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 the 25th 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 the 26th of August. Conditions in the Gulf, however, were favorable for development and Katrina began to intensify. By the evening of the 26th, Katrina was a Category 2 storm as it continued to move slowly west-southwest in the southeastern Gulf of Mexico. On the morning of the 27th, 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 has 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. The second image was taken at the same time and shows a 3D perspective of Katrina with a cut-away view throuth 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.

During the early morning hours of the 28th of August (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 the 28th, 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 recored 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 29th (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.

TRMM is a joint mission between NASA and the Japan Aerospace Exploration Agency (JAXA).

Images produced by Hal Pierce (SSAI/NASA GSFC) and caption by Steve Lang (SSAI/NASA GSFC).