TRMM Satellite Analyzes Hurricane Irene in Rainfall, Lightning, Eyewall

Three times this week (week of August 21, 2011) , the radar on the TRMM satellite has observed the three-dimensional structure of the rainfall regions under the cloud-tops of Hurricane Irene. At the same time, TRMM also observed Irene with passive microwave, infrared, and lightning instruments. In this image, the passive microwave estimates of surface rainfall are for all three overflights.

The Tropical Rainfall Measuring Mission Satellite known as TRMM has the ability to measure rainfall from space, and can also provide scientists with cloud heights, eyewall and lightning information. These images and captions of Hurricane Irene were provided by NASA hurricane scientist, Owen Kelley at NASA's Goddard Space Flight Center, Greenbelt, Md.

In general, lightning in the inner core suggests the presence of strong updrafts and large ice particles. By implication, lightning suggests that convective cells are pumping a lot of latent heat energy into the tropical cyclone's central vortex, which is favorable for intensification. Hurricane eyewalls often are devote of lightning, as is the case for Irene on 8/23. But cat-3 Irene (on 8/24) did have lightning flashes in the eyewall, and there were many flashes during Irene's tropical storm phase (on 8/21).

On Sunday, August 21, 2011, Irene was only a tropical storm, and the TRMM Precipitation Radar reveals that it had an impressive hot tower that reached over 9.5 miles high (15.75 km). Strong radar signals, perhaps from large ice particles are shown in red at the base of the hot towers, which suggests strong updrafts were present in order for the ice particles to have time to grow large. Strong rain had yet to completely encircle the eye, as is typical of tropical storms.

On Tuesday evening, August 23, 2011 Irene was a category 1 hurricane, in the process of intensifying to category 3 in less than 12 hours.Consistent with intensification, the TRMM radar showed that Irene had a complete eyewall circling its eye along with a region of intense convection on the eastern side of the eyewall (right side of image). Some studies suggest that bursts of convection in one portion of a hurricane's eyewall may help to pump latent heat into a hurricane and thereby assist with intensification.

On Wednesday afternoon, Irene had reached the status of a "major" hurricane, at category 3 strength. The observations of the TRMM radar show a compact eyewall, with the possibility that an outer eyewall is forming. Double eyewalls can be part of an eyewall replacement cycle associated with changes in hurricane intensity, either increases or decreases that are difficult to predict.

Text Credit: Owen Kelley, NASA Goddard Space Flight Center, Greenbelt, Md.

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