Tuesday, June 13, 2006

New Hurricane Model Seeks to Capture Warm Water Dynamics that Led to Hurricane Wilma's Shockingly Swift Intensification

Link: New Aid for Storm Forecasters.

New Aid for Storm Forecasters Computer Model to Give Greater Sense of Intensity and Size
By Peter Whoriskey
Washington Post Staff Writer
Monday, June 12, 2006

Scientists at the National Hurricane Center normally deliver findings in a just-the-facts style of prose: wind speeds, pressure readings, compass points. But their description of last year's Hurricane Wilma betrayed a sense of wonderment.

The storm, which strengthened unexpectedly and set records for intensity, was, in the words of the final report, "unprecedented," "explosive" and "incredible." More ominously, in an era in which the public has high expectations for meteorological pronouncements, Wilma had defied predictions.

"The bottom just dropped out," said Naomi Surgi, a hurricane scientist at the Environmental Modeling Center, which is part of the National Oceanic and Atmospheric Administration. "We had never in the Atlantic seen that kind of storm intensification. None of the models forecast that."

Meteorologists have made steady improvements in predicting the path of a hurricane, cutting errors roughly in half over the past 15 years or so. But they have long struggled to predict a storm's strength, a critical element because it determines who should and who will evacuate.

This hurricane season, forecasters have hopes of improving their record in predicting storm intensity.

A new computer model, developed by the Environmental Modeling Center and described as the "next generation" in tropical storm forecasting, will be at the disposal of forecasters at the National Hurricane Center here.

Built with $3 million in federal funds, according to Surgi, the Hurricane Weather Research and Forecast Model is expected to improve forecasts of hurricane intensity, size and rainfall.

In far more detail than its predecessors, the new computer model will envision the full three-dimensional hurricane, the circulation at its core and the varying winds from its bottom to its top, several miles up.

"We think it will have a more accurate physical representation of what goes on in the inner core of a hurricane," said Edward N. Rappaport, deputy director of the National Hurricane Center. "We're not sure we're going to see a monumental advance in the very first year, but this will set the framework for more accelerated improvements."

The model will also monitor and predict the waves and ocean temperatures beneath the hurricane, working in finer detail than previous attempts and seeking out "hot spots" in the ocean that might boost intensity. "It is very high-resolution," Surgi said.

Forecasters at the National Hurricane Center rely on about five to 10 models for any particular forecast, Rappaport said, with one known as the "GFDL" as the lead model. The new forecasting model is expected to supplant that.

Wilma eventually walloped the island of Cozumel, Mexico, last October as a Category 4 storm, then battered the Yucatan peninsula. It finally headed to South Florida, making landfall as a Category 3 and causing the largest disruption to electrical service in state history.

But what may be the lasting impression of Wilma is its rapid strengthening -- and what it reveals of forecasters' gaps in knowledge. Before striking Mexico, Wilma exploded over a 24-hour period from a tropical storm to a raging Category 5 hurricane.

It was superlative in many respects. The speed of its development is believed to have been unprecedented. It had at one point the smallest eye known to the National Hurricane Center staff -- two nautical miles in diameter -- and the central pressure at the time of peak intensity was a record low for an Atlantic hurricane.

"It is fortunate that this ultra-rapid strengthening took place over open waters, apparently void of watercraft, and not just prior to landfall," the tropical cyclone report on Wilma says in the understatement typical of such reports.

Forecasters did a relatively decent job at predicting its track, according to the report. But it noted that the errors in predicting intensity were "quite a bit larger than the average."

Predicting storm intensity is considered a critical challenge among forecasters because so many people depend upon forecasts of intensity to determine whether to flee an oncoming storm.

Emergency managers determine evacuation orders depending on the predicted intensity. Residents make their own calculations regarding whether to obey.

Coastal residents of the southeastern states have long been familiar with hurricane forecasts, and many simply ignore evacuation calls if the approaching tropical cyclone is a Category 1 or even a Category 2.

The uncertainty in the forecast intensity and track of storms forces emergency managers to make broader preparations than might be necessary, and when they prove unnecessary the inconvenience dulls the public's willingness to take precautions the next time.

"We have to base our evacuations on a worst-case scenario," said Jonathan Lord, an emergency manager in Miami-Dade County, noting that evacuation orders there assume that the storm will be one category stronger than forecast. "Look what happened to Wilma overnight."

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