A spectacular gravity wave train (or “undular bore“) was captured on a sequence of GOES-12 visible images (above) as it propagated southward across the northwestern Gulf of Mexico on 15 March 2008; as many as 16 separate cloud bands were evident (denoting individual wave crests along the wave train). Because of severe convection over the southeastern US on that day, the GOES-12 satellite had been placed into Rapid Scan Operations (RSO) mode, allowing images as frequently as every 5 minutes. Note how the patches of marine stratus cloud that were moving northeastward (ahead of the gravity wave train) are seen to dissipate very quickly as they encountered the wave. These undular bore features occur with some regularity over that particular part of the Gulf of Mexico — similar events were noted back in April 2007 and March 1998.

An AWIPS image combination using the MODIS 11.0 µm IR + MODIS sea surface temperature (SST) images (above) revealed that the gravity wave train was moving over increasingly warmer waters as it progressed southward across the Gulf of Mexico. The wave train was located north of a pre-frontal trough axis, and MADIS atmospheric motion vectors in the 900-775 hPa layer (red wind barbs) tracked the feature’s motion at 25-30 knots. The large plume of warmer SST values (75 – 80º F, darker orange colors) is a feature known as the Gulf of Mexico Loop Current (Reference #1 | Reference #2).

The MODIS Aerosol Optical Depth (AOD) product from the SSEC IDEA site (above) indicated the presence of elevated levels of particulate matter over the northwestern Gulf of Mexico on that day (presumably due to smoke from numerous small fires that had been burning across parts of Texas and northern Mexico during the previous day — or was it dust/sand from White Sands, New Mexico??).

A 250-meter resolution true color image from the SSEC MODIS Today site (below) showed that the wave motions of the gravity wave packet were also acting to organize the airborne smoke/haze aerosols into narrow banded features (similar to the cloud features seen on the GOES visible imagery).

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A tornado moved through the Atlanta, Georgia metro area  (this was the first tornado to strike downtown Atlanta since record-keeping began in the late 1800s)  around 01:40 UTC on 15 March 2008 (9:40 PM local time on 14 March 2008), causing EF-2 damage over a 6 mile long and 200 yard wide path (CNN report). An animation of GOES-12 10.7 µm IR images (above) showed the severe convection as it moved across northern Georgia (hail of 1.0 inch in diameter was also reported around 01:00 UTC). The storm’s appearance on satellite imagery was rather unremarkable, however — there was no “enhanced-v” storm top signature, and cloud top brightness temperatures were only as cold as about -50º C (light orange enhancement). The tropopause temperature on the 00:00 UTC Peachtree City, Georgia rawinsonde report (below) looked to be around -47º C.

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The visible image comparison shown above includes two images centered on Fairbanks AK (the green box in the center). The images are from the same time — 1730 UTC — one is from 14 March, and one is from 4 March, 11 days earlier. Sunrises [and daylight duration] at Barrow (yellow box), Fairbanks and Anchorage (red box) on 4 March were 17:41 [9 h, 56 m], 16:48 [10 h, 32 m], and 16:49 UTC [10 h, 45 m]. Eleven days later, sunrises [and daylight duration] were 16:52 [11 h, 31 m], 16:11 [11 h, 39 m] and 16:18 [11 h, 43m]. Thus the length of day increased by 95, 67 and 58 minutes, respectively, in those 11 days.

More daylight over northern North America has important consequences. When incoming radiation can exceed outgoing radiation — an event far more likely when the Sun is up — the land will not cool down. It is increasingly difficult to generate cold air over North America as the days lengthen, and at this time of year, the speed at which the days lengthen over Alaska approaches 10 minutes daily.

The later of the two images, shown here, is of a particularly clear day over southern Alaska, and the rugged terrain of both the Alaska Range and the Brooks Range is apparent. Denali, or Mt. McKinley, is visible just to the left of a line connecting Anchorage and Barrow.

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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.

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.

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?

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