Aug. 15, 2001 BOULDER -- Despite its tropical origin, the upper two-thirds of a typical hurricane is largely ice. Scientists from the National Center for Atmospheric Research (NCAR) are bringing unique cloud-profiling instruments into this mysterious realm in a NASA-sponsored project to help improve hurricane forecasting and modeling. Also supporting NCAR's participation is the National Science Foundation, NCAR's primary sponsor.
Based at Florida's Jacksonville Naval Air Station, the project runs from August 16 to September 24. NASA's DC-8 and ER-2 research aircraft will join satellites and other sensors to analyze the structure of hurricanes at sea and as they hit land.
NCAR's Andrew Heymsfield, one of the principal investigators, will fly seven instruments aboard the DC-8 to get the clearest-ever picture of frozen and condensed water within a hurricane. "A hurricane might extend 60,000 feet high, but only the bottom 15,000 feet is in the rain phase. The upper 45,000 feet or so is usually ice particles," explains Heymsfield, "and that's what we're going to be looking at."
The huge swirls of white cloud evident on hurricane satellite photos consist mainly of ice crystals. As water vapor freezes to form ice, it releases vast amounts of latent heat, which "helps to drive hurricanes," Heymsfield says. "You need to get the ice phase going to really intensify the hurricane." Typical hurricane-hunting flights operate below 20,000 feet, so they obtain only limited information on ice content.
Heymsfield's instruments will fly as high as 43,000 feet aboard the DC-8. One is a sophisticated cloud particle imager that shines a tiny laser beam on an array of photo diodes. Ice crystals passing in front of the laser leave a shadow on the array. The resulting photos, taken 40 times a second, show the crystal structure in fine detail. Heymsfield has taken the imager into cirrus clouds, but this will be its first foray into a hurricane. With the help of other sensors that measure overall moisture, Heymsfield and colleagues will study how much water a hurricane deposits in its upper levels and how much dry air it pulls down into the calm, clear eye. "For better forecasts of hurricane landfall and intensification, we need to know how much ice is transported into the upper two-thirds of a hurricane," says Heymsfield.
The project will also gain unprecendented detail on winds near the tops of hurricanes. Flying as high as 65,000 to 70,000 feet, the ER-2 will launch GPS dropsondes, NCAR-designed instrument packages that use the Global Positioning System. As they parachute down, the dropsondes will gather wind, temperature, and moisture data at upper levels about every 50 to 100 feet. The GPS dropsondes--never before released at these heights--will be launched from a new automated deployment system developed at NCAR. As many as ten DC-8 and ER-2 flights will take place, depending on where and when hurricanes develop. The aircraft range of more than 1,700 miles encompasses the entire Gulf of Mexico, most of the Caribbean Sea, and much of the western Atlantic. If the Atlantic season is unusually quiet, the aircraft may venture into the Pacific.
CAMEX-4, the fourth Convection and Moisture Experiment, is the first field project of the U.S. Weather Research Program, a multiagency effort to reduce the national impact of disastrous weather, particularly hurricanes. NCAR's Robert Gall is the USWRP's lead scientist. CAMEX is sponsored by NASA's Earth Science Enterprise.
NCAR is managed by the University Corporation for Atmospheric Research, a consortium of 66 universities offering Ph.D.s in atmospheric and related sciences.
On the Web: CAMEX-4 home page: http://camex.msfc.nasa.gov Two-minute Web video on Heymsfield/CAMEX: ftp://ftp.ucar.edu/communications/camex.qt
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