We computed power spectral density (PSD) for 1-hour long MOBB data segments and compared the results to the spectral wave density (SWD) measured at the nearby NOAA buoys. The SWD is computed at the buoys and measures energy of the ocean waves in $m^2$/Hz in the 0.01 Hz wide frequency bins that cover the 0.03 to 0.4 Hz range. The location of the buoys as well as comparison spectrograms for a 7-day period are shown in Figure 13.28. The infragravity peak can be observed in the PSD plot for the vertical MOBB channel (Figure 13.28, top panel). A rather sudden change of the infragravity peak width is indicated with a black line. The storm observed on day 344 was approaching from the WNW direction. Increased energy of the 10-20 s ocean waves on day 344 can therefore first be seen on buoy 46014, and last on buoy 46011. The arrival of these waves at buoy 46042 coincides with the increase of the infragravity signal on MOBB. This tells us that the infragravity waves observed at MOBB are primarily locally generated. The same can generally be observed throughout the deployment and for storms arriving from different azimuths.

Figure 13.28: Left: The location of some of the NOAA buoys closest to the MOBB. Right: The power spectral density (PSD) for the vertical MOBB channel as a function of period and time (top). Bottom panels show the spectral wave density (SWD) calculated at 3 buoys. The vertical line indicates a sudden change of the infragravity peak width.
\epsfig{file=dolenc05_1_1.eps, width=17.4cm}\end{center}\end{figure*}

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