Saturday, October 15, 2011

Mountain Induced Cirrus

We have a nice example this morning of upper-level cirrus clouds being generated near or downstream of the Sierra Nevada and Cascade Mountains of northern California.

In particular, note how the coverage of cirrus clouds increases near Lake Tahoe and how you can see a back edge to these clouds near or just downstream of the Sierra–Cascade crest.

Here's an even better image (thanks Dale Durran) of mountain induced cirrus downstream of the High Sierra from another event.

Most of us think about clouds forming on the windward side, but some mountain waves tilt upstream with height and, while the low-level flow might be sinking downstream of the mountains, at upper-levels it is rising.  For example, in the model simulation below, the flow moves from left to right and roughly parallels the black contours.  Note how contours slope downward in the lee of the barrier at low levels, consistent with sinking motion.  The mountain wave, however, slopes upstream with height, so that the strongest sinking motion sits over the barrier when you get to around 5000 m or higher.  Further, downstream of this subsidence, the air rises, much like one sees when water flows over a rock in a river.

Source: Jim Doyle, NRL
 It is in this area of downstream rising motion that mountain induced cirrus clouds form.  It is also in this area that gliders can reach extreme altitudes.


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  2. So this is what's ruining my cloud forecasts.