CYCLONE PERCY

February 28, 2005 0829 UTC	February 28, 2005 0829 UTC PR SLICE	March 2, 2005 0818 UTC

POWERFUL CYCLONE PERCY CHURNS THROUGH THE SOUTH PACIFIC

Percy, a powerful Category 4 cyclone, is now the latest cyclone to threaten the South Pacific. In early February Cyclone Meena hit the Cook Islands, and in mid-February Cyclone Nancy also skirted the Cook Islands while Cyclone Olaf brushed the islands of Samoa and American Samoa. Percy started out as a tropical depression on the 24th of February 2005 near Tuvalu just east of the dateline. The system moved east-southeast staying well north of Samoa and steadily increased in strength, becoming a cyclone on the 25th and a powerful Category 4 cyclone on the 27th with sustained winds estimated at 115 knots (132 mph) by the Joint Typhoon Warning Center. Percy then slowed down, weakened slightly and battered the atolls of Nassau and Pukapuka in the northern Cook Islands before turning south on the 28th. Percy regained Category 4 strength on the 1st of March and then further intensified into a powerful Category 5 storm on the 2nd with maximum sustained winds estimated at 140 knots (161 mph).

Back in November of 1997, the Tropical Rainfall Measuring Mission or TRMM satellite was launched to measure rainfall over the global Tropics. TRMM has also turned out to be an excellent observational platform for studying and monitoring tropical cyclones. The following images of Percy were obtained by TRMM. The first image was taken at 08:29 UTC on 28 February 2005 just as Percy was raking the Nassau and Pukapuka atolls. It shows the horizontal distribution of rain intensity as viewed by TRMM. Rain rates in the center swath are from the TRMM Precipitation Radar (PR), the first and only 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). TRMM shows an asymmetrical eye with intense rain (dark red area) in the northern part of the eyewall. However, this rain indicates where heat is being released into the storm. Known as "latent heat", it is the heat released when water vapor condenses into liquid cloud droplets. It is most effective in driving the cyclone's circulation when it is released near the center as is the case shown here by TRMM. The next image was taken at the same time and shows a vertical cross section through the center of Percy by the TRMM PR looking east. The intense rain (darker red area) is associated with the tallest towers in the northern eyewall. The broad yellow area indicating moderate rain is associated with an outer rainband. At the time of these images, Percy was a Category 3 storm with sustained winds estimated at 105 knots (121 mph).

The final image was taken at 08:18 UTC on 2 March 2005. At this time Percy was down to a strong Category 4 cyclone with maximum sustained winds estimated at 130 knots (150 mph). The center of the storm does not fall within the PR swath, which has a higher horizontal resolution than the TMI. However, the TMI is still able to show what appears to a double eyewall. Mature, intense tropical cyclones undergo what it known as eyewall replacement cycles wherein an outer eyewall forms as a ring surrounding the inner eyewall. The outer eyewall eventually contracts and replaces the inner eyewall. The two eyewalls are evident as the two concentric rings of moderate rain intensity (green areas).