Questions and Answers: Hurricane Andrew Winds
- How is the intensity of a hurricane estimated?
Estimating the intensity of a hurricane has always been difficult, because rarely are surface observations available in the eyewall, the strongest part of the storm. One of the best ways to estimate surface intensity is to take aircraft reconnaissance wind speeds, typically measured at an altitude of 10,000 ft, and adjust them.
- Why isn't the estimation of hurricane intensify straightforward?
Determining the proper adjustment, usually a reduction of winds at flight altitude to the surface has sometimes been controversial, mainly because observations in the hurricane eyewall were too limited to establish a broadly-accepted relationship between flight-level and surface winds in that part of the storm. In the early 1990's, reduction factors in common use ranged from 75-90%. Some scientists and wind engineers even maintained that surface winds were as low as 65% of the flight-level wind.
- What has changed in the science since Andrew's landfall in 1992?
In 1997, a new instrument, the Global Positioning System (GPS) dropwindsonde, became available and was deployed in the eyewall. The GPS sonde provides, for the first time, the ability to measure the strongest winds in a hurricane from the reconnaissance aircraft's flight-level all the way down to the surface. Over the last several years, several hundred dropwindsondes were released in hurricanes; and recently-concluded research has shown that typical adjustment factors are higher than previously thought with surface winds in the eyewall averaging about 90% of the flight-level wind. The dropwindsonde data also tell us that storms with little or weak eyewall thunderstorm activity tend to have somewhat lower smaller percentage factors, while intense storms, as well as those with vigorous thunderstorm activity, may have somewhat higher reduction percentage factors.
Based on this new understanding, the National Hurricane Center (NHC) typically now uses a factor of 90% to estimate a hurricane's maximum surface winds from flight-level observations. This means that the winds of some storms in the historical record may have been underestimated.
- How does this new understanding affect NHC's estimate of Andrew's landfall intensity?
A reconnaissance aircraft in Andrew at about the time of landfall reported flight-level winds of about 186 mph (162 kt). While the exact intensity of Andrew at landfall will never be precisely known, our new understanding of the structure of the hurricane eyewall suggests that Andrew's maximum sustained surface winds based on this particular measure were very likely of category five strength, in the vicinity of 165 mph (145 kt).
- Why wasn't the official intensity adjusted before now?
Because of the importance of Andrew's landfall and the implications of an intensity adjustment, NHC asked for, and considered input from the scientific community before such a change is officially made to the historical record. Efforts were completed between NOAA's NHC and the Hurricane Research Division to ensure that NHC's new estimate was consistent with other indirect measures of Andrew's intensity, such the few existing surface observations, and the observed storm surge values in south Miami-Dade County. These efforts were concluded August 20, 2002, when our senior management was briefed.
- What about the idea that hurricane winds diminish close to the coastline?
This is an interesting idea that could and should be studied further. Probably the best way to do this would be to deploy GPS dropsondes in the coastal waters during hurricane landfalls, as opportunities arise over the next several years. But because the necessary measurements haven't yet been made, we know very little about how strong this effect might be, or under what conditions it might occur, or whether it occurs at all. Until this idea can be documented and quantified, it would be inappropriate to incorporate it into operational practices. Consequently, it has been long-standing practice to presume that a hurricane's strongest winds over open water just offshore do reach at least the immediate coastline, and that is the standard being applied to Andrew.
- Why change the intensity if there is uncertainty?
Because there always is uncertainty. We never have the observations necessary to determine a hurricane's peak surface winds with precision, so the intensity must always be estimated by indirect means. NHC is currently involved in a project to re-evaluate the entire historical hurricane record; the goal is to produce a record that is as accurate as possible, based on current practices and understanding. The fact that there is uncertainty shouldn't prevent us from making the best possible intensity estimates for each storm, including Andrew.