Wednesday, March 30, 2011

Riming in Action

In the previous post, I suggested that we might be in for a period of riming in the mountains, which we often see in moist, northwesterly flow downstream of a low-amplitude upper-level ridge (a.k.a., the dirty ridge).

That riming appears to have come to fruition.  The Utah Avalanche Center reports "overcast skies and light snow, rime, and graupel" in the mountains this morning and there are multiple pilot reports along the Wasatch Front of light or light-to-moderate icing.

Maximum icing severity analysis with pilot reports of icing annotated.
Source: NOAA/NWS Aviation Weather Center
Frequent blog reader David clued me into the the product above, which is is produced by the NOAA/National Weather Service Aviation Weather Center and available here.  For you science geeks, the techniques used to produce the analysis are described by Bernstein et al. (2005).    I suspect lead author Ben Bernstein, who was my next door neighbor as an undergraduate at Penn State, never expected that it might be useful for ski forecasting!

Riming and aircraft icing occur in clouds that contain large amounts of supercooled liquid water.  Ice crystals have a difficult time forming at temperatures near freezing but warmer than -10C.  Since there is some snow forming in the mountains today, obviously we're able to form some ice crystals, but there's also a lot of supercooled liquid water.  The morning sounding shows why.  Note the strong inversion at about 640 mb and that cloud top temperatures are near or just below -10C.

Source: NCAR/RAL
The area downstream of a low-amplitude upper-level ridge is where one frequently finds shallow altostratus layers that contain large amounts of supercooled liquid water.  This year we've had several events of this type and now we have yet another.

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