Virtual hurricanes: NCAR model pushes frontier

July 31, 2001

BOULDER--In a key step toward improving the prediction of hurricanes, scientists at the National Center for Atmospheric Research have reproduced in a computer model the fine-scale structure that drives the birth and strengthening of tropical cyclones. NCAR scientists Jordan Powers and Christopher Davis will present imagery from their hurricane simulation on Thursday, August 2, in Fort Lauderdale, Florida, at the Ninth Conference on Mesoscale Processes of the American Meteorological Society (AMS). NCAR's primary sponsor is the National Science Foundation, which funded the research.

The simulation, which used the NCAR/Penn State Mesoscale Model, Version 5 (MM5), marks the first time a cloud-resolving simulation has been able to reproduce the formation of a tropical cyclone, given only information about atmospheric conditions on a scale much larger than that of the cyclone. (Tropical cyclones include tropical storms and hurricanes.) The breakthrough points toward future forecasting power that will soon be available to the National Weather Service. NCAR is part of a team now building a model similar to the MM5, but with more advanced capabilities, that will generate daily weather forecasts for the NWS beginning in 2004.

According to Davis, "One of the remaining mysteries about hurricanes is how they form, especially when theyre influenced by midlatitude weather systems that move into the subtropics and tropics. We hope that by analyzing the mechanisms behind storm formation in these simulations, we can make hypotheses of tropical cyclone formation that can be tested using aircraft, radar, and satellite data. We also hope to understand whats needed to predict storm formation in operational weather forecast models."

Operational computer models used for day-to-day weather prediction have become increasingly adept at projecting a hurricanes motion. Yet even the best models have little skill in predicting intensity, especially the rapid strengthening often noted in the most powerful hurricanes. Part of the problem is that the compact core of a hurricane, including the spiral bands of showers and thunderstorms that gather and focus energy, cant be modeled in sufficient detail on the computers and models used for everyday forecasting. For instance, the finest horizontal scale resolved in operational computer models is 10-30 miles, but spiral bands can be less than 10 miles wide.

To "see" the eyewall and precipitation bands within a tropical cyclone, Davis and Powers turned to the MM5, one of the worlds highest-resolution research models for reproducing storm-scale weather across a large area. The model's horizontal distance is as fine as 0.75 miles between computation points. For their experiment, Davis and Powers studied Hurricane Diana, which struck North Carolina in 1984. Diana was chosen because of ample surface data and because a well-defined nontropical low preceded its formation. The MM5 successfully reproduced several stages in Diana's development, from its original state as a nontropical low to its intensification to hurricane status more than a day later.

The Weather Research and Forecasting Model, now being developed for future use by the National Weather Service, is designed to regularly operate with resolutions from 0.6 to 6.2 miles. Together with more powerful computers, this will put the type of fine-scale detail in the MM5 into the hands of daily weather forecasters. The National Oceanic and Atmospheric Administration, the University of Oklahoma, and the U.S. Air Force are collaborating with NCAR on the project.
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NCAR is managed by the University Corporation for Atmospheric Research, a consortium of 66 universities offering Ph.D.s in atmospheric and related sciences. The AMS is the nation's leading professional society for scientists in the atmospheric, oceanic, hydrologic and related sciences.

Writer: Bob Henson

Note to Editors: Although AMS will not be operating an official press room, media are invited to attend any/all sessions and interview experts from around the world at the Mesoscale Processes and two other AMS conferences taking place this week in Fort Lauderdale. All media must sign in at the AMS registration desk at the Fort Lauderdale Mariott Marina to receive access to sessions. The Mariott Marina Hotel telephone number is 954-463-4000.

Visuals: Images are available at ftp://ftp.ucar.edu/communications. Filenames and captions:

diana.jpg This 48-hour model forecast of Hurricane Diana by the NCAR/Penn State MM5 model shows the predicted pressure pattern and location of rain water mixing ratio (which corresponds closely to rainfall). The rainbands detailed in the MM5 model are a key element in hurricane evolution.

diana.mov This QuickTime animation shows the hour-by-hour formation of Tropical Storm Diana (which later became a hurricane), as depicted by the NCAR/Penn State MM5 model. The two-day formation period is shown from several 3-D perspectives. Green and white colors correspond to rain and cloud water, which portray the bands of rainfall that spiral around the storm.

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