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. (Figure 1)
#. 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. (Figure 2)
#. 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. (Figure 2)
#. 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.
+----------+----------+
| |image1| | |image2| |
+----------+----------+
| Fig. 1 | Fig. 2 |
+----------+----------+
| 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 network.
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.
About the CDIP Wave Forecast Model
----------------------------------
These experimental coastal wave forecasts are a joint research effort by:
-------------------------------------------------------------------------
| `Office of Naval Research `__
| `California Department of Boating and
Waterways `__
| `U.S. Army Corps of Engineers `__
--------------
The CDIP coastal wave forecast model
------------------------------------
| The coastal wave height forecasts are actually the combined result of
two wave models:
- The ECMWF HRES-WAM global wave model. The ECMWF model is a wind-wave
generation and propagation model. That is, the global wave forecasts
are made based on global surface wind forecasts. More details are
available on the `ECMWF website `__.
[Prior to May 2022, NOAA Wavewatch III was used rather than ECMWF; please see the
`NOAA Wavewatch III `__ web site].
- The Coastal Data Information Program's implementation of a spectral
refraction wave model for shallow water (10m < depth < 500m). This is
a propagation-only model (no wind-wave generation). It models the
effect of bathymetry (underwater topography) on waves as they travel
from deep water towards the coast. [See the `CDIP swell
model <#id1>`__ web page]. This implementation is
slightly different than the swell version. We also include the
propagation (but not generation) of shorter period local seas based
on input from the ECMWF forecast model.
--------------
How the forecasts are made.
---------------------------
- Every 6 hours, CDIP receives a grid of 2D spectral wave forecasts from
ECMWF for deep water off of the West Coast and California.
- ECMWF's forecasts are used to initialize the CDIP wave propagation
model and make predictions of wave heights across the continental
shelf to the coast (10m water depth).
--------------
A Description of Forecast Model Products.
-----------------------------------------
- .. rubric:: Offshore Wave Height Forecast Plot
:name: offshore-wave-height-forecast-plot
| This is the plot displayed at the top of the CDIP Recent-Forecast
page. These are forecasted significant wave heights from the NOAA
global Wavewatch III model for the two deep water sites off
California used in the coastal wave models:
- Pt. Conception - (34N 121W, used to make coastal predictions in
southern and central California).
- San Francisco - (37N 123W, used to make coastal predictions from
Monterey Bay to Pt. Arena).
|
| The plot is designed to provide a quick look at whether there is a
big deep water wave event on the horizon that may impact the
coastline. The `storm threshold <#threshold>`__ line of
approximately 13 ft. is based on historical storms in southern
California. Forecasts of offshore waves exceeding this threshold do
not necessarily mean damaging coastal waves will occur (for example
the waves can come from the northwest and southern California is
sheltered by Pt. Conception). It is provided as a guideline for
when you may want to look at more detailed plots of coastal
forecasts.
- .. rubric:: Wave Height Maps (San Diego to Pt. Arena)
:name: wave-height-maps-san-diego-to-pt.-arena
| These are similar to CDIP real-time swell maps, but include short
period local seas. Regions along the coastline are clipped from the
larger modeling area and rotated so that 3 forecast days can be
stacked on a single plot. The wave height scale on these plots is
fixed between 0 and 27+ feet. As with the Southern California swell
maps, the time lag for waves to propagate from offshore waters to
the coast are not accounted for in these images.
- .. rubric:: Coastal Wave Height Plots (Southern California Only)
:name: coastal-wave-height-plots-southern-california-only
| These are also similar to CDIP's real-time swell predictions of
alongcoast wave height. They are a plot of the model results along
the 10m depth contour. They are not breaking wave heights.
Generally, the 10m depth contour is outside the surf zone (area of
depth-induced wave breaking). However, when the wave height exceeds
roughly 15 ft., 10m is the outer end of the surf zone and the
plotted height will be larger than what would actually occur at 10m
depth. Nevertheless, it does provide an idea of how much wave
energy is reaching the coast relative to other locations. Higher
wave energy generally leads to higher water levels and wave runup
at the adjacent shore.
- .. rubric:: Coastal Wave + Tide Plots (Southern California Only)
:name: coastal-wave-tide-plots-southern-california-only
These plots combine predicted tides and forecasted wave heights in
10m depth (described above) on a site by site basis in southern
California. The combined height is defined as the "Potential Flooding
Index". They provide a clear view of when forecasted storm waves are
going to be coincident with high tides.
--------------
Known Limitations
-----------------
- .. rubric:: Forecasting Local Seas
:name: forecasting-local-seas
| The CDIP coastal wave propagation models do not include wind-wave
generation by local winds between the ECMWF deep water sites
and the coast. Unlike the CDIP swell model, we have included the
propagation of seas in the coastal predictions using the ECMWF
forecasts of seas at the deep water sites. For the coastline
north of Pt. Conception, the resulting errors should be relatively
small during large wave events because the continental shelf is
narrow and there isn't much distance (fetch) between deep water and
the coast for additional wave generation.
| In southern California, the story is once again more complicated
given the distance between deep water and some coastal areas, and
the existence of numerous offshore islands. For the south-facing
coastline in Santa Barbara County the coastal forecasts will most
likely underpredict local seas. The CDIP wave propagation model
assumes almost complete blocking of seas from the south by the
Channel Islands and ignores local wind generation in the Santa
Barbara Channel.
--------------
Frequently asked questions
--------------------------
- *What is meant by 'storm threshold' in the offshore forecast plot?*
The definition of a major storm is arbitrary and is based upon
historical precendent. In Seymour et al (1984), a list was shown of
hindcast and measured large wave events in Southern California during
the first 83 years of the century when significant wave heights
exceeded 10 feet for more than 9 hours. Seymour (1996) updated this
list, but extended the height threshold to 13 feet to qualify as a
major storm (because it appeared that smaller storms had been
undercounted in the early part of the century when atmospheric data
to support wave hindcasts were sparse.) Because the forecasts used
here are based on 12 hour update intervals, a modification to the
threshold in Seymour (1996) has been made so that wave heights must
exceed 13 feet for 12 hours to be counted as a major event.
References:
Seymour, R.J., R. R. Strange III, D. R. Cayan, and R. A.
Nathan].1984. Influence of El Ninos on California's wave climate. In:
Proc. 19th Int. Conf. on Coastal Engineering, B. L. Edge, ed., ASCE,
Houston, Texas, 3-7 September, 1984, Vol 1: 577-592.
R. J. Seymour.1996. Wave Climate Variability in Southern California,
J. of Waterway, Port, Coastal, and Ocean Engineering, ASCE,
July/August 1996, Vol 122(4): pp. 182-186
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