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NDBC added additional data to its .drift file format on August 17. For more info, see the Drift File Format Change Notice.

Wave Steepness Algorithm

First, we compute a wave steepness parameter, , at all frequencies, f:

where Hs is the significant wave height, L is the wave length associated with Tz and Tz is the average wave period that are computed from the nthmoment of the wave spectrum by

where fu and f1 are the upper and lower frequency limits of measured wave spectra.

The separation frequency fs is estimated by

where fx is the frequency of maximum (f), and C=0.75, is an empirically determined constant.

Under light winds with swell present, fs often overestimates the contribution of wind seas. A modification to this method imposes a minimum separation frequency based on wind speed. Minimum separation frequency is determined from the peak frequency of the Pierson-Moskowitz spectrum of a fully-developed sea at the observed wind speed by

where fsu is separation frequency and 1.25/U is the peak frequency of the Pierson-Moskowitz spectrum at the locally observed wind speed adjusted to 10m1, U10, in m/s. C is an empirically determined constant equal to 0.90.

The higher frequency, fs or fsu, is used as the separation frequency.

For more information, refer to:

Gilhousen, D.B. and R. Hervey, 2001. Improved Estimates of Swell from Moored Buoys, Proceedings of the Fourth International Symposium WAVES 2001, ASCE: Alexandria, VA, pp. 387-393.

Note 1. Wind speeds on the NDBC web site are those actually observed by the anemometer (typically at 5 m) and have not been adjusted. If you have access to NDBC observations issued by the National Weather Service in WMO CMAN and SHIP code (FM12, FM13 format) wind speeds adjusted to 10 and 20 meters can be found in section 5, field identifiers 11 and 22.

The method used by NDBC to adjust wind speeds to the standard reference levels is described in a 1979 paper by W. T. Liu et al. appearing in the Journal of Atmospheric Science, 36, 1722-1735.