3-D model forecasts storm surges

September 13, 1999
Web posted at: 2:25 p.m. EDT (1825 GMT)

The CAPES model showed that Hurricane Emily pushed sea levels 10 feet above normal (red) around North Carolina's barrier islands in August 1993. The blue shows water levels below normal.

While Tropical Storm Dennis rocked North Carolina's coast last week, residents and tourists alike were accurately informed of flooding thanks to a 3-D forecasting model developed by scientists from North Carolina State University and warnings from the National Weather Service in Raleigh, N.C.

University meteorologists/oceanographers Dr. Lian Xie and Dr. Leonard Pietrafesa provided a forecast to the National Weather Service the morning of Aug. 30, updating it every four hours and taking into account when the storm was weak or strong, or changes in its speed or direction.

"Historically, storm flooding causes the greatest loss of life and property damage when a hurricane strikes," said Xie. "Advance warning of where and when the flooding will likely occur and how high it may go allows emergency management personnel, residents, tourists, marina owners and others to prepare for the worst."

One advantage of the model, called CAPES, over other forecasting tools, is that it takes into account the effects of inland rainfall as well as the surge of the ocean itself. Often, inland rainfall causes surprise flooding by filling up streams hours after the initial storm surge.

"Storm flooding can occur well after the storm surge itself recedes. Areas that experienced little flooding from the initial surge can be hard hit when rivers swollen from inland rains begin emptying into the estuaries," said Pietrafesa.

In the case of hurricanes Bonnie, Bertha and Fran, flooding occurred much later in Albermerle Sound near the mouths of the Roanoke and Chowan rivers, than in the lower Pamlico Sound. Xie and Pietrafesa's forecasts predicted this onset of inland flooding, which allowed public safety officials to warn communities along the western coast of Albemarle Sound that flooding might still occur.

Erosion is one of the effects created by a nor'easter on North Carolina's Outer Banks.

In 1986, Pietrafesa lead a team of scientists in devising the CAPES model. Pietrafesa first used an early version of the model for the NWS in 1993 to predict coastal flooding along the Outer Banks from Hurricane Emily. The model has also been used to predict flooding from strong winter winds called nor'easters.

Forecasts made from the model have been accurate to within 10 percent of actual observed water levels. Xie attributes this high level of accuracy to CAPES sophisticated computer code, which takes into account more detailed data than other storm flood forecast models.

For example, it computes water-flow speed and direction at 11 different depths, whereas the model developed by the National Oceanic and Atmospheric Administration computes just one average water-flow estimate for the entire water column. "Using our model, we have a much better idea of what's going on near the ocean bottom, which plays a major role in determining how powerful the storm surge will be," said Xie.

The model also allows users to view real-time computer images of where and when flooding will occur on a detailed topographical map of the North Carolina sounds.

Xie, Pietrafesa and their colleagues are also working on a 3-D coastal flooding model that will cover a part of the eastern coast from the Virginia border to Charleston, S.C. This model also will be able to incorporate the effects of tides and surface waves, "which can significantly increase the height of the storm surge locally, allowing for better forecasts of flooding on different parts of a coastline or even on different ends of a barrier island," said Pietrafesa.

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