Improving the accuracy of predictions with higher resolution models
For a tidal model, the variables of interest are the tidal current U (east‐west component) and V (north‐south component) and the surface elevation Z. It is convenient to stagger the variables on the model grid (i.e. the position of the current components and the elevation is different). This provides a way of computing the derivative functions that appear in the equations and is a
technique called 'finite differences'. Most of NOC’s tidal models are finite difference models.
The staggering of variables also ensures numerical stability (i.e ensures small errors in the computation are not amplified). In reality, physical variables like currents and elevations are continuous (they have some value everywhere in physical space). By dividing the model area up into a grid, and calculating the variables at only one place in each cell, we have distorted
this reality. The process of converting a real flow field onto the gridded one is called discretisation.
Model Accuracy, Resolution and Numerical Considerations
As only one value for each variable of interest is available for each grid cell, each value computed for each grid cell can at best represent the average value for that cell. Therefore the smaller the grid cell, the more accurately the model will be able to represent reality. This is particularly important when modelling parameters which can vary greatly over fairly small distances.
The only expense in having more grid cells is increased computer memory and computational time. A close analogy would be the resolution of the sensor in a digital camera – an 16
Mega Pixel camera will capture more detail in the image than a 4 Mega Pixel camera (it has a higher resolution) but the image files are a lot bigger and take longer to download and process.
The model resolution determines the types of motion that the model is capable of simulating.
The accuracy of a model can be increased by any of the following:
- Improved horizontal resolution (smaller grid)
- Improved vertical resolution (more layers)
- Improved resolution in the bathymetry
- More accurate boundary conditions
- Better temporal resolution of the boundary conditions
New models for Polpred are continually in development and it is worth checking that you always are using the best possible model to inform your operations. More details on Polpred models available to you can be found on our website or by contacting the Marine Information Products and Services Team.