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| |  | | | | Katrina Intensifies Into a Powerful Hurricane, Strikes Northern Gulf Coast
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08.30.05
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The 2005 hurricane season will long be remembered both for the record-breaking
number of early storms and for the emergence of a powerful Category 5 hurricane
in the central Gulf of Mexico--Hurricane Katrina. Katrina began as a tropical
depression (TD #12) in the central Bahamas on the afternoon of 23 August 2005,
before making landfall just south of Fort Lauderdale along the southeast coast
of Florida on the evening of August 25 as a Category 1 hurricane. Katrina was
blamed for 9 deaths in Florida, several as a result of falling trees. After
coming ashore, Katrina cut southwestward across southern Florida. The relatively
short amount of time the center spent over land combined with the wet marshy
composition of the Florida everglades kept Katrina from weakening all that much.
As a result, Katrina quickly regained hurricane status after emerging into the
Gulf of Mexico, becoming a Category 1 storm on the morning of August 26.
Conditions in the Gulf, however, were favorable for development and Katrina began
to intensify. By the evening of the August 26, Katrina was a Category 2 storm as it
continued to move slowly west-southwest in the southeastern Gulf of Mexico. On
the morning of August 27, Katrina became a Category 3 storm with maximum sustained
winds reported at 100 knots (115 mph) by the National Hurricane Center (NHC). The
central pressure continued to drop throughout the day, however, and the storm
began to shift to a more west-northwesterly direction.
In November of 1997, the Tropical Rainfall Measuring Mission satellite, known as
TRMM, was launched to measure rainfall over the global Tropics. TRMM, however,
has shown itself to be a valuable instrument for observing tropical cyclones.
This first image was taken at 03:24 UTC 28 August 2005 (11:24 pm EDT 27 August)
just as Katrina was about to become a Category 4 hurricane in the central Gulf of
Mexico. The image reveals the horizontal distribution of rain intensity within
Katrina as obtained from TRMM's sensors. Rain rates in the central portion of
the swath are from the TRMM Precipitation Radar (PR), the only radar capable of
measuring precipitation from space. The PR is able to provide fine resolution
rainfall data and details on the storm's vertical structure. 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 the time
of the image, Katrina was still a Category 3 storm with maximum sustained winds
reported of 100 knots (115 mph). TRMM reveals that Katrina had a closed eye
surrounded by concentric rings of heavy rain (red areas) that are associated with
outer rain bands. The intense rain near the core of the storm indicates where
heat, known as latent heat, is being released into the storm. This latent heat
release is what drives the storm's circulation. + Click for high resolution image.
The second image was taken at the
same time and shows a 3D perspective of Katrina with a cut-away view through the
eye of the storm. The vertical height of the isosurface (15 dBZ) is determined by
the height of precipitation-sized particles as measured by the TRMM PR. Two
isolated tall towers (in red) are visible: one in an outer rain band and the other
in the northeastern part of the eyewall. This area of deep convection in the eyewall
is associated with the area of intense rainfall in the eyewall. The height of the
eyewall tower is 16 km. Towers this tall near the core are often an indication of
intensification as was true with Katrina, which became a Category 4 storm soon
after this image was taken. + Click for high resolution image.
During the early morning hours of the August 28 (local time), Katrina's central
pressure continued to drop, and the storm intensified into a powerful Category 5
hurricane. By 11:00 am EDT on August 28, Katrina's sustained winds reached an
unbelievable 175 mph! At 17:55 UTC (1:55 pm), a NOAA hurricane hunter aircraft
measured a central pressure of 902 mb, the fourth lowest ever recorded in the Atlantic
Basin. Katrina now turned more to the northwest in response to a weakness in the
subtropical ridge to the north and an approaching trough from the west.
The final
image was taken by TRMM at 02:29 UTC August 29 (9:29 pm CDT August 28) as Katrina
bore down on the north-central Gulf Coast. The center of Katrina does not fall
within the PR swath in this image. However, the large eye of the storm is clearly
visible by the TMI by the large ring of moderate intenity rain, which is visible
by the green annulus. The first outer rain bands with embedded areas of heavy rain
(red areas) are already impacting the coast in southeastern Louisiana. At the time
of this image, Katrina was at Category 5 intensity with maximum sustained winds
measured at 140 knots (161 mph) by NHC. Katrina initially made landfall at 6:10 am
CDT south of Buras, Lousiana along the Mississippi delta as a strong Category 4
storm. The eye eventually crossed the coastline again along the Mississippi-
Louisiana border with the most dangerous part of the storm, the eastern eyewall
hitting along the same part of the Mississippi coastline that was wiped out by
Hurricane Camille back in 1969. Click on image to view animation.
Images credit: NASA
TRMM is a joint mission between NASA and the Japanese space agency JAXA.
Images produced by Hal Pierce (SSAI/NASA GSFC).
Steve Lang
Goddard Space Flight Center
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