Tilt Error in NDBC Ocean Wave Height Records

We identify and characterize an error in the National Data Buoy Center (NDBC) wave records due to the sustained tilt of a buoy under high winds. We use a standard, operational 3-m aluminum discus buoy from NDBC with two wave systems, one gimballed, and the other strapped down but uncorrected. By comparing the two, we find that the most extreme significant wave heights are systematically overestimated. The overestimation is shown to be confined to a region around the peak frequency in the spectra: 0.05-0.15 Hz. Wave direction and directional spread are unaffected. A bias due to tilt error can be observed starting at winds of 10 m s(-1) or wave heights of 4 m. The bias increases as a function of wind speed and wave height, i.e., the bias is +10% when winds are 20 m s(-1). Very high waves and winds are relatively rare, so while the tilt error does not affect overall statistics and basic analyses it could potentially affect analysis sensitive to the extremes. A correction is derived for significant wave height, which is a quadratic function of wind speed. The correction is shown to reduce wave heights in uncorrected records, but is found inadequate for general use. There is evidence of tilt error at other NDBC stations, but the full extent of prevalence in the record is not known at this time.

Automated summary

This study identifies and characterizes an error in the National Data Buoy Center (NDBC) wave records, showing that the sustained tilt of a buoy under high winds leads to an overestimation of significant wave heights. The error begins at wind speeds of 10 m/s or wave heights of 4 m and increases with higher wind speeds and wave heights. While this tilt error does not affect overall statistics and basic analyses, it could potentially impact analyses sensitive to extreme conditions.