CIMSS Satellite Blog

GOES-12 and GOES-13 visible images

GOES-12 and GOES-13 visible images (above) showed the development of a narrow band of terrain-forced “standing wave clouds” over extreme northeastern Minnesota on 21 November 2008. Surface wind barbs (plotted in cyan) indicated that the surface winds were generally from the northwest at speeds of 10 knots or less across the region; however, the cloud motions suggested that the northwesterly winds at higher altitudes were a bit stronger. This northwesterly wind direction was perpendicular to the higher terrain of the “North Shore Ridge” — where elevations rise to 2000-2100 feet — which runs from southwest to northeast across the Arrowhead Region of northeastern Minnesota (topography image courtesy of Rick Kohrs, SSEC).

The fact that a thin shadow was cast on the surface along the northern edge of the cloud band indicated that this cloud feature was either fairly deep, or was located at a fairly high altitude. Note that a cirrus plume can be seen that was apparently being sheared off the northern portion of the main standing wave cloud band, which was then carried south-southeastward across Lake Superior by the stronger winds aloft. AWIPS images of the MODIS visible, 11.0 µm IR window, Cloud Top Temperature, and Cloud Phase at 16:22 UTC (below) indicated that a significant portion of the aforementioned cirrus plume was colder than -30º C, with the Cloud Phase product indicating that Ice cloud was present (pink color enhancement).

MODIS visible, IR window, Cloud Top Temperature, and Cloud Phase

Vertical cross sections of RUC13 model fields (below, courtesy of Dan Miller, Science and Operations Officer at the Duluth MN National Weather Service forecast office) did a fairly realistic job of depicting a deep pocket of upward vertical velocity (Omega, purple contours) within the 800-300 hPa layer that was providing the forcing for the standing wave cloud band — and as moist layers (Relative Humidity greater than 50%, green shading) passed through the deep pocket of Omega, a standing wave cloud band formed that could then seen on satellite imagery. The higher-altitude moist layer arriving at the later time periods seems to correspond to the layer that produced the cirrus plume — and this higher layer was at temperatures colder than -30º C, in agreement with the temperatures seen on the MODIS IR image and Cloud Top Temperature product.

Vertical cross sections of RUC13 model fields

A closer view using 250-meter resolution MODIS true color and false color imagery from the SSEC MODIS Today site (below) actually depicted two separate standing wave cloud bands, with the high-altitude cirrus streaming off the upper portion of the bands showing up quite nicely. One could also see that many of the smaller lakes across northeastern Minnesota were either completely frozen or were in the process of becoming ice-covered.

MODIS true color and false color images

MODIS Fog/Stratus product + topography

A cold and moist upslope (northeasterly) flow had pushed up against the Front Range of the Rocky Mountains in Colorado during the pre-dawn hours on 20 November 2008 — a comparison of AWIPS images of the MODIS Fog/Stratus product and the topography (above) showed that the western edge of the stratus deck was backed up against the highest terrain that runs north-south across central Colorado.

AWIPS images of the MODIS Fog/Stratus product, Cloud Top Temperature (CTT) product, and Cloud Phase product (below) indicated that  CTT values were generally in the -8 to -15º C range, with the Cloud Phase product indicating that the cloud was composed of supercooled water droplets. Without the presence of ice crystals in the clouds, the resulting precipitation type across parts of eastern Colorado was freezing drizzle at locations whose surface air temperature had dropped below freezing, as was seen at Denver/Centennial Airport (KAPA) and Elbert Mountain (KMNH) at 10:00 UTC (4 AM local time). Winter Weather Advisories were issued for a number of counties, due to the freezing drizzle causing roadway icing.

MODIS Fog/Stratus product, Cloud Top Temperature product, and Cloud Phase product

This extensive stratus cloud deck could be further characterized by examining the GOES-12 Fog/Stratus product in conjunction with the Low Cloud Base product and the sounder-derived Cloud Top Height product (below) — the cloud bases were all below 1000 feet (green color enhancement), while the cloud tops were generally in the 12,000-15,000 foot range (light green to dark green color enhancement). Using the AWIPS “Sample Cloud Heights/Radar Skew-T” functionality, the cloud top height in the Elbert Mountain area (where the MODIS IR brightness temperature was around +9ºC) was placed at around 11,000 feet, in close ‘agreement with the GOES Sounder Cloud Top Height product.

GOES-12 Fog/Stratus product, Low Cloud Base product, and Cloud Top Height product

Finally, it is interesting to point out the improved accuracy of the 1-km resolution MODIS Fog/Stratus product compared to the 4-km resolution GOES-12 Fog/Stratus product (below). In particular, note how the MODIS imagery displayed a “hole” in the stratus deck in the area of Pike’s Peak (located just to the west of Colorado Springs, KCOS), directly  where the higher terrain would have extended above the top of the stratus cloud layer — on the GOES-12 Fog/Stratus product, the “hole” in the stratus deck was incorrectly placed a bit farther to the west.

MODIS and GOES-12 Fog/Stratus product + topography