Typhoon Kujira from TRMM (click to enlarge)

LARGE [1.9mb] Animation Fade showing eye changes on April 14, 2003. MEDIUM [.4 mb] mpeg Fade animation showing eye changes on April 14, 2003.

This remarkable mosaic, obtained using the TRMM Microwave Imager, shows the evolution of Super Typhoon Kujira's eye in the Western Pacific, during April 14-16. This storm, located over open water well to the north of Papua New Guinea, was steadily intensifying during the period of TRMM observations, to a maximum sustained wind of 130 kts classifying the tropical cyclone as a Typhoon-4. The times of these images are as follows: Upper left (April 14, 11 UTC); upper right (April 14, 21 UTC); bottom left (April 15, 20 UTC); and bottom right (April 16, 11 UTC). Earth's natural microwave energy upwelling from the lower layers of the atmosphere passes through clouds unimpeded, but is partially blocked by large precipitating ice particles (such as snowflakes) high in deep clouds. The TRMM Microwave Imager (TMI) measures the amount of energy scattered by the ice particles; the more ice there is, the more energy is scattered, meaning smaller amounts of energy are received by TRMM. Low values of returned energy (200 degrees Kelvin, shown by the color scale) represent the most intense clouds (colored red on the image). The dark blue color in the center of the eye signifies very warm temperatures, essentially the absence of any ice-producing clouds. In addition to adding artificial color, the energy values have been rendered in terms of three dimensionsSwith the coldest, tallest clouds rising above all others. Thus, the eyewall appears as a tall ring of deep, intense convective clouds at the center of the storm.

By following the sequence of images, we can watch the structure of the eyewall change with time. Initially on the 14th, Kujira's eye is quite symmetric and very circular. It's "healthy" appearance signifies continued intensification. Only a few hours later on the 14th (top right panel), the storm's circulation evolves into several heavy rainbands. More interestingly, the eyewall appears to assume a double structure, with a partial eyewall ring embedded within the larger outer eyewall. Very powerful tropical cyclones such as Super Typhoons frequently undergo one or more of these "eyewall replacement cycles", where a new eyewall develops and replaces an existing one. When this happens, the intensity of the tropical cyclone can dramatically fluctuate. The lower left panel (15 April) shows the Super Typhoon during its mature stage, and a single eyewall is again present. The extent of the circulation has also greatly expanded, with broad rainbands wrapping in from the south. However, on the 16th (lower right panel), the storm's circulation is finally beginning to weaken; notice the marked disappearance of rainbands, and the eyewall has become asymmetric, with a disruption in the southeast quadrant. Loss of a contiguous eyewall usually heralds a decrease in storm intensity.

Like a CAT Scan, TRMM peers through the clouds and shows us the evolution of a typical powerful typhoon in Pacific, with the associated structural changes that signify intensification, peak intensity and weakening phases over a multi-day period.