California Wave Models

Advantages of the new model include:

• coverage of the CA coast from Mexico to Oregon;
• maps of peak period, peak direction, and average period;
• output of parameter values for individual points; and
• properly time-lagged predictions for all areas.

 
In order to generate such detailed, point-by-point output, the new model is currently run at a somewhat lower resolution than the Southern California Swell Model. The improved accuracy and range of information from the new model, however, should more than compensate for the change in resolution.

More details on the CDIP MOP System.
More scientific details on the models.

About the Southern California Swell Model

Note: While this document specifically describes the Southern California model, most of the information and FAQs apply to all of the nowcast models.

For more detailed information please see "A Comparison of two spectral wave models in the So. Cal. Bight" by O'Reilly and Guza (Coastal Engineering, 1993, v19, p263-282).

The model only simulates waves arriving from outside the islands (wave periods of 8 seconds and longer). It doesn't consider any local generation of seas.

To create the image, deep water wave data is collected by a Datawell buoy in 549 meters of water, about 19 kilometers west of Pt. Arguello. This data is transferred to the Coastal Data Information Program at Scripps at approximately 30 minute intervals.

The buoy data is processed to produce an estimate of the deep water directional spectrum (shown at the bottom left of the image). Because there is a time difference between the time the swell reaches the buoy and the time it reaches the beaches, the image is not really a "snapshot" of estimated heights. For a swell from the W-NW, the lag between a swell reaching San Diego after being measured at the buoy is typically around 6 hours or so, making the image more of a short range forecast for San Diego. However, for a south swell, the waves have pretty much hit all the Southern California beaches before reaching the buoy, making the estimates of a south swell more of a hindcast.

Frequently asked questions

• What is the circle at the bottom left of the swell image?
  It's a wave spectrum (swell intensity as a function of period and direction). It shows the relative intensity (using color, red = highest relative energy density, or itensity), direction (on the compass) and period (by distance from the center) of the swell. The colors in the compass plot are not related to the values given on color scale for Hs at the top of the image.

• Sometimes the red peak in the spectrum does not match the peak period and direction with the largest wave height in the N. Pac/S. Pac swell table on the image. Why is that?
  The values in the table are calcualted by summing up wave energy over all swell periods for N. Pac directions (245-335 degrees) and S. Pac directions (155-245 degrees). It is not unusual for a swell event to have a large, sharp peak in the spectrum (i.e. its energy is concentrated in a narrow range of wave periods and directions), but have less total energy than another concurrent swell with a broader distribution.

• What does the Hz value inside the spectrum represent?
  Cycles per second. So a dot of color on the outer part of the circle indicates a swell at .12 cycles per second or more commonly "every 8 seconds." If a dot is near the center of the circle, the swell period is higher. So if it's around the .04 Hz value, that swell component is "every 25 seconds."

• Where is the center of this "radar" geographically?
  The swell measurements are from the CDIP Harvest buoy, which is indicated by the small buoy icon in the upper left of the image.

• What do Hs, Tp and Dp stand for?
  Hs=Significant wave height of swell, or roughly the average height of the 1/3rd highest waves (feet); Tp=Peak period of the swell (seconds); Dp=Compass direction from which the waves are arriving (degrees), e.g. 180=from the south, 270= from the west.

• What is the spatial resolution of the images?
  The image of the entire Southern California Bight has a resolution of 800m. The more detailed regional images have a resolution of 200m in water depths less than 60m (that is why these images look like they have smaller pixels close to the coastline).

• What is the spatial resolution of the bathymetry (ocean bottom topography) grid used by the model?
  100 x 100 meters.

• Why is there sometimes a rather large disparity between the Southern California Bight swell model and the local models (the San Pedro Channel model, the Long Beach model and the San Diego Bay model)?
  The local models are generally more accurate. They use "local" buoys to model both sea (short period waves) and swell (long period waves). The Southern California Swell model is for swell only (T=8 sec and longer) and uses our offshore buoy at Point Conception. They do look significantly different when a local wind sea is present.

• Why is the time on the model sometimes two hours old (or more)?
  Data from the buoys is collected in 30-minute files and marked with the start time, so in general the latest file has a time from one hour ago. And because the swell model products are updated just once an hour, the time on the models can be about two hours old before the next update runs. Lastly, note that even during daylight savings the models are labeled with PST times, making them appear yet another hour older. (E.g. a model with data from 10:30 PDT will be labeled 09:30 PST, the corresponding standard time.)