Saturday, February 8, 2020

A Trip to Bizzaro World

Today's ski tour was one of the more surreal that I can recall.

For better or worse, we decided to leave late, opting to head out mid morning.  It seemed pointless to deal with the canyon traffic, but with the weather of the past week, it seemed essential to get to high elevations in the hope of finding decent snow.

While driving south on I-215, the sign flashed "Little Cottonwood Closed", which we expected, but then "Big Cottonwood Closed" which we didn't. 

We opted to drive to the canyon mouth anyway, where the sign said the canyon was closed above mile post 11.  Hooray, we can find something to ski below that, even if it's just a walk in the woods.  We talked our way through the bottom roadblock and then another partway up the canyon.  We ended up skinning up the Mineral Fork road in complete solitude. 

The snowpack at elevations we skied was topped by a stout crust.  In places it was 5 cm thick. 

A close look at the crust, which my photo doesn't really reveal, showed that there were two layers, with the one on top relatively clear and perhaps 5 mm thick. 

Clear ice was also evident on tree branches. 

Rime ice is not uncommon in mountain areas, but is typically produced by smaller cloud droplets and has a white, translucent look.  Clear ice of the type above requires bigger droplets.  At least drizzle sized if not larger. 

Thus, the clear ice is evidence of either freezing drizzle or freezing rain.  The distinction between drizzle and freezing rain can be made based on size (drizzle droplets have diameters < 0.5 mm, rain > 0.5 mm) or process.  With regards to the latter, the pathway to freezing rain typically involves snow falling through a warm layer (>0˚C), melting into rain, and then falling into a subfreezing layer where it becomes supercooled but does not freeze until it contacts an object.  This didn't happen yesterday.

Instead, some hints at what happened are provided in the 0000 UTC sounding from yesterday afternoon.  Note how the dewpoint (green line) and temperature (red lines) meet near 700 mb and parallel each other to about 650 mb.  That is reflective of a shallow cloud layer.  At 650 mb, where the lines spread out again, is cloud top.  There, temperatures are just a bit higher than -10˚C. 

Source: SPC
Clouds that extend to altitudes where the temperature is lower than -10˚C typically glaciate, meaning they convert into a cloud that is part ice and part supercooled water.  However, clouds that do not extend to altitudes where the temperature is lower than -10˚C often consist of very little ice and instead are comprised primarily of supercooled liquid water droplets.

When such clouds intersect the mountains, they can cause riming, but something happened yesterday to cause large droplets to form.  I think this occurred for two reasons.  First, the rate of change of temperature in the cloud layer is about 0.65˚C/100 meters.  In a cloud, that results in air parcels that are statically "neutral," meaning if you give them a push up or down, they can easily move further up or down.

Second, there is wind shear in this layer.  Such shear, in a neutral environment, can easily cause turbulence and overturning.  That turbulence and overturning enables larger droplets to grow.  I suspect this led to large drizzle or even small rain droplets that in turn froze on contact with the snow (or any other surfaces). 

This mechanism was first proposed as a contributor to precipitation enhancement by Bob Houze and Socorro Medina of the University of Washington based on data collected in the Cascade Mountains and European Alps.  Their conceptual model is shown below and highlights the turbulent, overturning cells in the shear layer.  Below the melting band, this leads to precipitation enhancement as droplets grow through collision and coalescence.  Above it, through the riming of snowflakes. 

Their conceptual model assumes a deep cloud that is glaciated.  It appears that at some point yesterday we were dealing with a cloud that had a great deal of supercooled liquid water.  Thus, the turbulence and overturning led to large drizzle drops. 

At least this is my hypothesis.  I wasn't out yesterday to observe the event.  Please share your observations in the comments and perhaps they will support the hypothesis or suggest revision. 

One thing is for sure, that crust is supportive and it actually skied reasonably well on lower angle terrain.  It is also here to stay for a while.  It may be an issue for avalanche safety in the coming days or even weeks. 


  1. Observations from Powder Mountain: The crust was not as thick as what you observed. Clear ice on the chairs was up to a centimeter thick. Yesterday (2/8/19), the crust was not consistently supportive for skiing - boarders seemed to fair better. In some areas, a lot of small aspen limbs had broken from the weight of ice and rime. There was crust from top to bottom - 8900 feet down to 6900 feet - thicker up high. The groomers did a good job tilling and groomed slopes were skiing well by the end of the day. But the rest of it - yuck.

  2. Friday - Summit park to 8600 ft. I couldn't decide if it was rime or freezing rain- but it was windy! The shear process you describe above makes sense. The snow surface was breakable sheet ice on the ridge but dense, creamy fast powder in the NE facing trees. Saturday - skied upper BCC. Same ski conditions on S quarter but no evidence of a rain crust from 10k down to 8000. I was quite surprised about this. I expected more evidence of rain but the trees were rimed, not iced.

  3. Thanks for this post, it makes me feel very good about my decision to take a couple of days off touring and rest. Sat 2/8 I napped and read a Journal of Economic Perspectives symposium on populism, today (Sun 2/9) I will nap and read a symposium on financial stability regulation. You may have heard I retired last year and that I have been ski touring virtually every day this season. Your experience Sat, both snow and police roadblocks are what I expected, so napping, based on your post, and the UAC obs page, was a superior choice for me. As good a time as any to get recharged for more hiking.

    I was touring Porter Fork every day last week. If you haven’t seen them I posted two obs to the UAC page.

    Thursday 2/6

    Friday 2/7

    Thurs ob is a summary of the week, including the Monday storm and variation in snow amounts. Amazing the bottom of the Icebox got 2 feet Monday, Peak 9661 south got 1 foot, and westfacing Raymond got 6 inches. Snowfall declined from 2 feet to 6 inches in a half mile.

    I didn’t mention it in the Thurs ob, but there seemed to be a rime event on my exit at 2pm. You know what these feel like, the air around you suddenly moistens. But my ski poles and goggles didn’t ice up so I wasn’t sure.

    Sure enough, you’ll see in the Fri ob the same photos of icy branches and inch thick crust you experienced Sat. The icy branch occurred sometime between Thurs pm and Friday am. The surface Thurs 2pm was 6 inches dense snow on top of 18 inches of fluff which made for difficult skiing. Precip Thurs was wet snow, grauple/dendrite mix until riming started. Friday was graupel/dendrite/rime dust on a generally supportable crust at 9000, less so at 8000.

    As to the weather process, at 6000 feet there was standing water in the parking lot and an inch or two of slushy snow, not frozen, on the trail Friday morning. My guess is precipitation overnight Thurs at 9000 feet was that same graupel/dendrite/rime mix that didn’t freeze into a crust until a pause in the storm early Friday morning when air temps caused the surface to harden from the top down about an inch. The storm resumed w light mixed precip during the day Friday.

    All I know for sure is that going forward in the years ahead I was thinking Friday we in the Wasatch will need to get up to speed on the snow vocabulary of the Pacific Northwest.

    As to avalanches, I liked your UAC ob where you said the snow was “completely locked up” as I used the same phrase in my Fri ob. You really think the hazard was low? From the couch, I’m thinking its possible to initiate a fracture in steep rocky terrain that will propagate into a massive D3/D4 avalanche, 300 foot wide 6 foot deep crowns. So I’m thinking moderate going forward, but I’m staying away from steep rocky terrain for the time being.