Motion-correlated flow distortion and wave-induced biases in air–sea flux measurements from ships

Abstract. Direct measurements of the turbulent air–sea fluxes of momentum, heat, moisture and gases. are often made using sensors mounted on ships. Ship-based turbulent wind measurements are corrected for platform motion using well established techniques, but biases at scales associated with wave and platform motion are often still apparent in the flux measurements. It has been uncertain whether this signal is due to time-varying distortion of the air flow over the platform, or to wind–wave interactions impacting the turbulence. Methods for removing such motion-scale biases from scalar measurements have previously been published but their application to momentum flux measurements remains controversial. Here we show that the measured motion-scale bias has a dependence on the horizontal ship velocity, and that a correction for it reduces the dependence of the measured momentum flux on the orientation of the ship to the wind. We conclude that the bias is due to experimental error, and that time-varying motion-dependent flow distortion is the likely source.

Download Full-text

Motion-correlated flow distortion and wave-induced biases in air–sea flux measurements from ships

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-10619-2015 ◽

2015 ◽

Vol 15 (18) ◽

pp. 10619-10629 ◽

Cited By ~ 15

Author(s):

J. Prytherch ◽

M. J. Yelland ◽

I. M. Brooks ◽

D. J. Tupman ◽

R. W. Pascal ◽

...

Keyword(s):

Momentum Flux ◽

Time Varying ◽

Wave Interactions ◽

Flow Distortion ◽

Platform Motion ◽

Flux Measurements ◽

Wind Measurements ◽

Dependent Flow ◽

Measured Momentum ◽

Wave Induced

Abstract. Direct measurements of the turbulent air–sea fluxes of momentum, heat, moisture and gases are often made using sensors mounted on ships. Ship-based turbulent wind measurements are corrected for platform motion using well established techniques, but biases at scales associated with wave and platform motion are often still apparent in the flux measurements. It has been uncertain whether this signal is due to time-varying distortion of the air flow over the platform or to wind–wave interactions impacting the turbulence. Methods for removing such motion-scale biases from scalar measurements have previously been published but their application to momentum flux measurements remains controversial. Here we show that the measured motion-scale bias has a dependence on the horizontal ship velocity and that a correction for it reduces the dependence of the measured momentum flux on the orientation of the ship to the wind. We conclude that the bias is due to experimental error and that time-varying motion-dependent flow distortion is the likely source.

Download Full-text

Comparison of Direct Covariance Flux Measurements from an Offshore Tower and a Buoy

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-15-0109.1 ◽

2016 ◽

Vol 33 (5) ◽

pp. 873-890 ◽

Cited By ~ 10

Author(s):

Martin Flügge ◽

Mostafa Bakhoday Paskyabi ◽

Joachim Reuder ◽

James B. Edson ◽

Albert J. Plueddemann

Keyword(s):

Motion Correction ◽

Flow Distortion ◽

Wide Range ◽

Direct Covariance ◽

Flux Measurements ◽

Floating Platforms ◽

Buoy Data ◽

Wave Induced ◽

Good Agreement ◽

Platform Motions

AbstractDirect covariance flux (DCF) measurements taken from floating platforms are contaminated by wave-induced platform motions that need to be removed before computation of the turbulent fluxes. Several correction algorithms have been developed and successfully applied in earlier studies from research vessels and, most recently, by the use of moored buoys. The validation of those correction algorithms has so far been limited to short-duration comparisons against other floating platforms. Although these comparisons show in general a good agreement, there is still a lack of a rigorous validation of the method, required to understand the strengths and weaknesses of the existing motion-correction algorithms. This paper attempts to provide such a validation by a comparison of flux estimates from two DCF systems, one mounted on a moored buoy and one on the Air–Sea Interaction Tower (ASIT) at the Martha’s Vineyard Coastal Observatory, Massachusetts. The ASIT was specifically designed to minimize flow distortion over a wide range of wind directions from the open ocean for flux measurements. The flow measurements from the buoy system are corrected for wave-induced platform motions before computation of the turbulent heat and momentum fluxes. Flux estimates and cospectra of the corrected buoy data are found to be in very good agreement with those obtained from the ASIT. The comparison is also used to optimize the filter constants used in the motion-correction algorithm. The quantitative agreement between the buoy data and the ASIT demonstrates that the DCF method is applicable for turbulence measurements from small moving platforms, such as buoys.

Download Full-text