A Simpler Saffir-Simpson?
Posted June 1, 2009
Hurricane season is officially upon us today, although we did of course have our first Atlantic tropical cyclone of the year already in the form of Tropical Depression One, which failed to become strong enough in terms of wind speed to receive a name or the associated upgrade to Tropical Storm status.
One change that the National Hurricane Center is implementing this season on an experimental basis is to pare down the Saffir-Simpson Hurricane Scale that has been in use since the 1970s to categorize storms by their intensity. Since its inception, the scale has defined hurricane category based on maximum sustained (one-minute average) wind speeds at 10 meters above the surface. However, each category, from 1 through 5, was also assigned a range of associated minimum surface pressures and likely storm surge values. A big reason for including a surface pressure range was to help in establishing a category back when hurricane hunter aircraft could make reliable surface pressure estimates but did not have the ability to accurately measure surface winds. Since about 1990, dropsondes have enabled better surface wind measurements, and in the past few years an even better instrument called the stepped frequency microwave radiometer has been added to the aircraft.
The storm surge ranges have turned out to be a larger problem when it comes to the scientific validity and internal consistency of the Saffir-Simpson scale. Because surge levels can vary greatly depending on lots of variables other than maximum wind speed (forward speed and direction of storm motion, size of the storm, character of land and adjacent underwater topography), quite a few hurricanes through the years have produced surges well outside the range associated with the speed-based intensity of the storms. The Hurricane Center cites a few interesting examples, including last year's Ike, which struck Texas as a Category 2 system, but produced surge levels associated with Category 4-5 storms on the existing scale. Conversely, Hurricane Charley moved into Florida back in 2004 with Category 4 winds, but only managed 6-7 foot storm surges, which the old scale places at Category 2.
In response, the Hurricane Center is now posting the Saffir-Simpson Hurricane Wind Scale, which describes typical wind impacts from each of the categories, but leaves out any mention of associated central pressures or storm surges. This does not mean they are de-emphasizing the impact of rising waters pushed ashore by tropical cyclones. Instead, forecasts of storm surge that are tailored to the size and motion of the storms, along with the angle of approach and topographic/bathymetric features near the point of landfall, will continue to be produced and disseminated in an assortment of advisories and statements.
For a detailed rundown on the Saffir-Simpson Hurricane Wind Scale, see the attached link.