for most of the events.  Looking at a handful of sites across Alaska, here's the percentage of all observations (throughout the year) at which drizzle is reported along with a temperature below freezing:

Utqiaġvik/Barrow   0.39%
Bettles   0.08%
Fairbanks   0.06%
Anchorage   0.05%
Northway   0.03%
McGrath   0.03%
Tanana   0.02%
Gulkana   0.02%

I'm slightly suspicious of the data, because in more than 18 years of data there are zero reports of freezing drizzle in Eagle, Delta Junction, or Kaltag - all of which I understand to have the same ASOS instrumentation as the sites above.  Perhaps there are in fact some instrument differences that contribute to the variation in the results.

Here's the seasonal distribution of freezing drizzle reports in Fairbanks; less than 20 years is insufficient to build up a good climatology, but there does seem to be a preference for these events in early winter rather than mid-late winter.

Up in Utqiaġvik, where freezing drizzle is much more common, the majority of events occur in early winter and late spring; it is very unusual in deep winter, when moisture levels and temperatures are usually both too low to produce the phenomenon.