Aug. 25, 2011
NASA's TRMM Satellite Provides an In-Depth Study of Hurricane Irene's Rainfall
NASA's TRMM satellite identified large areas of heavy rainfall within Hurricane Irene affecting the Bahamas, and helped forecasters identify its "hidden" eye. Over the days of August 23 and 24, 2011, NASA's TRMM satellite provided key information on rainfall rates, hot towering clouds that act as hurricane heat engines, and cloud heights.
After becoming a minimal hurricane while passing over Puerto Rico, Irene re-emerged over the warm, open waters of the western Atlantic northwest of the Dominican Republic on the morning of August 22nd. Irene quickly showed signs of intensifying as deep convective towers arose near the center of Irene, releasing heat into core of the system. In response, the central pressure fell and the winds intensified, making Irene a Category 2 storm on the Saffir-Simpson hurricane intensity scale with sustained winds reported at 85 knots (~100 mph) by the National Hurricane Center by the end of the day. Irene remained at Category 2 intensity most of the following day as it brushed the northern coast of Hispaniola heading west-northwest in the direction the Turks and Caicos Islands. Late in the afternoon on the 23rd, Irene weakened slightly after ingesting some drier air into its circulation. But the effect was short lived as new convective towers signaled that Irene was again ready to re-intensify.
The Tropical Rainfall Measuring Mission (or TRMM) satellite passed directly over Irene late in the day on the 23rd and captured these two images as the storm was bearing down on parts of the Caicos Islands. Images were taken at 23:11 UTC (7:11 p.m. EDT) on August 23, 2011.
One image shows a top-down view of the rain intensities within Irene. TRMM revealed that although Irene does not have a visible eye, deep down under the cloud tops there is an eye surrounded by a complete eyewall of varying rain intensities as evidenced by the complete circle in the rain field at the center of the image.
Creating the image is complicated. Rain rates in the center of the swath are from the higher resolution TRMM Precipitation Radar (PR), while those in the outer swath are from the TRMM Microwave Imager (TMI). The rainrates are overlaid on visible and infrared (IR) data from the TRMM Visible Infrared Scanner (VIRS).
The image also showed that the northeast corner of the eyewall contains an area of intense rain (2 inches/50 mm per hour) and the southern portion only light rain with mostly moderate rain in between. The storm is still fairly asymmetrical, however, with most of the surrounding rain northeast of the center.
Another image taken at the same time shows a 3D view of Irene that shows "hot tower" clouds. The tops of these deep convection towers have precipitation-sized particles that are being carried higher into the atmosphere by strong thunderstorms. These storms within a storm can intensify tropical cyclones and hurricanes by releasing large amounts of heat, known as latent heat, via condensation. This heat can intensify the hurricane's circulation especially when released near its core.
TRMM showed several towers both near the center and in the outer rainbands with tops near 15 km (brighter red areas). These areas are associated with the areas of intense rain seen in the previous image. Also, evident at this time is an area of shallow tops (in blue) near to the center where the drier air had worked its way into the storm.
At the time of these images, Irene was a Category 1 storm with sustained winds of 80 knots (~90 mph). However, the central pressures were already beginning to fall and Irene re-intensified into a Category 2 storm just over 6 hours later.
Irene continued to strengthen overnight and by 8 a.m. EDT the next morning on the 24th, it had become a Category 3 storm with sustained winds of 100 knots (~115 mph).
TRMM next passed over Irene at 15:42 UTC (11:42 a.m. EDT) on the 24th as she was passing directly over Crooked Island in the southeast Bahamas. Although the eye is somewhat obscured by the island, Irene did have a visible eye before passing over the island. Unlike the previous day's TRMM image, the rainbands surrounding the center were now very tightly wound and are more evenly distributed about the center. These are characteristic of a larger, much more intense circulation as any rain features are quickly smeared around in a circle around the center by the stronger winds.
Another 3-D view of Irene was captured on August 24 the storm, revealed once again the presence of a deep convective tower within the eyewall. At the time of these last two images, Irene's maximum sustained winds were still 100 knots (~115 mph), but the storm's central pressures were continuing to fall, an indication the storm was in the process of intensifying. Just over two hours later, Irene's maximum sustained winds had risen to 120 mph. Irene is expected to pass through the central Bahamas and could become a Category 4 storm before threatening the US East Coast.
TRMM is a joint mission between NASA and the Japanese space agency JAXA.
Steve Lang, SSAI/NASA Goddard Space Flight Center
, Greenbelt, Md.