Shelf slope, estuarine dynamics and river plumes in a generalized vertical coordinate, unstructured grid model

Modeling the ocean from the open sea to the coasts requires an adequate choice of the vertical coordinate system.

A category of generalized vertical coordinates, the so-called z^∗ coordinates ([Stacey et al 1995; Adcroft and Campin 2004]), has been implemented into an unstructured grid, finite elements and layer integrated model which was previously z−geopotential coordinate based.

Idealized and realistic test cases are shown to assess the vertical coordinate impact on key physical processes, i.e. the internal tide generation at a continental slope, a river plume shape and intensity and the salt water intrusion.

To our knowledge this is the first work that uses the z∗ coordinates in an estuary and river plume with an unstructured grid model. Other studies have used the z∗ coordinates with unstructured grid models covering the global ocean ([Ringler et al 2013, Scholz et al 2019] as examples), but without assessing their performance for the small scale dynamics of estuaries and river plumes. Coastal ocean models are currently mostly based on the terrain following coordinates, used within optimised or hybrid approaches ([Zhang  et al 2015; Bruciaferri et al 2018, Fofonova et al 2021, Wise et al 2022] as examples).

The results show that the z^∗ coordinate model produces, with respect to z−geopotential coordinates, a stronger water column stratification. An idealized twin experiment configuration of the model to simulate the production of internal tides at the continental slope confirms that the z^∗ coordinate model reproduces the expected internal tide fields while the z−geopotential model fails. A set of realistic experiments in the Po river delta coastal region, demonstrates that the representation of the dynamical processes at the coastal scales benefits from a very fine resolution of a few tens of centimeters at the surface (four layers of 25 cm are considered in the upper water column), that is only allowed by the z^∗ formulation.