Today's Rime Event

 The models have advertised a possible riming event today and tonight in the upper elevations of the central Wasatch and it seems to be setting up as advertised.

As illustrated by the photo below, taken about 8:15 AM looking south from my office on the University of Utah campus, shallow altostratus clouds are now impinging on the highest peaks of the central Wasatch.

The GFS forecast sounding valid 2100 UTC (1400 MST) this afternoon suggests that by this afternoon, the altostratus layer will extend from about 750 mb (8000 ft) to 650 mb (11,775 ft), enveloping much of the upper elevations of the central Wasatch.  

Cloud base temperature will be 0˚C and cloud top temperature about -7˚C, so this cloud will largely be "subfreezing."  

However, water droplets in clouds do not necessarily freeze if the temperature is below 0˚C.  To freeze, water needs an ice nucleating particle to serve as the nucleus for ice crystal formation.  These particles are typically called ice nuclei or IN for short.

The number of particles that can serve as an IN varies depending on the types of particulate matter in the airmass and the temperature.  All else being equal, the lower the temperature, the more particles that can serve as IN.  

At temperatures just below 0˚C, however, there aren't typically many IN available.  As a result, clouds that don't extend to temperatures below about -10˚C tend to contain a lot of supercooled cloud droplets.  If these droplets can combine and grow into larger droplets, especially drizzle, they tend to freeze on contact with a sub-freezing surface, such as trees, the ground/snow, lift towers, buildings, etc.

In general, conditions in Utah do not favor extreme riming events like occurs in other parts of the world, especially in maritime regions like the Olympic and Cascade Mountains, southern Andes, etc. In Patagonia, spectacular rime mushrooms often form and represent a challenge and hazard for mountaineers.

Rime mushrooms on Cerro Torre (Photo: Rolando Garibotti).  Source: Whiteman and Garibotti (2013).

You can read more about them in this article by my college Dave Whiteman and mountaineer Rolando Garibotti.

It's probably a good thing we don't get such extreme riming in Utah, although even our light rime can be an issue for ski area operations, ski conditions, and future avalanche conditions.  

Although the cloud and temperature conditions today and tonight are generally favorable for rime, the severity of the event will depend to large degree on small-scale processes that are more difficult to anticipate.  In particular, with the winds, turbulence, and cloud microphysical processes favor the development of larger supercooled water droplets that rime more efficiently?  Will the clouds get just deep enough that some droplets can freeze and help reduce supercooled water concentrations?  These are questions I can't answer.  We will just have to see how things evolve today and tonight.