Abstract
The issue of appropriate resolution of coastal models is addressed in this paper. The quality of coastal predictions from three different spatial resolutions of a coastal ocean model is assessed in the context of simulation of the freshwater front in Liverpool Bay. Model performance is examined during the study period February 2008 using a 3-D baroclinic hydrodynamic model. Some characteristic lengthscales and non-dimensional numbers are introduced to describe the coastal plume and freshwater front. Metrics based on these lengthscales and the governing physical processes are used to assess model performance and these metrics have been calculated for the suite of downscaled models and compared with observations.Increased model resolution was found to better capture the position and strength of the freshwater front. However, instabilities along the front such as the tidal excursion led to large temporal and spatial variability in its position in the highest resolution model. By examining the spatial structure of the baroclinic Rossby radius in each model we identify which lengthscales are being resolved at different resolutions. In this dynamic environment it is more valuable to represent the governing time and space scales, rather than relying on strict point by point tests when evaluating model skill.
Highlights
Increasing spatial and temporal resolution appears to be an obvious route for getting more accurate forecasts in operational coastal models
POLCOMS is coupled to the General Ocean Turbulence Model (GOTM) (Holt and Umlauf, 2008), and POLCOMS–GOTM will be used for the highest resolution simulations presented here
The frontal width is represented by 1–2 grid cells in Atlantic Margin Model (AMM), of order 10 in the Irish Sea (IRS) model, and order 100 in the highest resolution Liverpool Bay (LB) models
Summary
Increasing spatial and temporal resolution appears to be an obvious route for getting more accurate forecasts in operational coastal models. Physical processes such as coastal baroclinic Rossby waves may need increased resolution (Garvine, 1995; Chant, 2011). There are penalties for increasing resolution, for example the cost of running a higher resolution model may increase by several orders of magnitude, because if the resolution in 1-D is doubled the resolution in 2-D is 22, plus usually there will be a related reduction in time step which may lead to an order of magnitude increase in the model run time for the same period of real time Another problem is the introduction of high-frequency variability which is not necessarily deterministic.
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