Exploring Corrections to Long-Term Wave Measurements for Climate Studies

As wind waves integrate characteristics of atmospheric dynamics over a range of scales, they potentially serve as an indicator of climate variability (IPCC AR5, 2013). Waves are measured from a variety of platforms, but it is the moored buoy that is most often used to calibrate remote sensing applications and verify numerical wave and coupled model hindcasts that can extend wave measurements temporally and spatially for climate studies, understanding air-sea fluxes, and gage the impact of wave runup to coastal inundation and erosion with sea-level rise due to climate change.

US National Data Buoy Center (NDBC) moored buoy wave measurements now span nearly 40 years. However, recent studies (Gemmrich et al, 2011, Livermont et al., 2015) indicate that changes in US and Canadian moored buoy measurements over time can be attributed to changes in the wave systems rather than geophysical changes. Furthermore, Durrant et al. (2009) found a bias in the satellite altimeter wave measurements between US and other buoy networks. In extreme seas, the Bender Effect (Bender et al. 2010) can cause wave measurements from some moored buoys using strapped down accelerometers to overestimate significant wave heights by 26 to 56% because of the tilt of the vertical accelerometer.

Three collaborative projects are currently underway to assess and correct the above issues with wave measurements for moored buoys to provide more stable, homogenous, long-term datasets for climate and climate variability studies:
a. NDBC Dual Wave System: A limited number of buoys were equipped with both strapped-down accelerometer systems and Datawell Hippy 40 sensors. The Hippy provides better stabilization of vertical accelerations, and thus can help assess the Bender Effect and the approaches used to mitigate the Effect.
b. Wave Evaluation and Testing Pilot Project of the Data Buoy Cooperation Panel (DBCP) consists of side by side comparisons with Datawell Waverider buoys with collocated Canadian and US buoys using standardized analysis methods. By comparing to a common standard, buoy wave measurements no longer will carry unknown national or system signatures to mask climate variability and trends.
c. Field Laboratory for Ocean Sea State Investigation and Experimentation (FLOSSIE) sponsored by the US Army Corps of Engineers Coastal and Hydraulics Laboratory fielded a large hull buoy with various US and Canadian wave sensors, as well as older and newer US wave systems. This will lead to understanding the bias between national wave systems and the continuity of wave measurements.

Preliminary results of the three projects show:
a. The uncorrected Bender Effect has an exponential growth and can be on the order of 30% within the current range of wave heights
b. The Bender Effect can be reduced to a linear growth by applying corrections for tilt to the acceleration time series.
c. The Bender Effect is not apparent on large hulls and small hulls in deep water.
d. Older NDBC systems on large hulls tend to overestimate wave heights.

Future, and so far unfunded, research efforts could be aimed at
a. Metadata rescue so system changes can be identified and corrections applied.
b. Actual re-analysis of the moored buoy wave measurements with methodologies derived from the current projects.

The paper will present more recent results and discuss future initiatives. The continuation of these projects can have far-reaching results in improving the quality and utility (NOAA, 2014) of the millions of wave observations held by NOAA’s archive centers and ensure they meet NOAA’s standards for Data Stewardship (NOAA, 2010).