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Hurricane Season 2008: Dongo (Indian Ocean)
Jan. 12, 2009

Dongo Had Its Ups and Downs in Intensity and is Now Extratropical

AIRS visible image of Tropical Cyclone Dongo from Jan. 12, 2009> Larger image
Credit: NASA JPL/ Ed Olsen
Over the last three days, Tropical Cyclone Dongo has had its ups and downs, but had now become extratropical in the Southern Indian Ocean.

On Saturday, January 10, Dongo continued to move south to southwest across the Indian Ocean, intensifying with sustained winds near 40 knots (46 mph). The next day, Dongo's sustained winds got as strong as 65 knots (74 mph) making it a short-lived Category One Typhoon. By Monday, January 12, Dongo weakened again as maximum sustained winds dropped to 30 knots (34 mph). That's the trend it will continue on until dissipation. Dongo continued to move south to southeast across the Indian Ocean and is already undergoing extratropical transition which will be done by the end of the day.

At 15:00 Zulu Time (10:00 a.m. EST) on Jan. 12, Dongo was near 29.4 degrees south latitude and 70.6 degrees east longitude. That's about 906 nautical miles east-southeast of La Reunion. Reunion is an island in the Indian Ocean, east of Madagascar, about 130 miles (200 kilometers) southwest of Mauritius, the nearest island. Dongo continues to track south into open waters.

AIRS infrared image of Tropical Cyclone Dongo from Jan. 12, 2009> Larger image
Credit: NASA JPL/ Ed Olsen
NASA's Aqua satellite was flying over Dongo on January 10 and the Atmospheric Infrared Sounder (AIRS) instrument onboard captured Dongo's cloud temperatures. AIRS produced this infrared image on Jan. 10 at 4:23 a.m. EST (9:23 UTC). Dongo is depicted as a comma like image in both the visible image and the infrared image (blue and purple).

The infrared image shows the frigid cloud top temperatures, giving forecasters a clue to the storm's strength. The coldest temperatures (and highest cloud tops) shown in purple are as cold as 220 degrees Kelvin or minus 63 degrees Fahrenheit (F) or colder. The lower clouds are depicted as the blue areas, which are around 240 degrees Kelvin, or minus 27F.

What Does "Conversion to an Extratropical Storm" Mean?

A conversion to "extratropical" status means that the area of low pressure (known as Dongo in this case) eventually loses its warm core and becomes a cold-core system. During the time it is becoming extratropical the cyclone's primary energy source changes from the release of latent heat from condensation (from thunderstorms near the storm's center) to baroclinic (temperature and air pressure) processes. When a cyclone becomes extratropical it will usually connect with nearby fronts and or troughs (extended areas of low pressure) consistent with a baroclinic (pressure) system. When that happens it appears the system grows larger while the core weakens.

Text credit: Rob Gutro/Goddard Space Flight Center

Jan. 09, 2009

Hello, Dongo! The First Tropical Cyclone of 2009

AIRS image of Tropical Cyclone Dongo from Jan. 9, 2009> Larger image
Credit: NASA JPL
For anyone wondering where in the world the first tropical cyclone of 2009 would form, we now know the answer: the southern Indian Ocean. Tropical Cyclone Dongo has won the honor by forming on Friday, January 9, 2009 close to the center of the southern Indian Ocean.

On Jan. 9 at 12:00 Zulu Time (7:00 a.m. EST) Dongo was located near 17.6 degrees south latitude and 67.9 degrees east longitude in open waters of the southern Indian Ocean. To get an idea where Dongo is, picture a world map of the Indian Ocean bordered by the African continent on the left, directly north is the country of India, and to the right are the islands that make up Indonesia. Dongo is almost right in the middle of it all, located about 655 miles south-southwest of the atoll called "Diego Garcia."

Diego Garcia is the largest atoll, in terms of land area, in Chagos Archipelago, part of the British Indian Ocean Territory. The island is located in the Indian Ocean, about 1,600 km (1,000 mi) south of the southern coast of India. Other countries in the vicinity of Diego Garcia include Sri Lanka and Maldives.

Dongo has maximum sustained winds near 30 knots (34 mph) with higher gusts and has tracked southwestward near 5 knots (6 mph). The Joint Typhoon Warning Center, the organization that issues forecasts for tropical cyclones in this area of the world noted "Animated multispectral satellite imagery depicts a consolidating low-level circulation center with deep convective banding wrapping into the eastern quadrant obscuring well-defined and tightly-wrapped low-level cloud lines." That means that the storm is getting better organized.

The upper-level environment has quickly improved as the system tracked southwestward with decreasing vertical wind shear (winds that tear a storm apart when they're strong). Because Dongo is entering a favorable environment that will power it up, it is expected to intensify over the next day or two. Dongo is expected to track south-southeastward over the weekend.

The infrared imagery of the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua satellite is used to identify the cloud temperatures in tropical cyclones. It captured this image of Dongo coming together on Jan. 9 at 3:41 a.m. EST (8:41 UTC). Although the circular shape of the storm is present, it's obvious that it is still not well-organized.

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, which mean a stronger tropical cyclone.

The infrared image also 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 Dongo. 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.

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)/Goddard Space Flight Center