A Simplified Empirical Model For Hurricane Wind Fields

ABSTRACT In many situations the design engineer is faced with the problem of the selection of wave and storm surge conditions due to hurricanes. The theories for storm surge computations and wave spectral computations under known wind fields are comparatively well established or currently being refined. The techniques involved require the use of digital computers. A common problem which remains is specification of the wind field. The preparation of tapes or cards continaing the hurricane wind field components over the entire grid and times of interest is tedious and time consuming. This paper presents a set of simplified empirical equations describing the hurricane wind fields to a close approximation for specific input data including radius to maximum wind, forward speed and position of the hurricanes. The algebraic functions for the wind speeds and directions are expressed relative to the hurricane center. The problem is then reduced to a transformation of coordinate systems for application to the particular study under investigation. The simplified equations are extremely efficient to use in a digital computer and yield wind field cross sections for space-time wave hindcasts and storm surges. The approach is intuitively justified by its ease of calculation and incorporation into overall wave and storm surge forecasting schemes. Comparisons are made between the wind fields given by these equations and a Standard Project Hurricane recommended by ESSA. Application to actual hurricanes has also been used with success. 1. INTRODUCTION The determination of wind fields in hurricanes is needed as an incidental step in the determination of extreme wind, wave and surge conditions, for many engineering problems associated with shoreline and offshore works. A complete analysis of the wind fields in hurricanes would require the solution of the equations for fluid flow in the earth';s atmosphere taking into account the earth';s rotation, thermodynamic effects and boundary layer (friction) effects. Many author s such as Myer sand Malkin (1961), Riehl and Malkus(1960), Rosenthal (1961) and others, have attempted to obtain wind fields by numerical solution of the governing differential equations after making various simplifying assumptions. Another group of authors such as Graham and Nunn (1959), Graham and Hudson (1960), have proposed empirical wind fields based on a combination of simplified equations correlated with observed historical data. This paper presents an even more empirical approach which essentially consists of the determination of a set of equations suitable for computing use. These equations are derived to match the Standard Project Hurricanes of Graham and Nunn (1960) as modified by Goodyear and Nunn (1965.) The equations used are purely algebraic (no differentials) and it has been found that good approximations to SPH windfields can be made extremely economically. The approach is intuitively justified by its ease of calculation and incorporation into wave and storm surge forecasting schemes. Waves and surges are all the result of an integrated effect of winds. The empirical model used does not predict the finer detail of the wind structure but its averaged effects.a