Ice forming in the nearshore waters of Lake Michigan

December 24th, 2013 |
Terra and Aqua MODIS true-color RGB images

Terra and Aqua MODIS true-color RGB images

After a cold night (morning low temperatures of -24ºF were recorded at several inland locations in Wisconsin, along with -12ºF in northern Illinois), ice was seen forming in the western and southern nearshore waters of Lake Michigan on 24 December 2013. The ice motion could be seen on consecutive 250-meter resolution Terra and Aqua MODIS true-color Band 1/4/3 Red/Green/Blue (RGB) images from the SSEC MODIS Today site (above).

A comparison of the Aqua MODIS true-color and Band 7/2/1 false-color RGB images (below) confirms that the brighter features seen along the western and southern nearshore waters were ice — ice and snow (as well as clouds composed of ice crystals) appear as varying shades of cyan in the false-color image, in contrast to supercooled water droplet clouds which appear as varyinng shades of white.

Aqua MODIS true-color and false-color RGB images

Aqua MODIS true-color and false-color RGB images

Freezing rain in Oklahoma, with a broad swath of snow to the northwest

December 22nd, 2013 |
Suomi NPP VIIRS 0.64 µm visible channel and false-color snow/ice-vs-cloud RGB images

Suomi NPP VIIRS 0.64 µm visible channel and false-color snow/ice-vs-cloud RGB images

A large portion of central and southwestern Oklahoma experienced a signifcant freeezing rain event during the 20 December21 December 2013 period, as abundant moisture moved over shallow cold air at the surface (NWS Norman OK event summary). On the following day, as clouds began to clear across the region, AWIPS images of Suomi NPP VIIRS 0.64 µm visible channel data and the corresponding false-color Red/Green/Blue (RGB) product (above) revealed a broad swath of snow on the ground that stretched from the New Mexico/Texas border northeastward across Kansas (where as much as 15 inches fell) and into southeastern Nebraska and northwestern Missouri.

The false-color “snow/ice-vs-cloud discrimination” RGB image used the VIIRS instrument 0.64 µm visible channel and the 1.61 µm “snow/ice channel”, taking advantage of the fact that snow and ice are strong absorbers of radiation at the 1.61 µm wavelength (and therefore appear darker on that particular image). The result of this image combination shows snow cover as varying shades of red, with significant ice accretion on the ground showing up as the darkest shades of red. These darker shades of red in central and southwestern Oklahoma corresponded to the area between Clinton (KCSM) and Oklahoma City (KOKC) which received as much as 0.75 to 1.00 inch of ice accretion. The weight of this thick layer of ice caused widespread downed trees and powerlines, with power outages at several locations. Note that the significant glaze of ice on the ground  — for example, in areas surrounding Clinton (KCSM) and Hobart (KHBR) — did not show up particularly well on the visible image, since an ice layer is generally translucent in appearance as viewed from above (unless covered with a thin layer of snow, as was the case farther to the north).

Through holes in the clouds, there were some hints of darker red seen in southeastern Kansas, where significant accrual of ice also occurred.

Blowing dust over southern Texas

December 19th, 2013 |
GOES-13 0.63 µm visible channel images (click to play animation)

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

Strong southerly winds (gusting over 30 mph) generated a plume of blowing dust that originated in far northeastern Mexico and moved over Deep South Texas on the afternoon hours of 19 December 2013. McIDAS images of GOES-13 0.63 µm visible channel data (above; click image to play animation) showed the plume as it moved northward; surface visibility dropped as low as 2.5 miles at Jim Hogg Country Airport (station identifier KHBV) at 23:35 UTC.

A signal of the airborne dust plume was evident on an AWIPS image of the MODIS 11-12 µm IR brightness temperature difference (below). At the time of the MODIS image the surface visibility had dropped to 5 miles at McAllen, Texas (station identifier KMFE).

MODIS 11-12 µm IR brightness temperature difference

MODIS 11-12 µm IR brightness temperature difference

The hazy tan signature of the blowing dust plume was also quite evident on the corresponding 250-meter resolution MODIS true-color Red/Green/Blue (RGB) image (below).

Aqua MODIS true-color RGB image

Aqua MODIS true-color RGB image

The POES AVHRR Cloud Top Height product at 20:55 UTC (below) indicated that the top of the blowing dust plume was at 2 km above ground level.

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

Orographic cirrus over Colorado and Wyoming

December 18th, 2013 |
GOES-13 10.7 µm IR channel images (click to play animation)

GOES-13 10.7 µm IR channel images (click to play animation)

AWIPS images of 4-km resolution GOES-13 10.7 µm IR channel data (above; click image to play animation) showed the development of a large shield of orographic cirrus clouds immediately downwind (to the east of) the high terrain of the Rocky Mountains in eastern Colorado and southeastern Wyoming on 18 December 2013. The development and persistence of such cloud features is important to monitor, due to their potential impact on daytime temperatures: if incoming solar radiation is significantly reduced by a canopy of dense cirrus, surface temperatures may not be as warm as forecast. Early in the cirrus shield development during the overnight hours, GOES-13 IR brightness temperatures were as cold as -66º C (darker red color enhancement) along the western edge of the cirrus shield, suggestive of a high and potentially dense ice cloud feature — but during the early morning hours the cirrus was seen to begin to rapidly dissipate after about 15 UTC.

While not directly related to the orographic cirrus cloud shield per se, the strong westerly winds interacting with the complex terrain of the Rocky Mountains produced some areas of turbulence across Colorado. At 14:05 UTC a pilot reported Moderate turbulence throughout a very deep layer (5,500 feet to 20,000 feet), and at 15:22 UTC a pilot reported occasional Severe turbulence between the altitudes of 13,000 and 16,000 feet.

Night-time comparisons of 1-km resolution Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) and 11.45 µm IR channel images at 08:12 UTC or 1:12 AM local time (above) and 09:51 UTC or 2:51 AM local time (below) revealed that the cirrus along the western (upwind) edge was quite cold on the IR images (with IR brightness temperatures as cold as -71º C at 08:12 UTC, and -76º C at 09:51 UTC), and also appeared much more optically thick along the western portion on the DNB images. However, on the DNB images the cirrus shield — although still exhibiting fairly cold IR temperatures — appeared to be much more optically thin along the eastern portion. Due to ample illumination by a nearly-full moon, the DNB provided vivid “visible images at night” to compliment the IR images; in the upper right corner of the DNB images, snow on the ground could also be seen across northern Nebraska into far southern South Dakota.

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

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

A 1-km resolution POES AVHRR Cloud Top Height product at 11:39 UTC or 4:39 AM local time (below) indicated that the tops of the cirrus shield were at 12 km (darker green color enhancement).

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

The 12 UTC Denver rawinsonde data (below) showed that the tropopause was around 13 km, with an air temperature of -70º C. Winds at that altitude were from the west at 100 knots (with strong westerly winds at all altitudes throughout the tropopause).

Denver, Colorado rawinsonde data

Denver, Colorado rawinsonde data

Additional details and imagery of this orographic cirrus event can be found on the CIRA RAMMB GOES-R Proving Ground Blog.