Monday, February 20, 2017

Orographically Forced Clouds and K-H Instability

Today featured strong south to southwest flow impinging on the central Wasatch, leading to the formation of a very pronounced cap cloud over Lone Peak and the Alpine Ridge separating Little Cottonwood and American Fork Canyons.  The cap cloud was illuminated in sunlight when I arrived home from my ski tour this afternoon.

Cap clouds of the type above form due to orographically (i.e., terrain) forced ascent as flow impinges on the mountain barrier and is forced to rise.  It is very common to see such clouds over Lone Peak and the Alpine Ridge in southerly or southwesterly flow.

This morning, the top of the cloud pattern featured a structure that looked liked breaking waves on a beach.  Such patterns are produced by Kelvin-Helmholtz instability (or K-H instability or just K-H for short).  K-H instability is named after Lord Kelvin and Hermann von Helmholtz, 19th century scientists who made important contributions to meteorology and many other scientific fields and is produced when strong vertical wind shear overwhelms the atmospheric stability, resulting in an overturning flow and turbulence. 

Such instabilities can occur in the absence of clouds, and lead to clear-air turbulence, which you have surely experienced.

For better or worse, we spent much of the day in the lee of the Alpine Ridge and just downstream of the cap cloud.  In wind-sheltered area, the turns were creamy and fun and reminded me a little bit of British Columbia.  For the most part, we didn't see much of the Alpine Ridge, but the Pfeifferhorn made a brief appearance before our last run.

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