Two cyclones, Olaf a powerful Category 4 cyclone and Nancy a weaker butstill strong Category 2 cyclone, are together threatening the Cook Islandsand the islands of Samoa in the South Pacific. This spectacular imagefrom TRMM, the Tropical Rainfall Measuring Mission satellite, shows CycloneOlaf in the upper left as it is approaching the islands of Samoa andCyclone Nancy in the lower right as it heads towards Rarotonga in the CookIslands.
The image was taken at 15:34 UTC on February 15, 2005, and showsthe horizontal distribution of rain intensity as seen from above by the TRMMsatellite. Rain rates in the center swath are from the TRMM PrecipitationRadar (PR), the first and only precipitation radar in space, while rainrates in the outer swath are from the TRMM Microwave Imager (TMI). The rainrates are overlaid on infrared (IR) data from the TRMM Visible InfraredScanner (VIRS).
At the time of this image, Olaf was a Category 4 stormwith maximum sustained winds estimated at 120 knots (138 mph) by the JointTyphoon Warning Center and was in the process of intensifying. Nancy,meanwhile, was a Category 2 storm with maximum sustained winds estimated at95 knots (109 mph) and was beginning to weaken. These characterizations aresupported by TRMM, which shows that Olaf has much better banding in the rainfield as evidenced by the green arcs of moderate rain intensity and a tight,closed eye (green circle). Nancy exhibits no well-defined eye, and the banding is much less evident.
When two cyclones comewithin close proximity and their circulations begin to interact with eachother, two possible interactions can occur. One interaction is known as the Fujiwaraeffect, wherein the cyclones begin circling around each another. The other possibility isthat the outflow from one storm impedes the outflow from the other storm, weakening it.
TRMM is a joint mission between NASA and the Japanese space agency JAXA.
References & Resources
Image produced by Hal Pierce (SSAI/NASA GSFC) and caption by Steve Lang (SSAI/NASA GSFC).
