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Summer 2001

MM5 pushes frontier of hurricane simulation

by Bob Henson

In a key step toward improving the prediction of hurricanes, NCAR scientists Jordan Powers and Christopher Davis have reproduced in a computer model the fine-scale structure that drives the birth and strengthening of tropical cyclones. Powers and Davis presented imagery from their hurricane simulation in Fort Lauderdale at the Ninth Conference on Mesoscale Processes, sponsored by the American Meteorological Society.

This 48-hour model forecast of Hurricane Diana shows the predicted pressure pattern and location of rain water mixing ratio (which corresponds closely to rainfall). The rainbands, shown in detail in the MM5 model, are a key element in the evolution of hurricanes.

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. According to Davis, "One of the remaining mysteries about hurricanes is how they form, especially when they're influenced by midlatitude weather systems that move into the subtropics and tropics. What we hope is that by analyzing the mechanisms behind storm formation in this and related simulations, we can formulate hypotheses of tropical cyclone formation that can be tested using aircraft, radar, and satellite data. We also hope to gain insight into what's needed to predict storm formation in operational weather forecast models."

Computer models used for day-to-day weather prediction have become increasingly adept at projecting a hurricane's motion. Yet even the best models have little skill in predicting intensity, especially the rapid strengthening often noted in the worst 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, can't be simulated in sufficient detail on the computers and in themodels used for everyday forecasting. For instance, the finest horizontal scale resolved in operational computer models is 16–48 kilometers (10–30 miles), but spiral bands can be less than 16 km wide.

To resolve the eyewall and precipitation bands within a tropical cyclone, Davis and Powers used the MM5. With a horizontal distance as small as 1.2 km between the model's computation points, the MM5 is one of the world's highest-resolution models for reproducing storm-scale weather across a large area.

Davis and Powers used the MM5 to study 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 the development of Diana, from its origins as a nontropical low to its intensification to hurricane status more than a day later.

The Weather Research and Forecast Model (WRF), a collaboration among NCAR, the National Oceanic and Atmospheric Administration, the University of Oklahoma, and the U.S. Air Force, is designed to regularly operate with resolutions of 1–10 km. Together with more powerful computers, this will allow for the type of fine- scale detail in the MM5 to be simulated on a day-to-day basis. A prototype is already being run, and the model is scheduled to be introduced for public weather forecasting by the National Weather Service as soon as 2004.


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Edited by Bob Henson, bhenson@ucar.edu
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Last revised: Wed Aug 8 17:05:07 MDT 2001