California Wave Models
In October 2016 the server running CDIP's original swell model code
reached its end of life; both the server and that model have been retired.
Now the latest version of CDIP's spectral refraction model is being used
to generate all model products. This model was released in 2010 as a
significant advance over the original model. With lower levels of numerical
noise, the spectral refraction model is able to accurately predict wave
periods and directions in addition to wave heights.
For scientific details on the models, please refer to our
model description page.
Since output from the original swell model remains highly popular among
users, CDIP has started generating the same products and formats using the
latest version of the wave model. There are, however, a number of ways in
which the new output differs from the old:
- Predictions in all regions incorporate proper time lags for the
propagation of swell. E.g. where the old model would show wave heights
all across the Southern California Bight instantaneously rising and
falling with readings from the Harvest buoy, the current model
shows offshore changes propagating across the bight hour-by-hour.
- While the old model used a single buoy to make predictions, the current
model combines input from multiple offshore buoys. This generates
better predictions but can sometimes result in minor discontinuities on
the wave maps, where input from one buoy starts or stops.
- Uncolored areas at the edges of the map represent locations
where model predictions are currently unavailable. Whereas the old model
would assume a common wave field at all deep water locations and make
predictions across the entire map, the current model assesses output
sites on a point-by-point basis, producing predictions only where
appropriate. A location's update status is determined by its position
relative to the buoy network, its swell exposure, and buoy data availability.
- The 'Deep Water' spectrum plot and the 'Deep Water Swell' summary text no
longer come from a single buoy. Instead they are estimated for the center
of the map from multiple buoys using the appropriate time lags.
- Model resolution has been standardized across the entire state. In many
areas - e.g. across all of Northern California - this results in improved
resolution. In a few areas, however, resolution will be lower than with
the old swell model.
More details on the
CDIP MOP System.
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 maps are created by a linear spectral refraction
wave model developed by Bill O'Reilly. The model is initialized
with input from the deep-water buoys in CDIP's wave monitoring
For more detailed information please see "The California coastal wave
monitoring and prediction system" by O'Reilly et al.
(Coastal Engineering, Volume 116, October 2016, Pages 118–132).
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 are collected by Datawell buoys
off the California coast. These data are transferred to the Coastal Data
Information Program at Scripps at 30 minute intervals.
The buoy data are processed and combined to produce estimates of the
directional wave spectrum at each point on the map. These estimates
are time-lagged to properly predict the arrival times of swell at
locations across the map.
The buoy data are also used to create an estimate of the deep water
directional spectrum for the middle of the model domain (i.e. for
the center point of the map). This 2D spectrum is shown at the bottom
left of the Southern California image, and is used to calculate the N/S
summary parameters given on the map.
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 (240-335 degrees) and S. Pac
directions (155-240 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 directional spectrum is estimated for the middle of the map, e.g.
for Southern California a point between Catalina and San Nicolas Islands.
This central point is most representative of the region as a whole.
- 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
0.01 degrees or approximately 1000m. The more detailed regional images
have a resolution of 0.001 degrees or approximately 100m 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.