California Wave Models
In 2010, CDIP started releasing output from its California-wide
wave model. This new model is based on the CDIP Monitoring and
Prediction (MOP) System. The new model represents a significant
advance over the old Southern California Swell Model. With lower
levels of numerical noise, the new model is able to accurately
predict wave periods and directions in addition to wave heights.
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.
scientific details on the models.
Below is information on the Southern California Swell Model, which
is still being run for some areas.
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.
The swell model is based on wave refraction-diffraction simulations
performed on San Diego Supercomputer Center in 1990. The model uses
methods developed by Jim Kirby at the University of Delaware.
The original computer code was written by Eloi Melo, Santa Catarina Federal
University, Brazil, while he was a graduate student at Scripps. The program
was later adapted for use with large regions by Bill O'Reilly.
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.)