Friday, November 30, 2012

Incredible Cloud Gallery

Some spectacular clouds could be found over the Salt Lake Valley this morning.  First, we have a marvelous example of an orographic cloud over Lone Peak.  It's been sitting there since yesterday and is produced by southerly and southwesterly flow being forced up and over Lone Peak and the high divide that separates Little Cottonwood and American Fork Canyons.

Even better are examples of Kelvin Helmholtz Billows, referred to by meteorologists as simply KH.  KH billows look like waves breaking on the beach.

Photo: Peter Veals
Water waves break because friction slows the wave more at the ocean floor than near the ocean surface.  This causes the wave to steepen and eventually break.

KH waves in the atmosphere are produced by strong vertical wind shear.  In the photo above, the flow at the top of the wave is stronger than the flow at the bottom.  This leads to wave breaking much like occurs in the ocean.  Note that wave breaking is evident in the altostratus cloud near the top of the photo, but also near the top of the orographic cloud that is parked over Lone Peak.

The morning sounding from the Salt Lake City airport shows regions where the wind speed increases rapidly with height.

The first is from the surface to about 775 mb (~2200 m MSL).  This is associated with a strong southerly low-level jet produced by flow channeling in the Salt Lake Valley.  While there could be wave breaking in this layer, there are no clouds, so you can't see it (but you might feel it if you were on a plane).

The second begins just above 700 mb (~3000 m MSL).  This is likely contributing to the wave breaking with the orographic cloud.  One of the shear layers above this level is responsible for the wave breaking with the altostratus cloud.

No comments:

Post a Comment