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 December – 21 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.

View only this post Read Less

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

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

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

View only this post Read Less

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

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

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

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

View only this post Read Less

GOES-15 3.9 µm shortwave IR images (click to play animation)

The Pfeiffer Fire (InciWeb | Wildfire Today) began to burn in the Big Sur area of central California just after 08:00 UTC or Midnight local time on 16 December 2013. McIDAS images of 4-km resolution GOES-15 3.9 µm shortwave IR data (above; click image to play animation) showed the initial appearance of a fire “hot spot” (darker black to red color enhancement) at 08:11 UTC or 12:11 AM local time, with IR brightness temperatures increasing to 339.3 K or 66.2º C (red color enhancement) at 10:15 UTC or 2:15 AM local time. Although the fire hot spot was generally identifiable during most of the following 48 hours (even through cirrus cloud features passing overhead), it never was seen to reach that early intensity again.

Spatial resolution is very important for the accurate location and characterization of fires — this is demonstrated by an AWIPS image comparison of 1-km resolution Suomi NPP VIIRS 3.74 µm and 4-km resolution GOES-15 3.9 µm shortwave IR data just after 10 UTC on 17 December (below). The warmest IR brightness temperature on the GOES-15 image was 15º C (darker black pixels) compared to 54.5º C (red pixels) on the VIIRS image.

Suomi NPP VIIRS 3.74 µm and GOES-15 3.9 µm shortwave IR images

Suomi NPP VIIRS 3.74 µm shortwave IR images

The growth of the fire from 16 December to 17 December could be seen by comparing night-time images of Suomi NPP VIIRS 3.74 µm shortwave IR data (above) and the corresponding VIIRS 0.7 µm Day/Night Band data (below). Signatures of the fire — both the size of the fire hot spot on the shortwave IR images, and the bright glow from the fire complex on the Day/Night Band images — were seen to increase in size on 17 December. The fire was estimated to have burned about 500 acres by the afternoon of 17 December, and was only 5% contained at that time. A number of homes and other structures were lsot to the fire, and some evacuations had to be carried out.

On the 17 December VIIRS images, note the observation of smoke on the surface report for Monterey (station identifier KMRY) north of the Pfeiffer Fire. At that time, the surface visibility there was only being reduced to 10 statute miles — however, later in the day on 17 December the surface visibility was as low as 4 statute miles at 23 UTC (3 PM local time) and 00 UTC (4 PM local time), causing some air quality problems.

View only this post Read Less