the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A flexible z-coordinate approach for the accurate representation of free surface flows in a coastal ocean model (SHYFEM v. 7_5_71)
Christian Ferrarin
Marco Bajo
Georg Umgiesser
Abstract. We propose a z-coordinate algorithm for ocean models which, thanks to the insertion and removal of surface layers, can deal with an arbitrarily large tidal oscillation independently of the vertical resolution. The algorithm is based on a classical two steps procedure used in numerical simulations with moving boundaries (grid movement followed by a grid topology change, that is insertion/removal of surface layers) which leads to a stable and accurate numerical discretization. With ad-hoc treatment of advection terms at non-conformal edges that may appear due to insertion/removal operations, mass conservation and tracer constancy are preserved. This algorithm, called z-surface-adaptive, can be reverted, as a particular case when all layers are moving, to other z-surface-following coordinates, such as z-star or quasi-z. With simple analysis and realistic numerical experiments, we compare the surface-adaptive-z coordinate against z-star and we show that it can be used to simulate effectively coastal flows with wetting and drying.
Luca Arpaia et al.
Status: open (until 06 Apr 2023)
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RC1: 'Comment on gmd-2023-13', L. Bonaventura, 08 Mar 2023
reply
The preprint presents a potentially interesting contribution to numerical modelling of coastal and oceanic flows. The overview of the literature is broad (even though it could be further improved) and the level of detail in the description of the proposed numerical procedure is appropriate for the scope of GMD. The proposed technique could be very useful for the improvement of the results of coastal and oceanic simulations and deserves to be made available to the community doing research on numerical models for coastal flows, along with a comprehensive assessment of its performance.
However, the detailed description of the numerical method is strongly affected by a number of unclear statements, undefined symbols and imprecise definitions. This makes the paper hard to read in its present form and its real value difficult to assess even for an experienced reader. The authors claim that the method they propose has superior stability properties with respect to those available in the literature, but no stability analysis is provided. The authors' attempt at generality in considering apparently several z-coordinate approaches in the same framework unfortunately ends up making the preprint more obscure, since it is often unclear whether the whole approach has general validity or whether parts of it only apply to some specific z-coordinate formulation. Finally, the truncation error analysis of the vertical discretization appears to be only marginally related to the main topic of the preprint and is affected by several mathematical inconsistencies. The authors may consider removing this part from the revised version of the preprint.
The preprint can be considered for publication after major revisions, that are detailed in the attached file.
Luca Arpaia et al.
Model code and software
SHYFEM version with z-surface-adaptive coordinates Luca Arpaia https://doi.org/10.5281/zenodo.7528682
Luca Arpaia et al.
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