Polar low over the western Bering Sea

March 3rd, 2013
Suomi NPP VIIRS 0.7 µm Day/Night Band images

Suomi NPP VIIRS 0.7 µm Day/Night Band images

AWIPS images of 1-km resolution Suomi NPP VIIRS 0.7 µm Day/Night Band data (above) showed the development of a small polar low in the far western Bering Sea during the 02 March – 03 March 2013 period. A string of breaking Kelvin-Helmholtz waves could be seen feeding into the circulation of the developing low. Station identifiers 25941, 25954, and 21956 denote the villages of Cemurnaut, Korf, and Apuka (respectively) located on the northern end of the Kamchatka Peninsula of Russia.

The corresponding Suomi NPP VIIRS 11.45 µm IR channel images (below) indicated that cloud top IR brightness temperatures were colder than -30º C (darker blue color enhancement) near the center of the polar low circulation.

Suomi NPP VIIRS 11.45 µm IR channel images

Suomi NPP VIIRS 11.45 µm IR channel images

Suomi NPP VIIRS false-color Red/Green/Blue (RGB) images (below) showed that the polar low was developing just south of the sea ice edge (snow and ice appear as darker shades of red). The appearance of red shading also indicated that the cloud tops along the string of Kelvin-Helmholtz waves were beginning to glaciate. Note from the distance scale plotted on the lower left that the diameter of the polar low circulation was less than 100 miles.

Suomi NPP VIIRS false-color Red/Green/Blue (RGB) images

Suomi NPP VIIRS false-color Red/Green/Blue (RGB) images

A larger-scale view using a Suomi NPP VIIRS 0.64 µm visible channel image (below) helped to emphasize the small size of the Bering Sea polar low, especially when compared to the much larger storm system that was located just south of the Aleutian Islands at that time. The tightly-packed isobars of another strong storm approaching from the North Pacific Ocean could also be seen.

Suomi NPP VIIRS 0.64 µm visible channel image + Surface reports and surface analysis

Suomi NPP VIIRS 0.64 µm visible channel image + Surface reports and surface analysis

McIDAS images of MTSAT-2 0.73 µm visible channel data (below; click image to play animation) indicated that the polar low was initially moving southeastward away from the Kamchatka Peninsula, but then began to reverse direction and move back northwestward due to strong southeasterly flow in advance of the large and intense storm over the North Pacific Ocean.

MTSAT-2 0.73 µm visible channel images (click image to play animation)

MTSAT-2 0.73 µm visible channel images (click image to play animation)

Mountain wave “banner cloud” over Montana

March 1st, 2013
GOES-15 (left) and GOES-13 (right) 6.5 µm water vapor channel images (click image to play animation)

GOES-15 (left) and GOES-13 (right) 6.5 µm water vapor channel images (click image to play animation)

McIDAS images of 4-km resolution GOES-15 (GOES-West) and GOES-13 (GOES-East) 6.5 µm water vapor channel images (above; click image to play animation) showed a well-defined high altitude mountain wave cloud (or “banner cloud”) immediately downwind of the high elevations of the Rocky Mountains in western Montana on 01 March 2013.

A comparison of AWIPS images of 1-km resolution Suomi NPP VIIRS 0.64 µm visible channel, 11.45 µm IR channel, and 3.74 µm shortwave IR channel image at 20:47 UTC (below) revealed that this banner cloud exhibited 11.45 µm IR cloud top brightness temperatures as cold as -70º C (darker black color enhancement), yet on the 3.74 µm shortwave IR image the cloud top brightness temperatures were as warm as +24º C. These very warm shortwave IR brightness temperature values indicated that the banner cloud was composed of ice crystals that were quite small, and were thus very efficient reflectors of incoming solar radiation (reference: Ackerman, S. A., C. C. Moeller, K. I. Strabala, H. E. Gerber, L. E. Gumley, W. P. Menzel, and S-C Tsay, 1998: Retrieval of Effective Microphyscial Properties of Clouds: a Wave Cloud Case Study, Geophys. Res. Lett., 25, 1121-1124.)

Suomi NPP VIIRS 0.64 µm visible channel, 11.45 µm IR channel, and 3.74 µm shortwave IR channel images

Suomi NPP VIIRS 0.64 µm visible channel, 11.45 µm IR channel, and 3.74 µm shortwave IR channel images

The 1-km resolution POES AVHRR Cloud Top Height product (below) indicated that portions of this banner cloud were as high as 12 km (or around 39,000 feet). While an AIRMET had been issued advising of the possibility of moderate turbulence below 20,000 feet across the region, there was one pilot report of moderate turbulence at 38,000 feet earlier in the day at 16:37 UTC in the vicinity of the banner cloud.

POES AVHRR Cloud Height product

POES AVHRR Cloud Height product

Strong westerly to southwesterly winds flowing across the higher elevations of the Rocky Mountains were causing a chinook wind event to occur, as adiabatic compression of the downsloping winds warmed the air. Some of the warmest surface air temeratures were seen in areas of southern Alberta, Canada that were free of snow cover — in that region MODIS Land Surface Temperature values exceeded 60º F (below).

MODIS Land Surface Temperature product + MODIS 0.65 µm visible channel image

MODIS Land Surface Temperature product + MODIS 0.65 µm visible channel image

===== 02 March Update =====

In a night-time comparison of Suomi NPP VIIRS 0.7 µm Day/Night Band, 11.45 µm IR channel, and 3.74 µm shortwave IR channel images at 09:02 UTC or 3:02 AM local time (below), note that in the absence of reflcted sunlight the coldest shortwave IR brightness temperatures seen within the banner cloud (-63º C) were much closer to those seen on the 11.45 µm IR image (-69º C).

Suomi NPP VIIRS 0.7 µm Day/Night Band, 11.45 µm IR channel, and 3,74 µm shortwave IR channel images

Suomi NPP VIIRS 0.7 µm Day/Night Band, 11.45 µm IR channel, and 3,74 µm shortwave IR channel images

Historic blizzard affects parts of Texas, Oklahoma, and Kansas

February 26th, 2013
GOES-13 6.5 µm water vapor channel images (click image to play animation)

GOES-13 6.5 µm water vapor channel images (click image to play animation)

A powerful winter storm brought historic snowfall amounts and widespread blizzard conditions to parts of Texas, Oklahoma, and Kansas during the 25 February26 February 2013 period (see additional information from the NWS forecast offices at Amarillo TX, Norman OK, Dodge City KS, Wichita KS, and Topeka KS). AWIPS images of 4-km resolution GOES-13 6.5 µm water vapor channel images (above; click image to play animation) showed the evolution of the storm system on 25 February, which included the development of well-defined dry slot and comma head signatures.

A comparison of 1-km resolution MODIS 0.65 µm visible channel, 11.0 µm IR channel, and 6.7 µm water vapor channel images (below) revealed a snapshot of the storm at 20:00 UTC or 3 PM local time on 25 February. A line of deep convection exhibiting cold cloud top temperatures extended from the Gulf of Mexico northward into Missouri, which produced large hail, damaging winds, and a tornado (SPC storm reports).

MODIS 0.65 µm visible, 11.0 µm IR, and 6.7 µm water vapor channel images

MODIS 0.65 µm visible, 11.0 µm IR, and 6.7 µm water vapor channel images

Very strong winds were associated with this storm, which created a large area of blowing dust across southwest Texas and southeastern New Mexico on 24 February — and GOES-13 0.63 µm visible channel images (below; click image to play animation) revealed additional areas of blowing dust across drought-stricken areas of southern Texas on 25 February, where winds gusted as high as 56 mph and visibilities were reduced to 1 mile or less in some locations (see NWS Brownsville TX summary).

GOES-13 0.63 µm visible channel images (click image to play animation)

GOES-13 0.63 µm visible channel images (click image to play animation)

During the following overnight hours the clouds had cleared across the Texas panhandle region, which allowed the Suomi NPP VIIRS 0.7 µm Day/Night Band (below) to provide a “visible image at night” (aided by bright illumination from the “full snow moon”) to display the areal extent of the fresh snow cover at 08:38 UTC or 3:38 AM local time. While the deep snow pack appeared somewhat colder on the corresponding VIIRS 11.45 µm IR image, the exact edges of the snow cover were easier to see on the Day/Night Band image.

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR images

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR images

During the afternoon hours on 26 February, a comparison of the Suomi NPP VIIRS 0.64 µm visible channel image with the corresponding false-color Red/Green/Blue (RGB) image at 20:02 UTC or 3:02 PM local time (below) aided in the discrimination between snow cover (varying shades of darker red on the RGB image) and supercooled water droplet cloud features (lighter shades of white). Glaciated (ice crystal) cloud features exhibit a lighter pink appearance in the RGB image.

Suomi NPP VIIRS 0.64 µm visible and False-color Red/Green/Blue (RGB) composite images

Suomi NPP VIIRS 0.64 µm visible and False-color Red/Green/Blue (RGB) composite images

Mountain wave turbulence over the Mid-Atlantic states

February 20th, 2013
MODIS 0.65 µm visible channel and 6.7 µm water vapor channel images

MODIS 0.65 µm visible channel and 6.7 µm water vapor channel images

A comparison of AWIPS images of 1-km resolution MODIS 0.65 µm visible channel and 6.7 µm water vapor channel data (above) revealed the presence of widespread mountain waves across parts of the Mid-Atlantic states on 20 February 2013. Parallel bands of rotor clouds helped to identify the location of these waves (caused by strong northwesterly winds interacting with the terrain of the Appalachian Mountains) where ample moisture was present, but in many areas the atmosphere at that altitude was too dry to support rotor cloud development — this demonstrated the advantage that water vapor imagery has in helping to know the total areal coverage of such mountain wave activity.

Mountain waves can be an aviation hazard, since they are capable of generating turbulence. Plots o pilot reports of turbulence within +/- 30 minutes of the MODIS overpass time indicated that there was one report of severe turbulence in the 7000-9000 foot altitude range over extreme northwestern Virginia, along with moderate turbulence likely associated with rotor circulation wind shear in northern Virginia and southern Virginia. In addition, there was a report of moderate clear air turbulence at 36,000 feet over eastern Chesapeake Bay, suggesting that these mountain waves might be vertically propagating.