CIMSS Satellite Blog

GOES-13 visible images

We created the “What the heck is this?” category just for the type of case that is shown here: GOES-13 visible images (above) displayed an interesting cluster of O-shaped clouds forming over the Eastern Pacific Ocean (near Isla Guadalupe, off the coast of Baja California) on 18 November 2008. A few hours later, an overpass of the QuikSCAT satellite allowed an overlay of WindSat surface wind data on a GOES-11 3.9 µm shortwave IR image  (below) — and the QuikSCAT wind data seemed to suggest that these O-shaped clouds were actually perturbing the general northwesterly marine boundary layer flow to some extent.

GOES-11 3.9 µm shortwave IR image + QuikSCAT winds

MODIS visible, 3.7 µm IR, and 11.0 µm IR images

AWIPS images of the MODIS visible, 3.7 µm shortwave IR, and 11.0 µm IR window channels (above, with an overlay of lower-tropospheric MADIS atmospheric motion vectors) provided two important clues about these cloud features: (1) they were composed of supercooled water droplets, which reflected large amounts of solar radiation leading to a display of very warm (> 30º C, darker gray shades) 3.7 µm brightness temperatures, and (2) they were shallow clouds within the marine boundary layer, with fairly warm cloud top IR window brightness temperatures in the 13-14º C range. These points were further confirmed by examining additional MODIS images (below): the MODIS Cloud Top Temperature (CTT) product showed CTT values of 16-17º C (red color enhancement); the MODIS Cloud Phase product indicated these clouds were composed of supercooled water droplets (blue color enhancement); and the GOES-11 Sounder Cloud Top Height product placed the cloud tops in the 3000-5000 foot range (tan to orange color enhancement).

MODIS visible, Cloud Top Temperature, Cloud Phase + GOES Cloud Top Height

A closer view using 250-meter resolution MODIS true color image from the SSEC MODIS Today site (below) showed impressive structure to the O-shaped clouds, with hints of fine-scale outflow boundaries along the outer edges of some of the cloud features. These cloud features somewhat resemble  Pockets of Open Cells that have been previously documented — these open cells are apparently related to the formation of areas of precipitation (in this case, drizzle) that then act to dissipate a portion of the cloud to the point that a hole forms in the cloud feature. The downdrafts created by the formation of these pockets of open cells may indeed have had enough of an impact on the surface winds to be apparent in the QuikSCAT surface wind data seen above.

250-m resolution MODIS true color image

As an aside, the MODIS Sea Surface Temperature (SST) product (below) showed that SST values were generally in the mid 60s F (darker green colors) over the area where the O-shaped clouds were forming — and there was a well-defined SST gradient just to the south, where SST values rose into the lower 70s F (lighter green to yellow colors).

MODIS Sea Surface Temperature product

GOES-12 10.7 µm and 3.9 µm IR images

GOES-12 10.7 µm “IR window” and 3.9 µm “shortwave IR” images (above) showed a large area of mountain wave clouds to the lee of the spine of the Rocky Mountains, spreading eastward across parts of Alberta and Montana on 12 November 2008. Note how quickly the 3.9 µm IR brightness temperatures increased once  the reflection of solar radiation commenced — the wave clouds almost seem to “disappear” on the 3.9 µm imagery as the daytime sun angle increased, even as 10.7 µm IR brightness temperatures as low as -60º to -70º C (red to black color enhancement) persisted.  While this case was in the same general region as another mountain wave event observed on 03 November, the mountain wave cloud was much larger in this case.

AWIPS images of 1-km resolution MODIS 11.0 µm “IR window”, 3.7 µm “shortwave IR”, 1.3 µm near-IR “Cirrus detection”, 6.7 µm “Water vapor”, and Visible channels (below) allowed a more detailed look at the mountain wave clouds around 19:50 UTC. The coldest 11.0 µm IR brightness temperature over western Montana was -71º C (black enhancement), which corresponded to the 39,200 foot (200 hPa) level according to the Great Falls, Montana rawinsonde data. However, the 3.7 µm brightness temperatures in that same area were around +22º C (about 90º C warmer!), due to the strong reflection of solar radiation by the very small ice crystals that comprised the mountain wave cloud.

MODIS 11.0 µm IR, 3.7 µm IR, Cirrus, Water vapor, and Visible images

The MODIS cirrus detection image with an overlay of MADIS atmospheric motion vectors (below) confirmed the presence of strong winds aloft over the region, with wind speeds of greater than 100 knots (with one target as high as 186 knots over northwestern Montana). The MODIS cirrus image also helped to highlight some subtle cloud-top striations that were present.

MODIS cirrus image + MADIS atmospheric motion vectors