About Cody Townsend's Sierra Rotor

I'm a big fan of Cody Townsend's The Fifty, which documents his efforts to ski all the lines in the book Fifty Classic Ski Descents of North America.   I've skied a few of these lines and dreamed of skiing a few others.  Some are beyond either my physical or technical abilities, or the suffering to powder ratio is too high to interest me.  Cody and cinematographer Bjarne Salén do a remarkable job capturing footage of their adventures and bringing me along for the tour.  I keep thinking about all of the parameters they are dealing with including the snowpack, weather, route finding, climbing hazards, and ski challenges, and yet they are also shooting some great footage (and carrying all that gear).  It's an impressive accomplishment.  

The latest episode, Tragedy and Triumph in the Split Couloir, documents their attempts to ski the extremely challenging (and often snow starved) Split Couloir on the 14,058 foot Split Mountain in California's Eastern Sierra.  In one attempt, they are blown off the mountain by strong winds.  In a recent Instagram Post, Cody attributed these winds to "the Sierra Rotor", an "extreme terrain-induced downslope wind."   

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I'm not sure what day they were on the mountain, but watching the episode a couple of weeks ago, I immediately thought that they were in a downslope windstorm.  Let's take a look at what may have happened and why the term "Sierra Rotor" is perhaps not best used for what they experienced.

Severe downslope windstorms occur on the lee-side of mountain ranges around the world.  They have many regional names including the Foehn (northern Alps), Bora (Croatia), and Chinook (Rockies).  In Utah they are sometimes referred to as "Canyon Winds." They are produced by what meteorologists call "high-amplitude mountain waves."  These are the atmospheric equivalent of a hydraulic in a river in which the flow accelerates downstream of a rock or rocks and then decelerates in a turbulent hydraulic jump.  

Source: Wikimedia Commons, Author: aokomoriuta

This also happens in some situations during flow across mountain ranges as shown in the schematic below. In some cases a rotor can form downstream of the hydraulic jump.

Source: Whiteman 2000

The rotor is a component of the downslope windstorm system.  In a well developed rotor, there is flow reversal at the surface.  Incredibly, you can go from severe downslope winds in one direction to relatively light flow from another direction over a short distance.  

The eastern Sierra and especially the Owens Valley are well known for severe downslope windstorms and rotors.  Some of the seminal research on severe downslope windstorms was conducted in this area during the Sierra Wave Project in the 1950s (see Sierra Wave Project Revisited by Vanda Grubišić and John M. Lewis) and the Terrain-induced Rotors Experiment (T-REX) in 2006.  

Below is a classic photo of a Sierra downslope windstorm taken by Robert Symons.  The view is toward the south with the eastern Sierra on the right.  The flow is from right to left.  Strong downslope winds extend from the Sierra across the Owens valley where they pick up dust.  This dust is then elevated in the rotor circulation.  The ascent is deep enough that clouds form near the top of the rotor. 

One of my favorite papers on rotors (and subrotors, which are embedded in rotors) is Doyle and Durran (2007).  In it, they present the average cross-mountain wind speed (top panel color fill with reds away and blues toward the mountain) and vectors (bottom panel).  Note that in this figure the mountain is on the left. One can see the intense flow descending the mountain, with the speed maximizing just before the rotor.  The strong winds then elevate in the hydrolic jump and follow an arc over the rotor.  Near the base of the rotor (i.e. at the ground), the flow is actually reversed and is moving toward the mountain. The transition from strong downslope flow to light reversed flow occurs over a distance of less than 2 kilometers.  

Source: Doyle and Durran (2007)

The rotor is not the cause of the downslope windstorm, it is a component of it.  Not all downslope windstorms produce rotors and in some the rotor circulation may wax and wane.  The rotor circulation is sensitive to the of the characteristics of the atmospheric flow impinging on the mountain range (i.e., wind and stability) and affected by the presence of downstream ranges like the Inyo Range in the case of the Owens Valley.  

So, Cody Townsend did not experience the rotor while on Split Mountain.  He experienced the downslope wind.  The rotor, if it existed, would have been over the Owens Valley.  It's probably best not to call this phenomenon the Sierra Rotor since the rotor is a component of the downslope wind system and isn't always present.  That said, it would be appropriate to refer to the circulation beneath the hydraulic jump and Owens Valley as the Sierra Rotor.  

Congrats to Cody for eventually skiing the Split Couloir.