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Hurricane Season 2008: 06B (Indian Ocean)
 
Dec. 08, 2008

Tropical Cyclone 07B Dissipates Quickly Over Sri Lanka

AIRS image of Tropical Cyclone 07B on December 8, 2008> Larger image
Credit: NASA JPL/Ed Olsen
Last week, Tropical cyclone 07B located in the Indian Ocean looked like it was heading for a landfall in southern India as a tropical cyclone, but environmental conditions changed, and when it made landfall on the island nation of Sri Lanka, it fell apart.

On Sunday, Dec. 7, 07B was moving over Sri Lanka and rapidly weakening. By Monday, Dec. 8 the storm lost its circular shape, circulation, and had dissipated. Some of its remnant thunderstorms were over Sri Lanka, and some made it over to southern India.

The Joint Typhoon Warning Center, the organization that forecasts tropical cyclones, issued its final bulletin on 07B on Sunday, Dec. 7. On that day, it was located near 7.7 degrees north latitude and 83.2 degrees east longitude. That put it about 200 miles east-northeast of Colombo, Sri Lanka, and it was crawling along at 4 knots (5 mph) west-southwest. Its maximum sustained winds at that time were down to 25 knots (28 mph).

NASA's Aqua Satellite Confirms 07B's Dissipation

NASA's Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua satellite captured 07B's cloud temperatures as it made landfall and began dissipating. AIRS produced this infrared image on Dec. 7 at 3:05 a.m. EST (8:05 UTC).

The infrared image shows the frigid cloud top temperatures, giving forecasters a clue to the storm's strength. The lowest temperatures (in purple) are associated with 07B's high, cold cloud tops. Those areas are as cold as 220 degrees Kelvin or minus 63 degrees Fahrenheit (F) or colder. The only purple area is a remnant of the system with thunderstorms over Sri Lanka. Meanwhile, the rest of the remnants are lower clouds depicted as the blue areas. Those areas are around 240 degrees Kelvin, or minus 27F.

Text Credit: Rob Gutro/NASA's Goddard Space Flight Center


Dec. 5, 2008

A One-Two Tropical Cyclone Punch for Southern India

Portions of Southern India are still recovering from Tropical Cyclone 06B's flooding and winds in the last weekend in November. Now, over the first weekend in December, the next storm that formed in the Indian Ocean Basin is heading for the same area.

AIRS infrared image of Tropical Cyclone 07B on December 5, 2008> Larger image
Credit: NASA JPL/Ed Olsen
Tropical Cyclone 07B is dealing with wind shear as it heads for a landfall in southeastern India over the weekend of Dec. 5-7. That means winds blowing in different directions at different levels of the atmosphere are going to chip away at its strength. On Dec. 5, Tropical Cyclone 07B has maximum sustained winds near 30 knots (34 mph) with higher gusts. It was moving west-northwest near 8 knots (9 mph). At 15:00 Zulu Time (10 a.m. EST) on Dec. 5, it was located about 515 nautical miles east-southeast of Chennai, India, near latitude 8.7 degrees north and 84.7 degrees east.

The Joint Typhoon Warning Center noted in their recent forecast "Recent animated infrared imagery and a microwave image continue to show convection (rapidly rising air creating clouds and rainfall) to the west of a partially exposed low level circulation center. Vertical wind shear remains moderate, so winds aren't allowing it to strengthen. Convection and rainfall also remain to the west of the center.

Tropical cyclone 07B will begin to track more west-northwestward as the subtropical ridge of High Pressure builds westward and strengthens over the next 24 hours (through mid-day, Saturday, Dec. 6). Then the storm will track to the northwest and then to the west-northwest. The system is expected to make landfall over southern India after Sunday morning (Eastern Time) and will weaken after this time.

Some of the satellite data that forecasters use to determine if a storm will hold together, dissipate or strengthen are cloud temperatures. They look at data from the Atmospheric Infrared Sounder (AIRS) instrument that flies on NASA's Aqua satellite. When cloud temperatures get colder, it means that clouds are getting higher. Building clouds indicate a lot of "uplift" in the atmosphere and stronger thunderstorms. The data from AIRS is also used to create an accurate 3-D map of atmospheric temperature, water vapor and clouds, all of which are helpful to forecasters.

The infrared image, taken on Dec. 4 at 2:47 p.m. EST (19:47 UTC) shows a large temperature difference between the tops of the clouds in a tropical cyclone and the warm ocean waters that power it. The lowest temperatures (in purple) are associated with high, cold cloud tops that make up the top of the storm. Those temperatures are as cold as or colder than 220 degrees Kelvin or minus 63 degrees Fahrenheit (F). The blue areas are around 240 degrees Kelvin, or minus 27F. In this image, you can see that Tropical Cyclone 07B's clouds are broken, indicating a weakening circulation, and a weakening storm.

AIRS infrared signal doesn't penetrate through clouds, so where there are clear skies AIRS reads the infrared (heat) signal from the ocean and land surfaces, revealing warmer temperatures (colored in orange and red). The orange temperatures are 80F (300 degrees Kelvin) or greater (the darker they are, the warmer they are). Tropical cyclones need sea surface temperatures of 80F to strengthen and maintain their strength.

Text Credit: Rob Gutro (from JTWC reports)/NASA's Goddard Space Flight Center


Dec. 4, 2008

Southern India Expecting Another Tropical Cyclone

Current and future forecast positions of Tropical Cyclone 07B, heading toward Sri Lanka Credit: JTWC
> Larger image
Southern India just dealt with Tropical Cyclone 06B between Nov. 25-27, and a little over a week later is about to be pounded with the follow-up storm, 07B.

On Dec. 4, Tropical cyclone 07B was in the Indian Ocean, 450 miles east of Sri Lanka, but headed for a landfall there, then onto the mainland of southern India. 07B was near 7.2 degrees north and 88.9 degrees east at 15:00 Zulu time (10 a.m. EST). It is moving west-northwestward at 4 knots (5 mph) and will continue to track westward.

This graphic from the Joint Typhoon Warning Center (JTWC) shows the current position of Tropical Cyclone 07B (the black open circle). The future forecast positions of Tropical Cyclone 07B are depicted as the blue circular icons as it progresses toward Sri Lanka.

The JWTC is the forecast organization responsible for forecasting tropical cyclones in that part of the world, and the latest advisory noted, "The system remains in a favorable environment with moderate vertical wind shear and strong difluence aloft." Difluence in an upper level wind field is considered a favorable condition for severe thunderstorm development (if other parameters are also favorable), which is an indication of a strengthening tropical cyclone, or one maintaining strength.

AIRS image of Tropical Cyclone 07B Credit: NASA JPL
> Larger image
NASA's Aqua Satellite Instrument Sees 07B's Clouds

NASA's Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua satellite has been keeping an eye on 07B's cloud temperatures. AIRS produced this infrared image on Dec. 4 at 7:29 a.m. EST (2:29 UTC).

The infrared image shows the frigid cloud top temperatures, giving forecasters a clue to the storm's strength. The lowest temperatures (in purple) are associated with 07B's high, cold cloud tops. Those areas are as cold as 220 degrees Kelvin or minus 63 degrees Fahrenheit (F) or colder. The blue areas are around 240 degrees Kelvin, or minus 27F.

The Joint Typhoon Warning Center uses cloud temperature as one factor in determining whether a tropical cyclone is strengthening. When cloud temperatures get colder, it means that clouds are getting higher. Building clouds indicate a lot of "uplift" in the atmosphere and stronger thunderstorms.

AIRS' infrared signal doesn't penetrate through clouds, so where there are clear skies AIRS reads the infrared (heat) signal from the ocean and land surfaces, revealing warmer temperatures (colored in orange and red). The orange temperatures are 80F (300 degrees Kelvin) or greater (the darker they are, the warmer they are). Tropical cyclones need sea surface temperatures of 80F to strengthen and maintain their strength.

Text Credit: Rob Gutro (from JTWC reports)/NASA's Goddard Space Flight Center