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.