Skip Navigation Links weather.gov 
NOAA logo-Select to go to the NOAA homepage NDBC Title GraphicClick to go to NDBC home page   Select to go to the NWS homepage
Home News Organization
 
 
    
    Station List

 Observations
    Mobile Access
    Obs via Google Maps
    Classic Maps
         Recent
         Historical
         DART®
         Oil & Gas ADCP
    Obs Search
    Ship Obs Report
    Gliders
    BuoyCAMs Image indicating link to BuoyCAMs page
    APEX
    TAO
    DODS
    HF Radar
    OSMC
    Dial-A-Buoy
    RSS Feeds Image indicating link to RSS feed access page
    Obs Web Widget
    Email Access

 Station Status
    NDBC Maintenance
    NDBC Platforms
    Partner Platforms

 Program Info
    Facebook Logo
    NDBC on Facebook
    About NDBC
     Met/Ocean
          Moored Buoy
          C-MAN
    TAO
    DART®
    VOS
    CSP
    IOOS® Program
    IOOS® DAC

 Publications
    NDBC DQC Handbook
    Hurricane Data Plots
    Mariners Weather
      Log

    Observing
      Handbook No. 1


 Science Education

 NDBC Director
 FAQ
 Media Inquiries
 Contact Us
 Visitor Information
 Links
 
USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.

Visit NDBC's new TAO Data Download page to access current and historical data for the tropical equatorial Pacific.

Check out our new BuoyCAMs Map - also available at the BuoyCAMs link on the left side menu.

How are estimates of wind-seas and swell made from NDBC wave data?

Estimates of wind-sea and swell are made by selecting a separation frequency that partitions the wave spectrum into its wind-sea and swell parts. The respective parts of the spectrum are then used to compute significant wave height, peak frequency (or period), and mean direction of the swell and wind-seas portions of the spectrum. Figure 1 depicts this process on a plot of a typical wave spectrum showing the distribution of wave energy with frequency. NDBC has been using the wave steepness method to estimate wind-seas and swell on its web site since 1997. This method determines a separation frequency by assuming that wind-seas are steeper than swell and that maximum steepness, or ratio of wave height to length, occurs in the wave spectrum near the peak of wind-seas energy. The steepness method performs well under conditions of building or sustained wind-seas, but the method often overestimates wind-seas under conditions of abating winds and seas, or under light winds when significant swell is present.

To improve performance, the steepness method was modified to limit the minimum allowable separation frequency based on the observed wind speed. This is possible because sustained winds can build waves with increasing heights and periods only up to a certain point. If the winds are sustained long enough, and since wave propagation speed increases proportionally with wave period, a point will be reached where wind and wave propagation speeds are approximately equal. The wind can exert no further force on the waves, and wind-seas are said to have become fully-developed. Since peak frequencies of fully-developed seas generated by a given wind speed are well known, this relationship can be used to set a lower limit on the separation frequency.

Positive results were obtained in tests of the modified method using measurements from directional buoys where swell and wind-seas can be easily identified by differences in propagation direction. Improved results were also obtained when the modified method was compared with wind-sea and swell estimates from the Navy's operational wave model (WAM), shown in Figure 2. While the modification presents a slight disadvantage in that it requires wind speed information, satisfactory results are obtained without knowledge of wave direction.

Wave Plot
Figure 1. A typical wave spectrum showing the separation frequency and the distribution of swell and wind-seas energy with frequency.

Back to Top

Comparison of Methods
Figure 2. Comparison of NDBC's modified steepness method with the estimates from the Navy's wave model: a) combined wind-seas and swell significant wave height (Hs), b) swell Hs, and c) wind-seas Hs.

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.

For a list of resources on wave spectrum partitioning and wind sea and swell wave discrimination, click here.

Back to Top