15D.4 An Algorithm for Estimating Maximum Wave Height and Wave Period inside Tropical Cyclones

Friday, 20 April 2018: 8:45 AM
Heritage Ballroom (Sawgrass Marriott)
Paul Hwang, NRL, Washington, DC; and E. Walsh
Manuscript (221.0 kB)

Hurricanes are among the most hazardous natural disasters. They cause tremendous damage and pose extreme challenges for operation. Presently, satellite monitoring addresses the wind velocity. To properly evaluate the damage potential of a hurricane, it is critical to consider not only wind but also sea state parameters such as the significant wave height and dominant wave period.

Through recent analyses of the wind and wave data collected in several hurricane reconnaissance missions, it has been established that the wave development inside hurricanes follows essentially the same fetch- or duration-limited principles governing the wave growth under steady wind forcing conditions.

This is an important development because at the foundation of fetch- or duration limited wave growth is a pair of dimensionless equations describing the growth of wave height and wave period as fetch or duration increases. The three most important wind and wave parameters (the wind-wave triplets): wind speed U10, significant wave height Hs, and dominant wave period Tp are thus connected by two equations for a given fetch or duration. Defining the fetch or duration of a hurricane wind field thus holds the key to accessing the robust and versatile wind wave growth functions for achieving [U10 or Hs or Tp] = [U10 and Hs and Tp].

Through reverse engineering using the simultaneous wind and wave data collected in four hurricane reconnaissance missions during Bonnie 1998 and Ivan 2004, a scaling model of the fetch and duration of hurricane wind fields has been developed. The key features of the fetch and duration scaling model are: (a) The fetch and duration increase linearly with the radial distance r from the hurricane center along any radial transect with a constant azimuth angle f referenced to the hurricane heading; (b) The slope and intercept of the linear function vary systematically with the azimuth angle referenced to the hurricane heading; (c) The systematic azimuthal variation of the slope and intercept can be represented by Fourier harmonic functions, the coefficients of which vary systematically with a small number of the characteristic hurricane parameters; and (d) The most important characteristic hurricane parameters relevant to the fetch and duration are (in descending order of significance): the radius of maximum wind speed (rm), the translation speed of the hurricane (Vh), and the maximum wind speed (U10m).

With the fetch and duration defined at any location inside the hurricane, Hs and Tp are given in simple algebraic equations of U10 and r. Because the radial dependence of the wind speed can be approximated by a simple power function of r, Hs and Tp along a radial transect are thus given as simple algebraic equations of r alone, and their maximum values can be obtained from seeking the locations where the gradient of the algebraic functions become zero. The final result of maximum wave height are wave period of a hurricane depends on the U10m and rm of the hurricane.

An algorithm of estimating the maximum wave height and wave period insider hurricanes is developed based on the above considerations. The predicted values are in good agreement with simultaneous wind and wave measurements conducted by hurricane reconnaissance aircraft during Bonnie 1998 and Ivan 2004.

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