Tropical Depression Nine Forms

September 1st, 2010 |

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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