Smoke and blowing dust in Mexico

March 18th, 2008 |

MODIS images (Animated GIF)

Winds up to 60 mph associated with a strong cold front moving across the mountains of northern Mexico on 18 March 2008 were creating areas of blowing dust and sand, as well as fostering an environment favorable for wildfire activity. A comparison of AWIPS images of the MODIS visible channel, 1.3µm “cirrus detection” channel, 3.7µm “shortwave IR” channel, 11.0µm “IR window” channel, and 6.7µm “water vapor” channel (above) revealed the following: (1) the visible image depicted large “hazy” features, especially to the northeast of Zacatecas (MMZC) and to the south and southeast of Monterrey (MMAN); (2) the hazy features in the Zacatecas region showed up as light gray streaks on the “cirrus detection” image (since this channel also is very sensitive to light scattered from airborne particles such as dust/sand) and on the IR window channel (since these features aloft were cooler than the surrounding bare ground); (3) the 3.7µm shortwave IR image indicated several “hot pixels” (yellow to red colors) due to fires burning just south of Monterey; (4) water vapor image showed a widespread pattern of interfering “lee waves” caused by the strong winds interacting with the complex terrain.

A 250-meter resolution MODIS true color image from the SSEC MODIS Today site (below) helps to differentiate the composition of the various hazy areas: the fire smoke near Monterrey appears light gray, while the blowing dust/sand near Zacatecas has a distinct beige to light orange color (due to the soil types found in the plume source regions) — note the light orange color of the undisturbed soils in that area on MODIS true color imagery 2 days earlier.

MODIS true color image (Google Earth)

The MODIS Aerosol Optical Depth (AOD) product from the SSEC IDEA site (below) showed that these plumes of smoke and dust/sand exhibited AOD values as high as 1.0 (red enhancement), extending all the way northeastward into Texas.

MODIS Aerosol Optical Depth

We received photos (below; photo 2; photo 3; photo 4; photo 5) from Jon Zeitler (Science and Operations Officer at the National Weather Service forecast office at Austin/San Antonio, Texas) showing the after-effects of “muddy rain” that fell there on the evening of 18 March; precipitation from a mesoscale convective system scavenged a good deal of the airborne particulates, bringing them to the surface along with the rainfall. A truck that dirty could possibly even show up on satellite imagery!

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Undular bore over the Gulf of Mexico

March 15th, 2008 |

GOES-12 visible images (Animated GIF)

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.

MODIS IR + SST image

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).

MODIS Aerosol Optical Depth

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).

MODIS true color image

Tornado strikes Atlanta, Georgia

March 15th, 2008 |

GOES-12 10.7µm IR images (Animated GIF)

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.

Peachtree City GA rawinsonde report

Daylight returns to Alaska

March 14th, 2008 |

AKmovie.gif
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.