Evaluating the leeway coefficient for different ocean drifters using operational models

TL;DR

This study compares the water-following behavior of six ocean drifters using two operational models, revealing that inertial currents significantly influence leeway coefficients and that models can accurately reproduce observed trajectories.

Contribution

It introduces a method to evaluate and compare leeway coefficients of different drifters across two operational models, accounting for variability in direction and time.

Findings

Leeway coefficients are consistent with previous observational studies.

Inertial frequency is the largest source of variance in leeway coefficients.

Models can precisely reproduce observed drifter trajectories using variable leeway coefficients.

Abstract

The water following characteristics of six different drifter types are investigated using two different operational marine environmental prediction systems: one produced by Environment and Climate Change Canada (ECCC) and the other produced by the Norwegian Meteorological Institute (METNO). These marine prediction systems include ocean circulation models, atmospheric models, and surface wave models. Two leeway models are tested for use in drift object prediction: an implicit leeway model where the Stokes drift is implicit in the leeway coefficient, and an explicit leeway model where the Stokes drift is provided by the wave model. Both leeway coefficients are allowed to vary in direction and time in order to perfectly reproduce the observed drifter trajectory. This creates a time series of the leeway coefficients which exactly reproduce the observed drifter trajectories. Mean values for the leeway coefficients are consistent with previous studies which utilized direct observations of the leeway. For all drifters and models, the largest source of variance in the leeway coefficient occurs at the inertial frequency and the evidence suggests it is related to uncertainties in the ocean inertial currents.