White Sands, New Mexico: a source of blowing…sand

March 14th, 2008 |

GOES-12 visible images (Animated GIF)

The large (275 square mile) White Sands National Monument is an very familiar landmark on satellite imagery — the world’s largest surface deposit of gypsum sand stands out as a prominent white feature against the surrounding mountains and valleys of southern New Mexico. Strong winds across that region on 14 March 2008 (gusts as high as 66 mph were reported at Ruidoso) created a plume of blowing sand whose obvious source was White Sands. An animation of GOES-12 visible images (above) shows the development of the plume during the 17:45 – 22:45 UTC (11:45 AM – 4:45 PM local time) period.

MODIS true color image (Google Earth)

A 250-meter resolution Aqua MODIS true color image from the SSEC MODIS Today site (viewed using Google Earth, above) shows greater detail of the plume at around 19:20 UTC (1:20 PM local time). The surface visibility at Alamogordo, New Mexico (station identifier KALM, located about 20 mi or 37 km east of White Sands National Monument) was reduced to 2 miles during the late morning hours on 14 March, as winds increased and gusted to 44 mph.

NOAA ARL forward trajectories

So where did this airborne dust/sand go? NOAA ARL HYSPLIT forward trajectories (above) suggest that lower-tropospheric air parcels originating over the White Sands area at 21:00 UTC on 14 March were transported eastward and then southeastward, reaching the extreme northwestern portion of the Gulf of Mexico on 15 March. Did some of this dust then get entrained into the circulation of a undular bore that moved southward across the Gulf of Mexico on 15 March?

Bennett Island plume

March 12th, 2008 |

NOAA-18 AVHRR false color image

[Hat-tip to Gregg Gallina (NOAA/NESDIS Satellite Analysis Branch) for bringing this interesting case to our attention…] A false-color RGB image derived from 1-km resolution NOAA-18 AVHRR data (above) showed a long, narrow cloud plume streaming northwestward from Bennett Island (or “Ostrov Bennetta”, a tiny island north of Russia in the East Siberian Sea; latitude 76.5°N, longitude 149°E ; maximum elevation 426 m or 1398 ft) on 12 March 2008.

A large number of similar “plume features” have been detected on polar-orbiting satellite imagery in that same region since the 1970s and 1980s, but the explanation for their cause and composition (volcanic ash? methane plume? secret Soviet nuclear weapons testing?) remained elusive until aircraft studies in the 1990s determined that they were orographically-induced cloud plumes, enhanced by a vertically propagating mountain wave (Reference #1 | Reference #2).

MODIS visible image (250m resolution)

A 250-meter resolution Terra MODIS visible image (above) shows a closer view of Bennett Island and the source of the cloud plume (note that this satellite image is not re-mapped, so the top of the image is oriented toward the northeast). You can also see that there is a polynya — an area of open water — just to the lee (northwest, or left) of the island; could this warm water have served as a source of low-level instability to help initiate vertical motions that contributed to the cloud plume formation?

NOAA-18 AVHRR images (Animated GIF)

A comparison of the five 1-km resolution NOAA-18 AVHRR channels (above) revealed several important points: (1) the visible channels (channels 01 and 02) showed that there was a large shadow being cast by the cloud plume feature, suggesting that the plume was at a high altitude; (2) the “darker appearance” on the 3.7 µm shortwave IR (channel 03) image was suggestive of reflection of solar radiation off the tops of a cloud that was composed of either supercooled water droplets or very small ice crystals; (3) the 10.8 µm and 12.0 µm longwave IR (channels 04 and 05) brightness temperature values were generally in the -28º C to -45º C range (darker blue to violet colors), again suggesting a high-altitude plume; and (4) at lower altitudes, small-scale wave clouds immediately downwind of Bennett Island confirmed that the topography of the island was having some effect on lee cloud formation.

The issue of cloud composition (supercooled water droplets, or ice crystals?) vs. cloud top brightness temperature is perplexing, however: supercooled water cloud droplets are known to exist at temperatures as cold as -25º to -35º C, but the spontaneous freezing of supercooled droplets (via homogeneous nucleation) is thought to occur around -36º to -40º C (smaller droplets freeze at colder temperatures). It is possible that the lower- to middle-altitude portions of the cloud plume did consist of supercooled water droplets (giving a relatively high 3.7 µm reflectance), but the higher-altitude portions of the cloud plume were likely made up of ice crystals. Note that the coldest NOAA-18 IR cloud-top temperatures (-47º C, red colors) were near the plume source, immediately downstream of the highest terrain of Bennett Island — this was due to a greater amplitude near the source of the vertically-propagating mountain wave, which was producing the highest cloud-top altitude at that particular location.

Terra MODIS IR difference (channel 29 - channel 31)

An IR difference product using Terra MODIS 8.5 µm 11.0 µm (channel 29 – channel 31) data suggests that the upper portion of the long cloud plume was composed primarily of ice crystals — a positive IR difference value of several degrees K (brighter white enhancement) indicates the presence of ice crystals. However, another item of curiosity is the initial “rotor cloud” feature immediately downwind of the island: note the darker “donut hole” appearance on the MODIS IR difference product, which suggests that the center part of the cloud top was composed primarily of supercooled water droplets; perhaps that rotor cloud was in the process of rapidly glaciating during that time?

AWIPS IR image + 700 hPa model winds

700 hPa wind fields (from a 00:00 UTC global model analysis) plotted on an AWIPS image of the 03:00 UTC global IR satellite composite (above) indicated that there was a southeasterly flow of 30-35 knots at that level over the Bennett Island region (located near the center of the image), which explains the southeast-to-northwest orientation of the cloud plume on the NOAA-18 AVHRR imagery.

NOAA AVHRR IR winds (Animated GIF)

Polar cloud-tracked atmospheric motion vectors (winds) derived from NOAA-15 and NOAA-17 AVHRR InfraRed data (above) also showed that a southeasterly flow aloft dominated across that region early in the day on 12 March. The Bennett Island cloud plume can be seen as a thin white streak on the final 05:42 UTC NOAA-17 image (since no wind vectors were plotted over the top of the cloud feature at that time).

Chokurdah rawinsonde report

The 00:00 UTC rawinsonde report from Chokurdah along the north coast of Siberia (above) indicated that there was a strong temperature inversion below the 850 hPa pressure level; air temperatures in the -28º C to -45º C range (similar to those seen in the NOAA-18 IR imagery above) were found between 700 – 450 hPa (where there was still a southeasterly flow, albeit somewhat slower in velocity).

It is still unclear why the cloud plume feature formed at a relatively high altitude, when the island itself has a maximum altitude of only 1398 ft. As stated at the conclusion of Reference #2 above: “But the Bennett Island plumes have not yielded all their mystery. Meteorologists must now explain why the plumes form at an unusually high altitude, more than 3 kilometers above the mountaintops, says Schnell.” I only wish that I were the meteorologist to do that…

Fire activity in the southeastern US

March 12th, 2008 |

MODIS true color images (Animated GIF)

[Image source: Liam Gumley, SSEC/CIMSS]

A comparison of two consecutive MODIS true color images from the same day (viewed using Google Earth, above) reveals that numerous small fires developed (and produced smoke plumes) within a relatively short amount of time across the southeastern US on 12 March 2008. The satellite overpass time of the earlier Terra MODIS image is around 16:18 UTC (12:18 PM local time), while the overpass times of the eastern and western portions of the later Aqua MODIS image is around 17:53 UTC (1:53 PM local time) and 19:32 UTC (3:32 PM local time), respectively.

The GOES-12 Wildfire ABBA product (below) confirms the presence of widespread “fire hot spots” that increased in number and coverage during the late morning and early afternoon hours on 12 March.

Wildfire ABBA (Animated GIF)

Wildfire ABBA legend

Snow cover in the Upper Midwest, and ice in Lake Michigan

March 11th, 2008 |

MODIS true color images (Animated GIF)

MODIS true color images (viewed using Google Earth, above) showed that significant snow cover still remained over parts of the Upper Midwest region (especially across northeastern Ohio) on 11 March 2008. With the highway overlays removed, you can better see that several of the smaller towns across northeastern Ohio have a slightly darker appearance than the surrounding rural areas, due to the higher concentration of trees, buildings, and roadways in those urban areas. Also note the widespread ice that covered much of Lake Erie (in the upper right portion of the images), as well as along the southern shore of Lake Michigan (in the upper left portion of the images).

The National Operational Hydrologic Remote Sensing Center (NOHRSC) snow depth data (below) confirmed that many locations in eastern Ohio still had significant snow cover (as deep as 10-14 inches, with 4 inches on the ground at Toledo and 8 inches on the ground at Columbus) — however, not far to the west there was no snow on the ground in parts of Indiana (Indianapolis had zero snow depth, thus that area exhibited a light brown appearance on the MODIS true color imagery).

NOHRSC snow depth

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MODIS visible image + surface observations

An AWIPS image of the MODIS visible channel (above) showed the coverage of narrow ice features that were floating in the nearshore waters of southern Lake Michigan on 11 March (as was also pointed out by the National Weather Service forecast office at Milwaukee/Sullivan). Southwesterly winds at the surface were increasing during the day and gusting to 20-25 knots in some locations, allowing air temperatures to rise into the upper 30s to low 40s F over the snow-covered areas of southern Wisconsin and northern Illinois.

The effect of these gusty southwesterly winds on the offshore ice could be seen when comparing 2 consecutive MODIS true color images (viewed using Google Earth, below) — the time difference between the 2 images is about 101 minutes (the earlier Terra MODIS image time was around 17:11 UTC or 12:11 PM local time; the later Aqua MODIS image time was around 18:52 UTC or 1:52 PM local time), and the ice is seen to drift some distance to the northeast during that short time interval. The long, narrow, straight cloud features that also appear in the images are aircraft contrails.
MODIS true color images (Animated GIF)

[Image source: Liam Gumley, SSEC/CIMSS]