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Wind Wave Directional Spectra


This page describes the wind wave frequency-direction spectra obtained from the nine-element linear array of bottom mounted pressure gauges located on the 8-m contour about 900 m offshore of the FRF. Spectra are grouped by month and year. From directory fdspec, http users should cd to yyyy/Mmm (e.g., 1992/Jun) to find results for a particular month. All results from 1 September 1986 to the present are posted in this way.

There is one data file for each spectrum. Files are named for their data collection start times, in the form layymmddhhmm.asc, where yymmddhhmm is (2-digit) year, month, day, hour, and minute (Eastern Standard Time). Each file is about 30 kbytes in length. A file called readascii.f, contains (FORTRAN) formats for reading the ASCII files, and has a header that identifies all the file variables, units and dimensions.

Directions depicted in frequency-direction spectra from the FRF 8-m array represent azimuths from which waves arrive in degrees counterclockwise from normal to the array, or from parallel to the FRF pier, or, approximately, from shore normal. Zero degrees represents waves propagating straight onshore, having wavenumber vectors aligned with the pier, and wave crests perpendicular to the pier. A direction of +45 degrees indicates waves coming from the northeastward side of the pier, and propagating along an azimuth 45 degrees to the left of the pier axis for an observer looking seaward. An angle of +90 degrees represents the (rather uncommon) condition of waves propagating straight down the coast from northish to southish. Negative angles represent waves from the southeastward side of the pier. Waves at -45 degrees propagate onshore and northward along an azimuth 45 degrees to the right of the pier axis for an observer looking seaward.

Alternatively, but equivalently, a depicted angle is the conventional trigonometric direction of a wavenumber vector for wave equations cast in a right-hand Cartesian coordinate system with horizontal x-axis aligned with the FRF pier and increasing landward, horizontal y-axis normal to the pier and increasing southward, and z-axis positive upward. In this system, wavenumber vector direction originates along the positive x-axis and increases counterclockwise when viewed from above.

All results are based on 8192-sec (2 hr 16 min 32 sec) time series of data collected at 2 Hz or, rarely, 4 Hz. The basic analysis algorithm is the iterative maximum likelihood estimator derived and described by Pawka (1983, "Island shadows in wave directional spectra," J. Geophys. Res., 88, p. 2579), using his recommended convergence parameters and a maximum of 30 iterations. A description of the FRF linear array is given by Long and Oltman-Shay (1991, "Directional characteristics of waves in shallow water," Technical Report CERC-91-1, US Army Engineer Waterways Experiment Station, Vicksburg, MS), and details of data analysis are given by Long and Atmadja (1994, "Index and bulk parameters for frequency-direction spectra measured at CERC Field Research Facility, September 1990 to August 1991,", Miscellaneous Paper CERC-94-5, US Army Engineer Waterways Experiment Station, Vicksburg, MS).

Questions or problems should be addressed to Chuck Long: phone (252) 261-3511, FAX (252) 261-4432, e-mail c.long@cerc.wes.army.mil


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