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Summer 2002

New probes paint a wet picture of high-altitude cirrus

by Bob Henson

Hidden within the cirrus clouds atop hurricanes is a lot of liquid water—far more than Andy Heymsfield expected. The NCAR scientist took a suite of cloud-particle probes into 4 of the 15 tropical cyclones that formed in the Atlantic last fall. Heymsfield is now readying the probes for a major NASA study of tropical cirrus as he and colleagues continue to sort through the results of last year’s experiment.
In a project led by Andy Heymsfield (right), a cloud particle imager collected thousands of ice-crystal images from Hurricane Humberto, including those pictured above. (Imagery courtesy Andy Heymsfield and Aaron Bansemer; photo by Bob Henson. NOAA satellite image of Humberto courtesy Applied Physics Laboratory, Johns Hopkins University.)

In that project, the fourth NASA-sponsored Convection and Moisture Experiment, the probes were mounted on the wings of a DC-8. Particles and droplets along the path of the instruments were detected and documented by five sensors, each using a laser beam and photodiode array or a CCD camera. Two other sensors produced a collective measure of the total water and ice content present along the path. Data were compiled in five-second blocks that represented about a kilometer of flight space. The highest-resolution imagery allows specific crystal types to be identified.

One flight, through Tropical Storm Chantal on 20 August, encountered a bumper crop of ice (perhaps with substantial amounts of supercooled liquid) so dense that it resisted measurement. "There was something serious happening at temperatures of about –48°C," says Heymsfield. According to NCAR associate scientist Aaron Bansemer, "Most of our instruments were swamped by the unexpectedly harsh conditions in the storm. Clogged inlets, electrical discharges, vibrations, and broken hot wires all claimed proven instruments."

Despite these glitches, the season’s other flights confirmed the notion that considerably more water flows through the tops of tropical cyclones than previously thought. In Hurricane Humberto, one of the probes found graupel (ice-coated snow crystals) as large as 5 millimeters in diameter at heights of 37,000 feet. Such pea-sized hail wouldn’t be unusual in a Midwest thunderstorm, but it has rarely been documented in hurricanes.

The geographic distribution of particles was surprising as well. At the center of Humberto, a greater number of large particles than expected was found, while two areas toward the northern fringe of the storm showed an unexpected lack of small particles (0.1–0.4 mm in diameter). The credibility of these findings was bolstered by their quantity: over 15 million particles were recorded by one of the probes. The Humberto results are being presented in June at the American Meteorological Society’s 11th Conference on Cloud Physics in Ogden, Utah.

Putting a high-resolution FACE on Florida cirrus

This summer Heymsfield and Bansemer are turning the cloud-particle probes away from tropical cyclones and toward other sources of high-altitude clouds. The Florida Area Cirrus Experiment (FACE) will operate out of Key West this July as part of the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL). The large cast of participants includes NASA, NOAA, NSF, the U.S. Department of Energy, the Office of Naval Research, and the U.S. Weather Research Program, along with a number of universities.

Over 150 scientists will use six aircraft in CRYSTAL-FACE to examine the near-daily thunderstorms in and around South Florida, especially the anvils that flow up and out from them. Tropical cirrus clouds play a key role in Earth’s energy balance, but they remain poorly documented. Among the questions being explored:

o How does thunderstorm intensity affect the size and longevity of anvils? Fierce storms can produce huge anvils, for example. By the same token, larger droplets and crystals may scour away the anvil more quickly than in weaker storms.

o What happens to anvils after their parent storms dissipate? Wind shear helps both to spread and to disperse storm-generated cirrus. If conditions are right, a large anvil at unusually high altitudes may be lofted to the more stable realm of the tropopause (the troposphere-stratosphere boundary), where the remnant cloud—possibly too thin to be seen by the naked eye—could persist for days. CRYSTAL-FACE will take a close look at clouds, water vapor, and radiation at these little-studied heights.

Data from CRYSTAL-FACE will help validate measurements from NASA’s growing complement of Earth-sensing satellites. They’ll also help improve the treatment of cirrus clouds in both small- and large-scale models.

Heymsfield, who has studied high-altitude clouds for 30 years, is eager to see whether this year’s crop of cirrus can match the surprises found in last year’s hurricanes. CRYSTAL-FACE brings together "the largest assemblage of cirrus investigators to date," he notes, as well as the most complete set of instruments ever trained on cirrus and their environment.

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Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Carlye Calvin
Last revised: Thurs June13 17:05:07 MDT 2002