Waves and Satellites: Chasing the Big Ones (January, 2014)

Tuesday, January 14, 2014


Photo: ASP/Kirstin (provided by surfline.com).

Waves are the most commonly viewed feature of the ocean.  Forcing of wind on the ocean surface can generate waves that propagate across entire ocean basins.  Wave height is dependent upon wind speed and fetch (or distance over water which a given wind may blow without interruption). Waves represent the exchange of energy from the atmosphere into the ocean and impact air-sea gas exchange and upper ocean mixing.  Observations and predictions of wave generation, height, and direction of propagation throughout the global oceans are critical for safe and efficient commercial shipping, fisheries, mitigating potential hazards in coastal regions, and recreational activities.

Measuring waves has been the long-term job of moored buoys, which are spread along continental shelves with limited availability in the open ocean. Satellite observations provided by radar altimeters, such as the currently operating Ocean Surface Topography Mission (OSTM)/Jason-2 mission, provide global wave height observations on a daily basis.  Another key satellite observational capability is global ocean vector winds provided by radar scatterometers, such as the Advanced Scatterometer (ASCAT) onboard the Metop-A/B satellite. Both ocean vector winds and wave height observations from satellite are used to generate wave fields in global wave forecast models, such as NOAA’s Wavewatch III
In the Northern Pacific Ocean, some of the highest waves are generated in the fall and winter months. Key locales have been identified throughout the world, where extreme waves will arrive from a certain location and direction to form and break over shoaling ocean bottom topography. Such locations became key destinations for big wave surfers, including Mavericks near Half Moon Bay, CA; Waimea Bay on the North Shore of Oahu, HI; Cortes Bank, 100 miles due west of San Diego, CA.  Recently, big wave contests are now held in several of these locations. In big wave and other surfing contests, surfers and contest organizers examine surf wave model forecasts to predict where and when optimum conditions of wave height and propagation direction will occur. When the forecasts appear optimal, surfers travel to the contest sites to challenge their mettle, astonish viewers, and attempt to ride the largest waves of the season. Over the last 15 years, surfers have developed and refined methods to ride the largest waves, such as being towed behind jet skis at sufficient speeds to catch the cresting waves.
To support surfers in determining where and when to surf, multiple services have developed detailed surf forecast products for popular and remote surf locations throughout the world. These specialized products utilize global wave models, which are augmented to include more detailed coastal locales and conditions.  Like global wave forecasters, these services also incorporate satellite data for assimilation into the models as well as forecast validation and nowcasting.  According to Graeme Rae, head of the modeling effort for Surfline, based in Huntington Beach, CA, satellite data plays a key role in their operations.  In particular, the most recent products used have been JASON-1 and OSTM/JASON-2 wave height data, and ocean vector winds  from ASCAT and the Quick Scatterometer (QuikSCAT). After QuikSCAT’s primary operating mode halted in November 2009, Rae said, “QuikSCAT was a big loss for our operations that we haven’t recovered from yet, even with the use of ASCAT”, noting that ASCAT has comparatively limited coverage. Surfline incorporates multiple data sets from buoys, regional higher resolution wind forecast models (if available), weather information, and satellites (most recently from ESA’s CryoSat-2 mission) to continuously improve their products and forecast skill.
A recent example of satellite data and a related wave forecast are shown in the following figures. Figure 1 shows wind speed and wave height conditions for 27-28 November 2013 in the northern Pacific Ocean.  Note that the highest wave heights correspond with the greatest wind speeds. In support of a big wave contest that was taking place on the northern shore of Oahu, HI, Surfline generated a forecast for 29 November 2013 that described “good to epic conditions…awesome tube rides…that will bring select big wave sites to life” (G. Rae, personal communication). Figure 2 shows a model wave height forecast for 28 November 2013.  The local wave forecast animation in Figure 3 depicts the large waves approaching Hawaii on 29 November 2013 with wave heights over 14 feet, well illustrated by this stunning image of a surfer cutting back into a wave at Sunset Beach (Figure 4).


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  Figure 1.  (top) Daily ASCAT wind speed (m/s) on 0.25° grids on 28 November 2013 (Data from CERSAT/IFREMER). (bottom) 3-day averaged multi-mission (Jason-1 and OSTM/Jason-2) significant wave height (m) on 1° grids on 28 November – 1 December 2013 (Data from AVISO).





  Figure 2.  A wave height model forecast for 00 hr 28 November 2013,  generated by Surfline.com.





  Figure 3.  A local wave height forecast animation for Hawaii, generated by Surfline.com.



  Figure 4. A surfer cutting back into a wave at Sunset Beach, taken November 29, 2013. Photo:ASP/Kirstin (provided by surfline.com).


PO.DAAC Science Team,
Jet Propulsion Laboratory, Pasadena, Calif.