Rigorous Evaluation of a Fraternal Twin Ocean OSSE System for the Open Gulf of Mexico

TLDR

This study introduces the first ocean observing system that leverages design criteria and rigorous evaluation procedures originally developed for atmospheric OSSEs, with a nature run that realistically reproduces Gulf of Mexico climatology and variability. The goal is to validate a fraternal twin OSSE system in the Gulf of Mexico and to ensure, a priori, that it neither overestimates nor underestimates observing system impacts. The system comprises a nature run representing the true ocean, a forecast model coupled to a new data assimilation system, and software that simulates observations with realistic errors, following a fraternal twin configuration and applying rigorous atmospheric OSSE evaluation procedures, performing observing system experiments and identical OSSEs that differ only by assimilating synthetic observations. The OSSE validation showed that impact assessments from observing system experiments matched those from OSSEs, confirming the system’s validity without calibration.

Abstract

Abstract A new fraternal twin ocean observing system simulation experiment (OSSE) system is validated in a Gulf of Mexico domain. It is the first ocean system that takes full advantage of design criteria and rigorous evaluation procedures developed to validate atmosphere OSSE systems that have not been fully implemented for the ocean. These procedures are necessary to determine a priori that the OSSE system does not overestimate or underestimate observing system impacts. The new system consists of 1) a nature run (NR) stipulated to represent the true ocean, 2) a data assimilation system consisting of a second ocean model (the “forecast model”) coupled to a new ocean data assimilation system, and 3) software to simulate observations from the NR and to add realistic errors. The system design is described to illustrate the requirements of a validated OSSE system. The chosen NR reproduces the climatology and variability of ocean phenomena with sufficient realism. Although the same ocean model type is used (the “fraternal twin” approach), the forecast model is configured differently so that it approximately satisfies the requirement that differences (errors) with respect to the NR grow at the same rate as errors that develop between state-of-the-art ocean models and the true ocean. Rigorous evaluation procedures developed for atmospheric OSSEs are then applied by first performing observing system experiments (OSEs) to evaluate one or more existing observing systems. OSSEs are then performed that are identical except for the assimilation of synthetic observations simulated from the NR. Very similar impact assessments were realized between each OSE–OSSE pair, thus validating the system without the need for calibration.