Spotter Data Partitioning for "sea" and "swell"

Chris Ayan
By: 
Chris Ayan

In addition to the standard bulk parameters, Spotter can now provide the same parameters over two ‘sea’ and ‘swell’ wave partitions

Effectively, this means that we break up the total sea state, which is the superposition of all the waves in the wave field, in a part that is primarily forced by the local wind (the ‘sea’) and a part that is primarily decoupled from the local wind (the ‘swell’). The standard Spotter bulk parameters are calculated for the complete sea state (or the complete wave spectrum). The partitioned bulk parameters are calculated for their respective parts of the wave spectrum. More specifically, we define all waves with a period shorter than 8s to be part of the sea partition, and all waves with periods longer than 8s to be part of the swell partition.

The local sea state, wholly represented by the wave spectrum, is made of multiple individual wave components of various origins, either locally or remotely generated, with unique periods and directions. By partitioning the total measured wave field into its sea and swell components, we can calculate their individual parameters. 

The ‘sea’ partition thus represents the shorter waves (less than 8s period), also commonly known as ‘wind chop’, and forced by local winds. These short waves are generally short-crested and can lead to white-capping. The bulk parameters for the sea partition can change on fairly short time scales as a response to changes in the local wind in speed, direction, or fetch (distance over which the wind is affecting the ocean surface) and duration (time over which the wind is affecting the ocean surface). 

The ‘swell’ component is comprised of waves over 8s of period, and near the coast these waves are sometimes referred to as ‘ground swell’, indicating that they interact with the seabed. The swell field consists of waves originating from remote storms. The wave field is more long-crested, typically more narrow-band, which is visually seen as a fairly coherent and organized surface pattern. These waves could have travelled hundreds or even thousands of miles before reaching the point of observation. As a result, they are decoupled from local winds, and changes in this wave field are generally much slower than for the sea partition. 

The separating out of sea and swell partitions provides better insight into the nature of the sea state than an average parameter for the whole sea state. For instance, the sea roughness (or choppiness) is primarily dictated by the energy in the sea partition. Whereas nearshore shoaling and refraction effects are stronger for the longer waves in the swell partition. 

Please note that the total significant wave height is not a simple sum of the sea and swell height combined.

Our current partition scheme separates the sea and swell components as having shorter or longer periods than 8s, respectively. This is a common threshold for sea/swell separation, but admittedly somewhat arbitrary. Currently, this is fixed. We plan to provide user-adjustable and self-tracking thresholds in the near future as well.

We will keep you posted! If you have questions, please reach out to us at support@sofarocean.com. 

For the technical manual, please click here.


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