Extended Abstract (236K)10D.5
The climatology of hurricane wind profiles
Hugh E. Willoughby, Florida International University, Miami, FL; and M. E. Rahn
In such applications as windstorm underwriting or storm-surge forecasting, hurricane wind profiles are often approximated by a single continuous function (e.g. Holland 1980) that is zero at the vortex center, increases to a maximum in the eyewall and then decreases asymptotically to zero at large radius. Comparisons of currently accepted profiles with aircraft observations reveal a systematic pattern of errors. Although the winds around the peak are too strong, away from the peak they decrease with distance too rapidly--both inside and outside the eye. A pressure-wind relation produced by integration of the gradient wind using this profile overestimates the maximum wind for a given minimum pressure.
A promising alternative is a family of piecewise continuous profiles in which the wind increases as a power of radius inside the eye and decays exponentially outside the eye after a smooth polynomial transition across the radius of maximum wind. Based upon a sample of 493 observed profiles, the mean exponent for the power law is 0.79 and the mean decay length is 243 km. The database used actually contains 606 aircraft sorties, but 113 of these failed quality-control screening, largely because the radius of maximum wind was too large a fraction of the sampling domain. Hurricanes stronger than Saffir-Simpson category 2 generally require a superposition of two exponentials in order to match the observed relatively fast decrease of wind with radius just outside the eye and slower decrease farther away. After some experimentation, we found that a fixed value of 25 km was satisfactory for the faster decay length and the mean value of the slower decay length, which was fitted by least squares to each hurricane, was 295 km. The mean contribution of the faster exponential to the outer profile was 0.10, but for the most intense hurricanes it sometimes exceeded 0.5.
The power-law exponent and proportion of the faster decay length in the profile outside the eye increased with maximum wind speed and decreased with latitude; whereas the slower decay length decreased with intensity and increased with latitude. These results are consistent with the qualitative observation that more intense hurricanes in lower latitudes have more sharply peaked wind profiles than do weaker hurricanes in high latitudes. Key consequences of this parameter variation are that the maximum wind is proportional to a power > ½ of the pressure fall from the environment to the storm center and that, on average, a greater pressure fall is required to sustain the same maximum wind in higher latitudes.
Session 10D, tropical cyclone observations and structure V
Wednesday, 5 May 2004, 10:15 AM-11:45 AM, Napoleon III Room
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