June 05, 2024

A Very Wavy Atmosphere

The high-resolution MODIS satellite image taken mid-day yesterday was remarkable, with extensive wave-like cloud features east of the Cascade crest in Washington and Oregon (see below).    More waves over Montana as well.


Here is a closer look at the waves over eastern Oregon.  Very regular features with north-south cloud lines and clear zones between them.


Over southern Washington, there were still waves of clouds but they were distorted east of Mt. Rainier into more of a "v" shape.


What is going on here?

You are looking at mountain lee-wave clouds or, more precisely, trapped lee-wave clouds.  You will learn about the "trapped" business in a second.

When air approaches a mountain barrier with sufficient wind speed and under stable conditions, air is pushed up by the mountains and then oscillates up and down downwind of the terrain (on the lee side).  Think of a swing--you give a push and the swing goes back and forth for a while.

When the air goes up it cools (moving to lower pressure causes air to expand and cool), which can result in air becoming saturated (cool air can hold less water vapor than warm air). leading to withcloud formation.  So one can get a series of clouds with clear zones in between (where air is sinking) downwind of a mountain barrier.


The mountain waves are stronger in the lower atmosphere and weaken with height.  Thus, they are "trapped" in the lower atmosphere downwind of the mountain barrier.

If the mountain barrier is an elongated linear feature that is oriented north-south  (like the Cascades), one ends up with north-south lines of clouds and clear zones downstream of the barrier.  Exactly, what we see downstream of the north-south Cascades.

If the barrier is more isolated, the mountain wave clouds appear more like the wake of a ship (see below).  The mountain wave clouds downstream of Mt. Rainier looked like the  "ship wake" clouds.

Can high-resolution weather prediction models correctly simulate and forecast such waves?

I am proud to say that the answer is definitively yes.  Below is a nine-hour prediction of the cloud field from the UW ultra-high resolution WRF prediction model (the kind of thing I work on in my real job) for 2 PM yesterday.

Wow.   Near perfection.


Trapped mountain lee wave clouds are popular destinations for glider pilots, who try to stay in the upward motion associated with clouds.  Free vertical lift courtesy of Mother Nature!


1 comment:

  1. Cliff, any thoughts on the warming central pacific ocean 'The warm Blob' and how it could effect our summer weather?.

    ReplyDelete

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