Calculations of unbiased estimates of the magnitude of residual velocities from a small number of drogue trajectories

Sanderson, Brian G.; Pal, Badal K.; Goulding, Allan

doi:10.1029/jc093ic07p08161

Cited by 8 publications

(2 citation statements)

References 5 publications

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“…Velocity error has two sources (Okubo and Ebbesmeyer 1976;Kirwan 1988): GPS instrument noise or processes noise by assuming spatially uniform velocity gradients in the Taylor series expansion. The velocity error has been a posteriori estimated from the LS misfit (Okubo and Ebbesmeyer 1976;Sanderson et al 1988). Kirwan and Chang (1979) investigated the role of instrument noise on DKP error.…”

Section: Introductionmentioning

confidence: 99%

The Effect of Drifter GPS Errors on Estimates of Submesoscale Vorticity

Spydell

Feddersen

MacMahan

2019

Journal of Atmospheric and Oceanic Technology

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Differential kinematic flow properties (DKP), such as vertical vorticity, have been estimated from surface drifters. However, previous DKP error estimates were a posteriori and did not include correlated errors across drifters. To accurately estimate submesoscale (≤1 km) DKPs from drifters, errors must be better understood. Here, the a priori vorticity standard error is derived that depends upon the number of drifters in the cluster, the drifter cluster major and minor axes lengths, the instrument velocity error, and the cross-drifter error correlation. Two stationary GPS experiments, with zero vorticity, were performed at separations of O(101–103) m to understand vorticity error and test the derivation using 1 Hz position differences and Doppler shift velocities. Vorticity errors of ±5f (where f is the local Coriolis parameter)were found for ≈40 m separations. The frequency-dependent velocity variances and GPS-to-GPS correlations are quantified. Vorticity estimated with a “blended” velocity has reduced error. The stationary vorticity error can be well predicted given velocity error, correlation, and minor axis length. Vorticity error analysis is applied to submesoscale-sampling in situ GPS drifters near Point Sal, California. The derivation predicts when large high-frequency vorticity fluctuations (indicating noise) occur. Previously, cluster area or ellipticity were used as criteria to distinguish error. We show that the drifter cluster minor axis (narrowness) is a key time-dependent factor affecting vorticity error, and even for velocity errors <0.004 m s−1, the vorticity error exceeds ±5f when cluster minor axis <50 m. These results will aid submesoscale drifter deployment planning.

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Section: Introductionmentioning

confidence: 99%

The Effect of Drifter GPS Errors on Estimates of Submesoscale Vorticity

Spydell

Feddersen

MacMahan

2019

Journal of Atmospheric and Oceanic Technology

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“…the statistical degrees of freedom in the cluster average and the sum is over all drifters in the cluster [seeSanderson et al, 1988]. Note that by definition…”

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Notes on the cluster method for interpreting relative motions

Kirwan¹

1988

J. Geophys. Res.

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It is shown that the so‐called cluster method for determining the horizontal gradients of the horizontal velocity has some basic limitations caused by restrictive physical assumptions. For large‐area clusters there is aliasing of these estimates by large‐scale shear, while for small‐area clusters, the displacement of the statistical cluster center from a physically significant flow center can produce seriously aliased cluster model parameter estimates. These effects are independent of typical experimental problems such as measurement error and statistical reliability. An alternative to the cluster paradigm is proposed. This, however, requires solving a nonlinear inverse problem to recover estimates of the horizontal velocity gradients.

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Drifter observations of a cold filament off Point Arena, California, in July 1988

Swenson¹,

Niiler²,

Brink³

et al. 1992

J. Geophys. Res.

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We use the position fixes and temperature readings from 56 Tristar‐II mixed‐layer drifters, in conjunction with advanced very high resolution radiometer images, to provide a description of the mesoscale variability of the flows associated with cold‐water filaments in the California Current system off northern California in July 1988. We find that the northern, offshore temperature front of the filament is convergent and is closely associated with the core of a high‐speed (>100 cm/s) jet that has a broader spatial scale than the cold‐water filament. Mesoscale features are observed to be related to all developments of the cold‐water filament on a variety of time scales (1–30 days). In one well‐observed feature, estimates of vertical velocity (w) based on a vorticity budget are −20 m/d over an area of 150 km2 and are associated with a gain of vorticity. Estimates of w based on a local heat budget following a drifter are about 3 times larger over an area of 60 km2. These intense downwelling features are also places where horizontal stirring occurs.

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Calculations of unbiased estimates of the magnitude of residual velocities from a small number of drogue trajectories

Abstract: Sea, J. Phys. Oceanoor., 5, 361-368, 1975. Okubo, A., and C. C. Ebbesmeyer, Determination of vorticity, divergence, and deformation rates from analysis of drogue observations, Deep Sea Res., 23, 349-352, 1976.

Cited by 8 publications

References 5 publications

The Effect of Drifter GPS Errors on Estimates of Submesoscale Vorticity

The Effect of Drifter GPS Errors on Estimates of Submesoscale Vorticity

Notes on the cluster method for interpreting relative motions

Drifter observations of a cold filament off Point Arena, California, in July 1988

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