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CYCLONE SIGNALS START OF SEASON IN NORTH INDIAN OCEAN
The recent formation of Tropical Cyclone Nargis in the Bay of Bengal coincides with
the start of cyclone season in the North Indian Ocean, which typically runs from
April through December. Despite the long season, the region, which includes the
Bay of Bengal and the Arabian Sea, on average has just over 5 named storms per year
with 2 becoming full tropical cyclones. Last year there were two storms, both of
which were notable. In June, Gonu, an extremely rare Category 5 cyclone in the
Arabian Sea, became the strongest storm ever recorded in that part of the basin.
Despite weakening before landfall, it went on to kill 72 people in the area as a
result of flooding. In November, a slightly less powerful but far more deadly
storm struck the southern coast of Bangladesh in the Bay of Bengal. The region,
which is known for it's high death tolls, suffered over 3400 fatalities. As a
result, Cyclone Nargis is being closely watched.
Nargis formed into a tropical cyclone on the 27th of April 2008 in the central Bay
of Bengal about 360 miles off of the southeast coast of India. Initial movement
remained northwesterly. The Tropical Rainfall Measuring Mission satellite (known
as TRMM) was placed into service in November of 1997. From its low-earth orbit,
TRMM can provide valuable images and information on tropical cyclones around the
Tropics using a combination of passive microwave and active radar sensors. The
first image shows Nargis just as it was becoming a tropical storm. The image was
taken at 04:37 UTC (10:07 am local time) 27 April 2008 and shows the horizontal
pattern of rain intensity within the storm. Rain rates in the center swath are
from the TRMM Precipitation Radar (PR), the first precipitation radar in space,
while rain rates in the outer swath are from the TRMM Microwave Imager (TMI). The
rain rates are overlaid on infrared (IR) data from the TRMM Visible Infrared
Scanner (VIRS). At this stage the center is not yet very well defined, although
a fledgling circulation is evident by the slight curvature in the rain bands. At
the time of this image, the estimated sustained winds had not yet reached 35 knots
(40 mph), the minimum threshold for tropical storm intensity. Nargis officially
became a tropical storm less then 8 hours after this image was taken.
Over the next day and a half, the system continued to steadily strengthen, becoming
a Category 1 cyclone during the day on the 28th and a Category 2 cyclone later that
night. The next image from TRMM was taken at 18:32 UTC 28 April (12:02 am 29 April
local time) and shows a much different looking storm. A large, well-defined eye is
now readily apparent. In the western semi-circle, a strong eye-wall can be identified
by intense rainfall (dark red arc). The storm is very symmetric with rain bands now
tightly curving around the center (green rings indicating moderate intensity rain).
At the time of this image, Nargis was a Category 1 storm with maximum sustained winds
estimated at 75 knots (86 mph) by the Joint Typhoon Warning Center. The system was
upgraded to a Category 2 storm with sustained winds of 85 knots (98 mph) few hours
later.
This last image was taken simultaneously with the previous and shows a 3D picture
of Nargis courtesy of the TRMM PR. A half ring of tall towers (highlighted in red)
reveals areas of deep convection (e.g., thunderstorms) that are associated with the
intense rainfall in the western eyewall that was seen in the previous image. These
tall towers can be a sign of future intensification as they release substantial
amounts of heat into the core of the storm. At present, Nargis has begun to recurve
towards the east-northeast and has weakened slightly. However, some intensification
is forecast before the system makes an expected landfall along the western coast of
Myanmar (Burma).
The Tropical Rainfall Measuring Mission (TRMM) is a joint mission between NASA and the Japan Aerospace Exploration Agency (JAXA) designed to monitor and study tropical rainfall.
Images by Hal Pierce (SSAI/NASA GSFC) Caption by Steve Lang (SSAI/NASA GSFC)
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