September 2001

Driven by ice: the mysterious tops of hurricanes

Even though hurricanes are sustained by the warm waters of the tropics, much of their behavior is driven by a little-understood zone of ice that forms in the upper two-thirds of the storms. In an attempt to learn more about this ice and ultimately improve hurricane forecasting, NCAR scientists are using unique cloud- profiling instruments and dropping high-precision instrument packages as part of a sophisticated weather study.

This satellite photo of 1998's Hurricane Mitch shows the vast sheets of ice-crystal cloud extending outward at high levels (courtesy NASA-GSFC, data from NOAA GOES).

The ice crystals may shed light on important processes in hurricane evaluation (crystal photograph courtesy Andy Heymsfield).

Andy Heymsfield. (Photo by Bob Henson.)

Andrew Heymsfield (MMM), one of the principal investigators, is flying 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," Andy explains. "The upper 45,000 feet or so is usually ice particles, 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," Andy 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.

This device, designed and built by ATD and the NCAR machine shop, is allowing the pilot of a high-flying ER-2 airplane to deploy GPS dropsondes with the flick of a switch. (Photo courtesy Terry Hock.)

One of MMM's instruments aboard the DC-8 is a sophisticated cloud particle imager, built by Boulder-based SPEC Inc., 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. Andy 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, Andy and his colleagues are studying 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," Andy says.

The project will also provide unprecedented detail on winds near the tops of hurricanes. Flying as high as 65,000 to 70,000 feet, the ER-2 is launching GPS dropsondes, which are NCAR-designed instrument packages that use the Global Positioning System. As they parachute down, the dropsondes gather wind, temperature, and moisture data at upper levels about every 50 to 100 feet. The GPS dropsondes—never before released at these heights—are being launched from a new automated deployment system developed at NCAR in collaboration with NASA. Unlike other dropsondes, which have to be released manually, the GPS sondes can be released remotely by the pilot in the cockpit, thereby allowing one-person planes such as the ER-2 to deploy them. The new system was designed and built by members of ATD's Research Technology Facility and Design and Fabrication Services. The group included: Hal Cole, Jack Fox, Terry Hock, Dean Lauritsen, Ken Norris, Chip Owens, Steve Rauenbuehler, Dean Smith, and Jack Fox.

Researchers may be flying as many as ten DC-8 and ER-2 missions, 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 be venturing into the Pacific.

Collaborators with Andy on CAMEX-4 include Aaron Bansemer (MMM) and Cindy Twohy (Oregon State University), along with SPEC staff. Andy's brother, Gerry Heymsfield (NASA Goddard Space Flight Center), will also be on hand, overseeing the Doppler radar aboard the ER-2.

Edited by David Hosansky, hosansky@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Mon Sep 17 15:38:00 MDT 2001