Saturday, September 22, 2012

More Interesting Morning Clouds

Regarding the previous post, Mike Crimmins commented that the clouds looked like, from the satellite perspective, celluar convection over the cold waters of the Pacific. Indeed, from the top, the morning clouds the last several days have looked a bit strange. This morning they were around again (visible image above from 7:15 am MST), and they were producing quite a bit of virga, and one or two slight higher towers had mammatus. The photo below shows how shallow these clouds were this morning.

The morning sounding from Tucson (above is skewT plot of 12 UTC data from Univ. of Wyoming) shows that the atmosphere is essentially split in half. The old boundary layer from yesterday afternoon fills up the lower half, reaching up to about 500 mb with light winds, generally from the south. Above that the top half of the atmosphere is probably weakly subsiding with northwesterly flow around an upper-anticyclone, and producing a nasty inversion just above 500 mb. So, the morning clouds are trapped within a very shallow layer centered around 500 mb. The temperatures in the cloud-layer are a bit colder than -10C, so that the weak updrafts are very effective in producing ice crystals. This leads to some significant virga (see photo below) as snow falls from the shallow clouds. We see things like this a number of times during very hot and dry conditions, but rarely are such clouds so widespread as they've been the couple of mornings.

Off to the far south, the NHC is now tracking TS Miriam (see below). The NHC predicts that Miriam will become a hurricane as it moves toward the northwest, staying fairly far out in the Pacific. The storm is predicted to turn on a more northerly course after it weakens mid-week - much like TS Kristy did. Thus, the details of how the storm might affect the Southwest remain nebulous. Several of the GFS ensemble members bring the storm much closer to Baja than does the NHC forecast. However, the GFS ensemble members have a very large spread wrt the forecasted center of Miriam at 120 hours - so we'll just have to watch as things evolve at lower latitudes.

No comments:

Post a Comment