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Full Documentation

CDIP Wave Model Prediction Database

CDIP has developed a wave model prediction database for the CA coast. This document describes the current CDIP wave models, the wave information they provide, and some of the ongoing work being performed to improve the online database.

1. General Wave Climate Description.

2. California Coastal Wave Modeling

  • Offshore Directional Wave Measurements
  • The Spectral Refraction-Diffraction (RD) Model
  • The Spectral Refraction (R) Model

3. Developing a Coastal Wave Prediction Database

  • Creating The Best Offshore Wave Spectra Database
  • Including Local Seas

4. FAQs

  • How can I access the database of model predictions?

1. General CA Wave Climate Description

Waves along the CA coastline are typically described as sea and swell. Seas are waves generated by local winds near the coast. They are characterized by relative short wave lengths and wave periods ( < 10 seconds ). Swell are longer wavelength, longer period waves ( > 10 seconds) that are generated by distant storm winds, and then propagate to the CA coastline with little influence from the local winds. Often times, the sea and swell components of the wave field on the CA coast are quite distinct. However, when North Pacific storms make landfall in CA during the winter, the wave field tends to be more "fully developed" and there is no clear distinction between the sea and swell components.

CA surfers will often refer to "ground swell", which usually refers to swell arrivals from very distant storms in the Southern Ocean between Australia and New Zealand, or the Gulf Of Alaska in the North Pacific. You may also hear the term "wind swell" which refers to relatively short period swell ( 8 -12 seconds) that are generated relatively close to the CA coast, but far enough offshore that the coast is not directly in the wind field, or "fetch", that generated the waves. NW wind swell are common in the Pt. Conception area owing to the pocket of strong winds that frequently develops off the Central CA coast.

Real world wave conditions are defined by their frequency-directional wave spectrum, or the distribution of wave energy in the wave field as a function of the wave frequency (or period) and direction. The full frequency-directional spectrum is often distilled down to a few wave parameters such as height, peak period, and the mea wave direction at the peak period for many applications, but the current state-of-the-art wave models transform complete spectra, not wave parameters, to provide the most accurate predictions of coastal waves.

2. California Coastal Wave Modeling

From a wave modeling perspective, the CA wave climate can be divided into two regions:

A) Central/Northern CA open coastline: The coast north of Pt. Conception to the OR-CA border. A relatively straightforward wave transformation problem.

B) Southern CA sheltered coastline: The coast and islands south of Pt. Conception to the US-Mexico border. A more complicated island sheltering problem with local seas development between the islands and the mainland coast.

The general CDIP wave modeling approach is to use linear, wave propagation models to transform offshore (unsheltered) deep water buoy measurements of the wave field to shallow or sheltered locations along the coast. These are referred to as "buoy-driven" model predictions, as opposed to initializing the coastal model with offshore predictions from an ocean basin-scale wind-wave model like NOAA's WaveWatch III.

CDIP has been making offshore directional wave measurements in Southern California since November 1992. Initially, these measurements were made with an array of pressure sensors attached to Harvest Platform, west of Point Conception. Because of the distance between the pressure sensors, the array was only used to predict the directional spectrum of the wave field for periods of 8 seconds and longer. In March 1998, the Harvest Platform array was damaged by a service vessel and a Datawell Directional Waverider Buoy was deployed near the Platform as a replacement.

Two different linear wave propagation wave models have been used by CDIP: a spectral Refraction-Diffraction (RD) model, and a spectral Refraction (R) model. These propagation models do not simulate the generation of wave energy by the wind. They assume any additional wave energy generation by winds between the offshore buoy and a coastal prediction point is small. As a result, the models have primarily been used to date to predict coastal swell conditions, not local seas.

Both models are linear, which means that the results are independent of wave energy, and model calculations only have to be performed once for a particular offshore wave direction and period. The resulting relationship between the offshore and shallow wave spectra for this particular period-direction combination can be saved as a series of linear transformation coefficients. CDIP maintains a database of the transformation coefficients and uses them to generate the various real time wave model products.

The RD model was retired in October 2016. Currently CDIP is only running the R model.

The two models differ in two main ways:

A) Model Physics
The RD model simulates both the refraction and diffraction of waves, while the R model only simulates refraction. However, comparisons between the models in Southern California have shown that the models produce similar wave height results throughout the Bight, and it is believed that diffraction plays a minor role in the overall evolution of wave field characteristics across the CA continental shelf (prior to reaching coastal structures such as jetties and breakwaters).

B) Model Numerics
The numerical methods used in the RD and R models are dramatically different and this has proven to be a crucial issue when applying them to large, geographically complicated coastal regions.

The RD model uses a finite-difference propagation scheme, which means that you initialize the model along an offshore boundary, and when the model run is complete, you have estimates of wave conditions throughout the model domain. Southern California is one such model domain, and the RD model was previously used to produce the widely-used Southern California swell wave height maps.

The R model is a "ray-based" model. For a specific sheltered or shallow water location, wave rays are backward refracted to unsheltered deep water for all wave periods and directions of interest. The resulting starting and ending angles of the rays can then be used to transform, or map, an offshore wave directional spectrum to the starting location.

Because the R model produces results on a point by point basis, it is not easily used to make regional wave height maps like the RD model. However, the numerics of RD model has some serious limitations when applied to complicated areas like Southern California. Wave dissipation and propagation around islands and extreme bathymetry (eg. submarine canyons) results in low levels of numerical noise in the RD model results. This noise has relatively little impact on wave height estimates in most situations, but make it very difficult to extract useful directional wave information, or details about the overall distribution of wave energy with wave frequency or period, from the RD model output.

The R model, while containing less propagation physics, is more robust from a numerical noise perspective, and is required to predict the directional properties of the CA coastal wave field from offshore buoy measurements. Given recent advances in computing power, it has become feasible to use it for generating wave height maps and all of CDIP's model products, leading to the retirement of the RD model in October 2016.

3. Developing a Coastal Wave Model Database

The development of a coastal wave model database had two main components: 1) Developing a standard historical offshore wave spectra database for initializing the models, and 2) Including local seas in the prediction where possible.

Creating an offshore spectrum database requires blending wave measurements from several different stations over time. To do this, CDIP's models use a combination of offshore, deep-water buoys as input sources for swell predictions. For instance, in the Southern California Bight most swell predictions combine input data from the Harvest, San Nicolas Island, and Point Loma buoys. Since Harvest is best exposed to NW swell, and Point Loma to S and SE swell, using a combination of inputs improves model skill.

In Southern California, the local generation of seas in the regions between the offshore islands and the coastline limits the propagation model predictions to swell when using offshore buoys. To monitor and model local seas, a local deep water buoy network is now maintained by CDIP. The buoys are deployed at the shelf break within the islands and are used with the propagation models to predict both sea and swell near the coast and at harbor entrances. The CDIP local buoy network began with the Oceanside Buoy in May 1997 and was completed with the Anacapa Passage Buoy in June 2002.

Combined sea and swell predictions using the spectral R model with local wave buoys are now available along the entire California coast.

4. Frequently Asked Questions

  • How can I access the database of model predictions?

  • Alongshore predictions from the spectral refraction model are available for the California coast, datasets for some sites run from 2000 to the present. Currently access to the database interface is password-protected; please contact CDIP for more details.
    Official UCSD Web Page