Percy, a powerful Category 4 cyclone, is 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 February 24, 2005, near Tuvalu, just east of the international dateline. The system moved east-southeast staying north of Samoa and steadily increased in strength. It became a cyclone on February 25 and, two days later, grew into a powerful Category 4 cyclone 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 February 28. Percy regained Category 4 strength on March 1, and then further intensified into a powerful Category 5 storm on the 2nd with maximum sustained winds estimated at 140 knots (161 mph).
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, as shown by this series of images of Cyclone Percy. The top left image, taken at 08:29 UTC on February 28, just as Percy was raking the Nassau and Pukapuka atolls, shows the horizontal distribution of rain intensity. 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. 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 right image was taken at the same time by the TRMM Precipitation Radar (PR), and shows a vertical cross section through the center of the storm 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 lower left image was taken at 08:18 UTC on March 2, 2005. At this time Percy was 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).
TRMM is a joint mission between NASA and the Japanese space agency JAXA.