Typhoon Usagi continues to gather strength in the West Pacific as it continues on a course for southern Japan. The Tropical Rainfall Measuring Mission satellite (known as TRMM) was placed into its low-earth orbit back in November of 1997 with the primary mission of measuring rainfall from space; however, it has also served as a valuable platform for monitoring tropical cyclones, especially over remote parts of the open ocean. This series of remarkable images from TRMM captures the growth of Usagi in the Pacific. Usagi started out as a tropical disturbance in the northwest Pacific about midway between Wake Island and the Northern Marianas. After drifting generally westward for a couple of days the system began to organize and was classified as a tropical depression on the 28th of July 2007 and was soon after named a tropical storm. The first image above was from TRMM data taken at 10:35 UTC on 27 July 2007, about 24 hours before the system became a tropical depression. The image shows the horizontal distribution of rain intensity looking down on the storm. Rain rates in the center of the swath are from the TRMM Precipitation Radar (PR), while those in the outer portion are from the TRMM Microwave Imager (TMI). The rain rates are overlaid on infrared (IR) data from the TRMM Visible Infrared Scanner (VIRS). At this stage, the system appears as a relatively small cluster of storms. The circulation is very weak with no evidence of banding (curvature) in the rain field. However, the next image, taken at the exact same time, shows a 3D perspective of the system courtesy of the TRMM PR and reveals the areas of heavier rain visible in the previous image are associated with deep convective towers (shown in red). These can be a sign of future development as they release heat into the system.

On the 29th, Tropical Storm Usagi continued to develop as it propagated westward. At 18:00 UTC on the 29th, Usagi was upgraded to a typhoon and began a more northwesterly track. The next image was taken by TRMM about 24 hours later at 17:33 UTC on the 30th. Usagi was still a Category 1 typhoon. In contrast to the first image, Usagi is now a much larger storm with a well-developed circulation as evidenced by the many spiraling rainbands (blue and green arcs indicating areas of light to moderate rain, respectively). Usagi has a rather large, ragged eye at this time (central blue and green ring), but the storm is still fairly asymmetrical with most of the rain southeast of the center. At the time of this image, maximum sustained winds were estimated to be 75 knots (81 mph) by the Joint Typhoon Warning Center.

CLICK HERE to see a large 3-D "flyby" animation (1.5 mb) of USAGI courtesy of TRMM Precipitation Radar.

Soon after the last TRMM image was taken, Usagi was upgraded to a Category 2 cyclone. The next image was taken at 10:05 UTC 31 July. TRMM reveals that Usagi has continued to strengthen and organize. The eye (dark center) is now smaller and less ragged than in the previous image. The storm is also more symmetric with concentric rain bands surrounding the center. There is also a large area of intense rain (dark red area) on the western side of the eye. The final image shows a 3D view of Usagi at the exact same time. Again, the areas of intense rain are associated with deep convective towers, indicating Usagi is likely to continue to intensify as they release heat into the storm's core. The ring of deep convective towers (red tops) farther out but nearly surrounding center of the storm could be the beginning of an outer eyewall. At the time of these final two images, Usagi's sustained winds were estimated at 90 knots (104 mph). Soon after, Usagi was upgraded to a Category 3 cyclone with sustained winds estimated at 105 knots (121 mph). Usagi is expected to strengthen even further before possibly making landfall in southern Japan.

TRMM is a joint mission between NASA and the Japanese space agency JAXA.
Images and animations produced by Hal Pierce (SSAI/NASA GSFC) and caption by Steve Lang (SSAI/NASA GSFC)