Mountain wave clouds in western Montana

September 20th, 2015 |


As pointed out in a Tweet from NWS Great Falls (above), a “chinook arch” mountain wave cloud feature had formed in response to strong westerly winds interacting with the high terrain of the Rocky Mountains of western Montana on 20 September 2015. GOES-13 visible (0.63 µm) images (below; also available as an MP4 movie file) showed the development and evolution of the lower-altitude parallel bands of mountain wave clouds, as well as the larger patch of higher-altitude cloud immediately downwind of the eastern edge of the highest terrain (comparison of Suomi NPP VIIRS visible image and terrain) .

GOES-13 visible (0.63 µm) images [click to play animation]

GOES-13 visible (0.63 µm) images [click to play animation]

The corresponding GOES-13 infrared (10.7 µm) images (below; also available as an MP4 movie file) revealed that the large patch of high-altitude cloud (sometimes referred to as a “banner cloud”) began to grow in areal coverage after about 12 UTC, eventually exhibiting cloud-top IR brightness temperatures in the -50 to -55º C range (yellow to orange color enhancement).

GOES-13 infrared (10.7 um) images [click to play animation]

GOES-13 infrared (10.7 um) images [click to play animation]

The GOES-13 water vapor (6.5 µm) images (below; also available as an MP4 movie file) did not show the common signature of a jet streak axis (a well-defined moist-to-dry gradient) until later in the day.

GOES-13 water vapor (6.5 um) images [click to play animation]

GOES-13 water vapor (6.5 um) images [click to play animation]

A comparison of Aqua MODIS visible (0.65 µm) and “cirrus detection” (1.375 µm) images at 1949 UTC (below) demonstrated how the cirrus channel imagery can be used to better discriminate between the high-altitude ice clouds (brighter white features) and the low-altitude water and/or supercooled water droplet clouds.

Aqua MODIS visible (0.65 um) and

Aqua MODIS visible (0.65 um) and “cirrus detection” (1.375 um) images [click to enlarge]

A comparison of Suomi NPP VIIRS visible (0.64 µm), infrared window (11.45 µm) and shortwave infrared (3.74 µm) images at 2057 UTC (below) showed the high-altitude banner cloud feature as it was beginning to enter its dissipation phase. The 11.45 µm cloud-top IR brightness temperatures were as cold as -57º C; however, note that the 3.74 µm shortwave IR brightness temperatures were significantly warmer (in the +5 to +10º C range). These warm shortwave IR temperatures indicated that the banner cloud feature was composed of very small ice crystals, which were effective at reflecting incoming solar radiation back toward the satellite sensors.

Suomi NPP VIIRS visible (0.64 um), infrared window (11.45 um), and shortwave infrared (3.74 um) images [click to enlarge]

Suomi NPP VIIRS visible (0.64 um), infrared window (11.45 um), and shortwave infrared (3.74 um) images [click to enlarge]