November « 2007 « CIMSS Satellite Blog

Saharan dust outbreak

November 11th, 2007

A major outbreak of Saharan dust was noted during the 06-11 November 2007 period — the Meteosat-9 Saharan Air Layer (SAL) tracking product (above) revealed an extensive signal of thick dust (orange to red enhancement) that was being transported westward across the tropical Atlantic.

The MIMIC Total Precipitable Water (TPW) product (below) showed the dry character of this large Saharan dust plume — TPW values as low as 10-20 mm (brown to violet enhancement) were seen over eastern and central portions of the tropical Atlantic basin as the dust and dry air layer streamed westward off the African continent.

An animation of daytime Meteosat-9 “true color” image composites at 12 UTC and/or 18 UTC (below) shows the hazy appearance of the large Saharan dust plume. The appearance of such a strong Saharan dust event in November is curious in light of two recent studies by Evan et al.: (1) November appears to be the minimum of the seasonal Saharan dust cycle, and (2) August-September 2007 showed little to no dust storm activity over the tropical North Atlantic basin.

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Noel transitions to an extratropical cyclone

November 3rd, 2007

As Hurricane Noel made the transition to an extratropical cyclone during the 02 to 03 November 2007 period, it remained a very large and very powerful storm system that significantly impacted the Atlantic coast regions of the northeastern US and southeastern Canada with hurricane force winds (91 mph at Horseshoe Shoal MA), heavy rain (4.83 inches at Bass Harbor ME), high seas (46.0 foot waves at Buoy 44011), and even heavy snow (6.0 inches at Fort Kent ME). AWIPS images of the GOES-12 6.5 µm “water vapor” channel (above) showed a very pronounced warm/dry band (orange enhancement) just south of the main cloud shield along the southern quadrant of the storm. This particular water vapor signature often indicates a potential for very strong winds at the surface as momentum from aloft is transferred downward into the boundary layer; the southern flank is also a favored area for strong surface winds in an extratropical cyclone.

An AWIPS image of the 6.7 µm MODIS water vapor channel around 02:45 UTC (below) with 03 UTC surface reports reveals that Fixed Buoy 41048 (located west of Bermuda, station ID TXKF) was situated near the leading edge of the water vapor dry band, and experienced a peak wind gust (from the south) of 62 knots near the time of the MODIS image. Buoy 41048 recorded a peak wind gust of 69 mph at 01:50 UTC, and a peak wave height of 37.1 feet at 02:50 UTC.

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Hurricane Noel QuikSCAT winds

November 2nd, 2007

Hurricane Noel remained a minimal Category 1 hurricane early in the day on 02 November 2007, although the satellite presentation was starting to deteriorate (GOES-12 IR animation) as the storm began the transition to an extratropical system. SeaWinds scatterometer data from the QuikSCAT satellite around 10:48 UTC (above, and below) did show two wind retrievals of 74 knots just to the northwest of the storm center. However, those two 74 knot wind speed points had the “Rain Flag” active (all the wind barbs with circles on the CIMSS Tropical Cyclones TCTrak display have the Rain Flag active), meaning that the presence of rainfall may have contaminated the data at those points — energy backscattered from large rain drops can constitute a significant (but unknown) portion of the measured echo energy.