Validation of National Data Buoy Center Directional Wave Measurements Using Swell Waves from Distant Storms

Accuracy of swell wave direction measurements derived from Datawell Hippy 40-second Mark II and National Data Buoy Center angular rate sensor (ARS) are determined using two NDBC west coast stations: 46042 in Monterey Bay and 46028 near Cape San Martin. These are located in deep water 66 nautical miles apart off the central coast of California. Hippy and ARS spectral wave directions are compared to corresponding directions inferred from strong cyclonic wind fields represented by data obtained from National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/ NCAR) Reanalysis project of NOAA Earth System Research Laboratory (ESRL). Wave energy is assumed to propagate along great circle routes at its frequency-dependent group velocity. Time of wave generation is computed by the ridgeline method. Patterns found in contours of non-directional wave energy on time-frequency plots yield precise information on the time of swell generation. Frequency-dependent wave direction is determined using NDBC techniques. Swell origin position from buoy information is compared to true swell origin, determined to be where high winds exceeding 15 meters per second are found directed toward the station. We present the twelve and only cases from November 2004 to December 2005 in which peak wave period exceeded 20 seconds at NDBC station 46042. In the twelve cases, swell energy reached both stations nearly simultaneously. Storm centers were all deep extra-tropical cyclones, and these ranged across the entire Pacific Ocean. The most distant storm was 3,005 kilometers away in the southern Pacific near 60 degrees south 150 degrees west. Together, these cases show ARS-derived directions marginally exceeding NDBC error limits of 10 degrees. Accuracy of Hippy direction is estimated to be 7 plusmn 10 degrees. The viability of the validation method as a general technique for any directional wave system is discussed. The last of the twelve cases, the one with the least - - spectral energy density, representing a wave of period 21 seconds with an amplitude of 13 centimeters, is shown requiring further research owing to apparently weak winds around projected point of swell origin