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Surge in Atlantic Activity Coincides With Peak of Hurricane Season
The recent flurry of tropical cyclones in the Atlantic is a reminder that we are now entering the climatological peak of the Atlantic hurricane season. The 2008 season is turning out to be well above average so far. Following close on the heels of Hurricane Gustav, which hit the southern Louisiana coast as a category 2 storm on the morning of September 1, are Hanna, Ike and Josephine, the eighth, ninth, and tenth named storms of the season. On average there are 10 named storms for the entire season, which officially runs through the end of November. Ike became a category 4 storm on the evening of September 3rd, making it the third major and fifth overall hurricane of the season; typically there are 6 hurricanes in the Atlantic in any given year with 2 or 3 becoming major. Currently, Hanna, Ike and Josephine all remain active in the Atlantic.

TRMM image of Hanna on Aug. 28, 2008
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The first of these storms, Hanna, originated from an area of low pressure associated with an African easterly wave (AEW) that had propagated across the central Atlantic. There are about 60 of these westward-moving waves in a given year, but only a fraction develop into tropical cyclones in the Atlantic. This particular wave became more organized as it neared the Leeward Islands in the Lesser Antilles and was designated a tropical depression (TD #8) by the National Hurricane Center (NHC) on the morning of August 28. Soon after it was upgraded to a tropical storm and given the name Hanna. The Tropical Rainfall Measuring Mission satellite (also known as TRMM) has been in service for over 10 years now and continues to provide valuable images and information on tropical cyclones around the Tropics using a combination of passive microwave and active radar sensors. TRMM was able to capture the recent surge in tropical activity in the Atlantic.

TRMM obtained this first image of Hanna at 22:58 UTC (6:58 pm EDT) 28 August 2008 as the storm was nearing the northern Leeward Isles. It shows the horizontal pattern of rain intensity within the storm. Rain rates in the center swath are based on the TRMM Precipitation Radar (PR), and those in the outer swath on the TRMM Microwave Imager (TMI). The rain rates are overlaid on infrared (IR) data from the TRMM Visible Infrared Scanner (VIRS). At this time, Hanna is still only weakly organized. No eye is present and the rainbands (lines of blue and green areas with isolated areas of red indicating light to moderate and heavy rain, respectively) show very little evidence of banding (curvature).

TRMM image of Hanna on Sept. 1, 2008
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The storm is also highly asymmetric with most of the rain occurring in the eastern half of the storm. The center of circulation is actually located near the northwestern-most area of moderate rain in the TMI swath. These characteristics all indicate that Hanna's circulation is still in the developing stage and still fairly weak. At the time this image was taken, Hanna's maximum sustained winds were reported at 35 knots (~40 mph) by NHC, equivalent to a minimal tropical storm.

Over the next few days, Hanna continued to move to the west-northwest remaining well north of Puerto Rico and Hispaniola as a moderate tropical storm. After reaching the Caicos and Turks Islands north of Haiti on September 1, Hanna came under the combined influence of a large low pressure system located off of the northeastern US and a strengthening area of high pressure over the eastern third of the country. A channel of northerly flow wrapping around the backside of the low and ahead of the high pressure began to push Hanna southward towards Haiti.

TRMM image of Hanna on Sept. 2, 2008
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The next image from TRMM was taken at 14:18 UTC (10:18 am EDT) on September 1 just as Hanna was beginning to turn south towards Haiti. Although the storm is still highly asymmetric (note most of the rain is still east of the center), there is a substantial area of intense rain (shown by the darker reds) near the center. Associated with this area of intense rain are deep convective towers. Known as a convective burst, these are often a sign that the storm is about to strengthen as they release large amounts of heat (known as latent heat) into the storm. This heating is what drives the storm's circulation. So despite coming under the influence of northerly wind shear, some of which was due to outflow from Gustav in the Gulf of Mexico, Hanna managed to intensify into a category 1 hurricane about 3 hours later. Hanna remained a hurricane until the next morning when it finally succumbed to the wind shear and was downgraded to a tropical storm north of Haiti.

The next TRMM image was taken at 13:22 UTC (9:22 am EDT) on September 2 after Hanna was downgraded to a tropical storm. The storm is still asymmetric (the center is located near Great Inagua Island in the southeastern Bahamas above the Windward Passage between Haiti and Cuba), but some of the banding that was evident in the previous image is now gone. Despite losing some of its intensity, Hanna turned out to be devastating for Haiti as the center of the storm meandered just north of Hispaniola for 2 days, bringing torrential rains to the area.

TRMM can be used to calibrate rainfall estimates from other satellites. The TRMM-based, near-real time Multi-satellite Precipitation Analysis (TMPA) at the NASA Goddard Space Flight Center provides estimates of rainfall over the global Tropics.

Rain accumulation of Gustav and Hanna from Aug. 25 through Sept. 5, 2008
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TMPA rainfall totals associated with the recent tropical cyclone activity are shown here for the period 25 August to 5 September 2008 for the western tropical Atlantic and southeastern US. The highest rainfall totals for the period are over southern and central Haiti and exceed between 300 and 400 mm (~12 and 16 inches, shown by the orange and red areas, respectively). Some of this rain can be attributed to Gustav, which tracked across Cuba and the Gulf of Mexico into Louisiana. So far the death toll from Hanna stands at 137 in Haiti with most of the deaths reported near the port city of Gonaives.

Hanna finally began to move northward away from Haiti on September 3 as the upper-low off of the northeastern US pulled away and high pressure built back in across the western Atlantic. The storm then took a turn to the northwest on the evening of September 3, which placed it on a path near the central and northern Bahamas and eventually to the southeastern coast of the US.

TRMM image of Hanna on Sept. 4, 2008
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The last TRMM image of Hanna was collected at 19:42 UTC (3:42 pm EDT) on the September 4 as Hanna was moving to the northwest through the northeastern Bahamas. Due to southerly wind shear, the bulk of the rain is now located north and east of the low-level circulation (identified by the swirl in the low-level clouds shown in gray as opposed to the high clouds shown in white that are collocated with the rain). At the time of this image, NHC reported that Hanna's maximum sustained winds were 55 knots (~63 mph). Hanna is expected to gain a little more strength as it passes over the warm waters of the Gulf Steam, making it a strong tropical storm or possibly a minimal hurricane before it reaches the Carolina coast Friday night.

Meanwhile, Hurricane Ike, a much more powerful storm, continues to loom in the central Atlantic. Ike also began as an AEW that emerged off of the coast of Africa on the night August 28. Ike became a depression on September 1 while it was in the central Atlantic far from land. It was upgraded to a tropical storm later that same day. Ike remained a tropical storm for the next two days as it steadily made its way to the west- northwest through the central Atlantic. On the afternoon of September 3, however, Ike began a rapid deepening cycle, which brought it from a tropical storm to a category 4 hurricane with sustained winds estimated at 125 knots (~144 mph) in less than 24 hours.

TRMM image of Ike on Sept. 3, 2008
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TRMM snapped this image of Ike just as it was becoming a hurricane. The image was taken as 19:03 UTC (3:03 pm EDT) on September 3. Ike is a rather small storm, but it already has a well-defined eye in the rain field (the hole in the center) with tightly- curving rainbands surrounding the center. Areas of intense rain (dark red) are embedded in the northern and western eyewall. Ike's cloud pattern is very symmetric with good outflow. All of these are signs that the storm already has a well-developed circulation.

Ike remained a small but powerful category 4 storm on September 4 before weakening back down to a category 3 storm on the morning of the September 5 due to increasing northeasterly wind shear. Ike is expected to take a more west-southwesterly track through the Turks and Caicos and southern Bahamas. It could regain some strength and possibly make landfall in southern Florida or even enter the Gulf of Mexico.

The last storm in the Atlantic is Josephine. It too began as an AEW that came off of Africa on the night of August 31. Unlike the previous two storms, Josephine developed quickly and was already a tropical depression (TD #10) while it was still passing south of the Cape Verde Islands on September 2. The Cape Verde Islands are located about 700 km west of the coast of West Africa. Storms that form in this region are known as "Cape Verde" storms. They typically form in the middle of the hurricane season (August or September) when water temperatures become more favorable there. These storms have the potential to become quite powerful due to the vast expanse of open ocean. Ike is also a Cape Verde storm as it formed well out into the central Atlantic. Although Josephine started off quickly, it had trouble maintaining itself due to rather strong southerly vertical wind sheer brought about by a large upper-level trough of low pressure located in the central Atlantic west of the storm. So despite the early development, Josephine only reached a maximum estimated intensity of 55 knots (~63 mph), which occurred on September 3.

TRMM image of Josephine on Sept. 5, 2008
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TRMM captured this image of Josephine at 09:06 UTC (5:06 am EDT) on 5 September while the storm was heading northwest towards the central Atlantic. At this time, most of the rain within the storm is weak (blue areas) and there is very little evidence of banding. As a result of wind shear, the upper-level cloud shield (shown in white) is displaced northeast of the low-level circulation. Josephine is expected to weaken and continue heading into the central Atlantic well away from any land areas.

TRMM is a joint mission between NASA and the Japanese space agency JAXA. Credit: Images produced by Hal Pierce (SSAI/NASA GSFC) and caption by Steve Lang (SSAI/NASA GSFC).

Kathryn Hansen
NASA's Goddard Space Flight Center