Evaluation of nonidentical versus identical twin approaches for observation impact assessments: an ensemble-Kalman-filter-based ocean assimilation application for the Gulf of Mexico

Abstract

Abstract. Assessments of ocean data assimilation (DA) systems and observing system design experiments typically rely on identical or nonidentical twin experiments. The identical twin approach has been recognized as yielding biased impact assessments in atmospheric predictions, but these shortcomings are not sufficiently appreciated for oceanic DA applications. Here we present the first direct comparison of the nonidentical and identical twin approaches in an ocean DA application. We assess the assimilation impact for both approaches in a DA system for the Gulf of Mexico that uses the ensemble Kalman filter. Our comparisons show that, despite a reasonable error growth rate in both approaches, the identical twin produces a biased skill assessment, overestimating the improvement from assimilating sea surface height and sea surface temperature observations while underestimating the value of assimilating temperature and salinity profiles. Such biases can lead to an undervaluation of some observing assets (in this case profilers) and thus a misguided distribution of observing system investments.

Approximate continuous data assimilation of the 2D Navier-Stokes equations via the Voigt-regularization with observable data

“…The phrase identical twin here refers to how both simulations use the same underlying physics, even though the initial data for both simulations is different. For a detailed look at twin experiments see, e.g., [60].…”

“…We note that in several validation tests the physics were adjusted (as detailed below), which would classify those tests as "fraternal twin" experiments instead. For a detailed discussion of twin experiments see, e.g., [50].…”

Coupling an agent-based model and an ensemble Kalman filter for real-time crowd modelling

“…The use of pseudo-truth data is common for exploratory studies [17,20] and has the advantage that we can precisely attribute any uncertainty to its underlying causes, but leaves open the question of how well this approach might work in the real world. Investigations in other fields have compared the use of 'identical twins' against the use of 'non-identical twins' [50], finding that in those cases the use of 'identical twins' resulted in an overestimation of the effectiveness of the DA approach. Immediate future work will begin to test this by incorporating data that are derived from the movements of real people such as Zhou et al [42], along with the use of more complex models of pedestrian behaviour such as a social force model [51].…”

Assimilating synthetic Biogeochemical-Argo and ocean colour observations into a global ocean model to inform observing system design

“…-The scheme used for advection of biogeochemical variables was changed from total variance dissipation (TVD) (Zalesak, 1979) to the monotone upstream scheme for conservative laws (MUSCL) (Van Leer, 1977;Lévy et al, 2001).…”