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Sept. 2, 1998: (this is the thirteenth in a series of stories covering the ongoing CAMEX mission to hunt hurricane data in a way not done since the 50s. Other stories are linked in below.)
Hurricane Bonnie cut a towering figure twice as tall as Mount Everest a week ago when it was scanned by the first spaceborne rain radar as a part of a multi-level field campaign. On Saturday, Aug. 22, the radar on the Tropical Rainfall Measuring Mission (TRMM) observed a chimney of clouds as Bonnie slowed and gathered strength north of Hispaniola and east of the Bahamas. The chimney formation was not observed by the Convection and Moisture Experiment (CAMEX-3) science team - their crews were resting and planning in anticipation of two days of eye wall penetration flights - but a similar effect was observed on Aug. 25 when a dome-shaped cloud formed and snowed on the DC-8 Airborne Laboratory.
Meanwhile, Bonnie, which is churning the North Atlantic far away from the CAMEX-3 study area, continues to surprise scientists as they review data collected from the large array of instruments used to studying the storm. Although the GOES-8 weather satellite was watching Bonnie the whole time, it was looking almost straight down on the storm and could not see the interior structure. Thus, the grandeur of the cloud chimney was difficult to appreciate - except for TRMM's radar.
"It looks like a skyscraper in the clouds," said Dr. Christian Kummerow, TRMM Project Scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "This is the first time that TRMM's precipitation radar has seen a structure of this type in a hurricane approaching the U.S. East coast." "Clouds this tall are rarely observed in the core of Atlantic hurricanes," said Dr. Bob Simpson, former director of the National Hurricane Center in Miami and the National Hurricane Research Project. "This huge cloud probably happened because, at the time the data was collected, Bonnie was moving very slowly. The lack of movement kept funneling warm moist air into the upper atmosphere, thus raising the entire height of the tropopause, which is normally at around 45-52,000 feet (13.7-15.9 km). The tropopause marks the upper limits of Earth's densest layer of atmosphere, the troposphere, the layer starting at the surface where we live. "The vast amount of warm, moist air being raised high into the atmosphere, and the subsequent release of latent energy as this tropical air mass condensed into rain drops, is thought to be the precursor of hurricane intensification, which was observed in Bonnie in the 24 to 48 hours after these data were collected," Simpson continued.
Many scientists believe that towering cloud structures, such as the one observed by TRMM, are probably a precursor to hurricane intensification. This was the situation with Bonnie, whose central pressure dropped from 977 to 957 millibars in the subsequent 24 hours. Lower air pressure is associated with higher wind speeds and overall storm strengthening.
"TRMM has flown over 100 tropical cyclones since its launch in November of 1997," said Kummerow. "This enormously enhances our database of cloud structures within tropical storms during their growth and decay phases. It also greatly improves the more restricted observations we have obtained from aircraft radar and allows for the systematic study of this hurricane behavior which appears to precede their intensification." As the height of the hurricane season approaches, TRMM scientists are looking forward to the continuing analysis of Atlantic hurricanes. TRMM, launched Nov. 27, 1997, is the first dedicated to measuring tropical and subtropical rainfall through microwave and visible infrared sensors, including the first spaceborne rain radar. TRMM fills an enormous void in the ability to measure world-wide precipitation because so little of the planet is covered by ground-based radars. Presently, only 2 percent of the area covered by TRMM is covered by ground-based radars or surface rain gauges. By studying rainfall regionally and globally, and the difference in ocean and land-based storms, TRMM is providing scientists the most detailed information to date on the processes of these powerful storms, leading to new insights on how they affect global climate patterns. |
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