The parade of tropical impulses moving westward off of Africa into the tropical Atlantic that has produced Hurricanes Danielle and Earl, and Tropical Storm Fiona, has now yielded a new Tropical Depression, Number 9, that will become Gaston if it achieves Tropical Storm status. Some environmental conditions favor intensification, and some work against it.

The tropical depression is moving over warm sea surface temperatures, and is in an environment of low shear, two factors that argue for slow intensification of the system. However, an analysis of Saharan Air using Meteosat data (diagnosed as a split-window technique using 10.8 micron and 12.0 micron data) shows very dry air surrounding the storm. (See image below). Saharan Air Layers greatly impede the development of tropical cyclones. MIMIC Total Precipitable Water (from this site) also shows very dry air surrounding half of the developing storm. Water vapor imagery from GOES-East and from GOES-12 both show very dry air surrounding the tropical depression.

Visible and Infrared imagery from the storm this morning (below) show the impact of forward scattering and back-scattering on the detection of thin clouds. The visible imagery at 0815 UTC (left), when the sun is low in the sky and forward scattering dominates, suggests far more cloudiness than at 1745 UTC (right) when the sun is high in the sky and backscattering dominates. Infrared imagery, however, shows little change in the amount of detected cloudiness. Thin cirrus detection by visible satellite is easiest for very low sun angles; as the sun rises higher in the sky, cirrus clouds become less distinct in visible imagery. Note that GOES-R will include a detector sensitive to radiation at 1.3 microns to highlight cirrus clouds regardless of the Sun’s position (for example, see this comparison of MODIS visible, 1.3 µm near-IR, and 11.0 µm IR images).

For up-to-date information on the tropical systems in the Atlantic, visit the CIMSS tropical website, or the National Hurricane Center website.

(Added: TD #9 became Tropical Storm Gaston as of 5 PM EST on 1 September)

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AWIPS images of 1-km resolution POES AVHRR visible channel 1 (0.63 µm) and channel 2 (0.86 µm) along with IR channel 4 (10.8 µm) data (below) showed Hurricane Earl moving northwestward, just to the northeast of the Bahamas, on 01 September 2010. Earl’s intensity decreased from a Category 4 to a Category 3 storm during the day.

POES AVHRR 0.63 µm visible images

POES AVHRR 0.86 µm visible images

POES AVHRR 10.8 µm IR images

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GOES-13 6.5 µm "water vapor channel" images

Evidence of a large region of mid-tropospheric dry air on AWIPS images of GOES-13 6.5 µm “water vapor channel” data (above; also available as a QuickTime movie) in tandem with an increase in southwesterly deep layer wind shear was helping to erode the upper level cloud canopy over the western portion of Category 4 Hurricane Earl — this allowed a well defined low-level outflow boundary to be seen on McIDAS images of GOES-13 0.63 µm visible channel data (below; also available as a QuickTime movie) on 31 August 2010.

It is also interesting to note the westward-propagating “shock waves” that were emanating from Hurricane Earl, which could be seen on the water vapor imagery in the dry region (denoted by the predominantly yellow color enhancement) over the western Atlantic Ocean.

GOES-13 0.63 µm visible channel images

A comparison of AWIPS images of the POES AVHRR 0.63 µm visible and 10.8 µm IR channel data (below) seemed to support the fact that this was indeed a low-level feature, with the narrow cloud band of the outflow boundary feature exhibiting fairly warm IR brightness temperatures of 0º to +5º C.

POES AVHRR 0.63 µm visible and 10.8 µm IR images

The POES AVHRR Cloud Top Height (CTH) product (below) gave maximum CTH values of 3-4 km (red color enhancement) for the outflow boundary feature  — maximum CTH values near the center of Hurricane Earl were around 16 km (cyan color enhancement).

POES AVHRR 10.8 µm IR image and Cloud Top Height product

Further confirmation that this was a low-level feature was provided by an examination of the AVHRR Cloud Type product (below), which indicated that the narrow outflow boundary cloud band was composed of water droplets (cyan color enhancement).

POES AVHRR 0.63 µm visible image and Cloud Type product

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GOES-15 imagery shows the steady development of a distinct eye feature with Earl as the storm slowly moves away from the Leeward Islands. The 11 AM 30 August National Hurricane Center discussion notes peak sustained winds of 105 knots; Earl is a category 3 storm that is moving over warm water in an environment of low shear. Strengthening is forecast. Interests along the east coast of the United States and Canada should monitor the evolution of this system.

The infrared imagery shows cold cloud tops surrounding much, but not all, of the developing eye. Coldest cloud top brightness temperatures — the light purples within the greys — are values around -80 C.

In the loop above, GOES-13 and GOES-15 imagery are combined to give a stereoscopic view of the storm, allowing a three-dimensional perspective.

GOES-13 0.63 µm visible images

A closer view of the eye of  Hurricane Earl is shown using GOES-13 0.63 µm visible images (above; also available as a QuickTime movie) — the satellite was in Rapid Scan Operations (RSO), providing images as frequently as every 5-10 minutes. Near the end of the time of the visible image animation, a comparison of 1-km resolution NOAA-15 AVHRR and 4-km resolution GOES-13 IR images (below) revealed that the coldest cloud top IR brightness temperatures were located in the northeastern quadrant of the eyewall region (as cold as -86º C on the NOAA-15 image, and -80º C on the GOES-13 image). By this time, Hurricane Earl had reached Category 4 intensity on the Saffir-Simpson Hurricane Scale.

NOAA-15 10.8 µm IR and GOES-13 10.7 µm IR images

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