Optimal Boundary Conditions for ORCA-2 Model

TL;DR

This paper uses 4D-Var data assimilation to optimize boundary conditions in the ORCA-2 ocean model, leading to improved jet stream representations and insights into boundary parametrizations.

Contribution

It introduces a method to optimize boundary conditions in the ORCA-2 model using 4D-Var, highlighting the impact on jet stream simulation and discussing automatic adjoint code generation.

Findings

Optimal boundary conditions enhance jet stream strength and detail.

Bottom boundary parametrization significantly influences jet reinforcement.

Tapenade software effectively generates adjoint code with memory optimization.

Abstract

A 4D-Var data assimilation technique is applied to a ORCA-2 configuration of the NEMO in order to identify the optimal parametrization of the boundary conditions on the lateral boundaries as well as on the bottom and on the surface of the ocean. The influence of the boundary conditions on the solution is analyzed as in the assimilation window and beyond the window. It is shown that optimal conditions for vertical operators allows to get stronger and finer jet streams (Gulf Stream, Kuroshio) in the solution. Analyzing the reasons of the jets reinforcement, we see that the major impact of the data assimilation is made on the parametrization of the bottom boundary conditions for lateral velocities u and v. Automatic generation of the tangent and adjoint codes is also discussed. Tapenade software is shown to be able to produce the adjoint code that can be used after a memory usage optimization.