November 2nd, 2006 02:45 EST
NOAA Research of Hurricane Isabel Reveals Intensity Clues
While over the open western Atlantic Ocean during the 2003 hurricane season and before striking the U.S. East Coast, powerful Hurricane Isabel reached Category 5 strength and afforded NOAA researchers and their university partners the opportunity to collect a wealth of data. With this greater insight, researchers are able to better understand how the lower portion of a hurricane's eye and small circulations within the eyewall combine to influence intensity and sometimes lead to extreme wind speeds.
"Hurricane Isabel shared some of its secrets with us," said Sim Aberson, a meteorologist at the NOAA Atlantic Oceanographic and Meteorological Laboratory's Hurricane Research Division in Miami, Fla. "We were able to directly observe some unusual aspects of its structure and to use the data to gain understanding about how they contributed to the hurricane's extreme intensity."
Researchers were drawn into studying Isabel more in-depth upon detecting extremely high velocity winds within the hurricane that were observed by surveillance flights. Their findings are highlighted in two articles published in the October issue of the Bulletin of the American Meteorological Society, or BAMS.
Michael Montgomery, a professor at the Naval Postgraduate School's Department of Meteorology in Monterey, Calif., and a research scientist at HRD, authored the second NOAA article. Other co-authors are Michael Black of HRD and Michael Bell of the National Center for Atmospheric Research and Colorado State University. Montgomery and Aberson said the two papers compliment one another.
In one paper, the scientists use dropwindsonde and in-situ aircraft data from NOAA's P-3 hurricane hunter aircraft and an Air Force aircraft to create a composite of the storm's inner-core structure. The composite suggests a reservoir of high energy in the form of heat available to the hurricane. The reservoir is created by the transfer of moisture to the atmosphere from the underlying ocean. Previous theories have assumed that the low altitude portion of the eye plays no role in determining the storm's maximum winds. The Isabel research suggests something quite different, Aberson said.
"This high-energy air is transported and mixed into the eyewall where the strongest winds are. The 'extra heat' added to the eyewall leads to an acceleration of the swirling wind field," Montgomery said. "This extra boost is like a driver stepping on the gas pedal to make a car go faster."
The second paper focuses on a unique set of observations into an extraordinary small-scale cyclonic feature inside the inner edge of Isabel's eyewall. A small, tube-shaped instrument package called a dropwindsonde released into this feature measured the strongest known horizontal wind in a tropical cyclone—240 mph (107 m/s or 208 kt.)
The authors believe that these features are the result of what was described in the first paper and that similar "little whirls" may have been responsible for catastrophic wind damage during the landfall of Hurricane Andrew in 1992 in South Florida and in other lesser-known cases. Understanding the meteorological conditions that cause these intense vortical features will allow scientists to assess high resolution intensity forecast models that are in development at NOAA.
Aberson said these observations should help improve intensity forecasts. NOAA flew 11 research flights into Hurricane Isabel using its two NOAA P-3 research aircraft and the Gulfstream-IV jet. The U.S. Air Force also contributed by flying operational hurricane reconnaissance with their C-130 aircraft.
"This research is a great investment," Montgomery said. "It will repay us many times over in improving our understanding of the physics of intense storms and ultimately will lead to better forecasts and warnings. More research of this sort needs to be carried out using NOAA's unique resources."
The American Meteorological Society is the nation's leading professional organization for those involved in the atmospheric and related sciences. Founded in 1919, the AMS has more 11,000 international members, organizes nearly a dozen scientific conferences annually and publishes nine peer-reviewed journals.
In 2007 NOAA, an agency of the U.S. Commerce Department, celebrates 200 years of science and service to the nation. Starting with the establishment of the U.S. Coast and Geodetic Survey in 1807 by Thomas Jefferson much of America's scientific heritage is rooted in NOAA. The agency is dedicated to enhancing economic security and national safety through the prediction and research of weather and climate-related events and information service delivery for transportation, and by providing environmental stewardship of the nation's coastal and marine resources. Through the emerging Global Earth Observation System of Systems (GEOSS), NOAA is working with its federal partners, more than 60 countries and the European Commission to develop a global monitoring network that is as integrated as the planet it observes, predicts and protects.