Flow distortion in flux measurements with IMST/SA-CNRS Lyman-alpha hygrometer

1988 ◽  
Vol 6 (7) ◽  
pp. 443-454 ◽  
Author(s):  
P. Mestayer ◽  
C. Rebattet
Keyword(s):  
Flow Distortion ◽  
Lyman Alpha ◽  
Flux Measurements

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

2015 ◽  
Vol 15 (11) ◽  
pp. 15543-15570 ◽  
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.

scholarly journals Direct Flux Measurements from Mobile Platforms at Sea: Motion and Airflow Distortion Corrections Revisited

2015 ◽  
Vol 32 (6) ◽  
pp. 1163-1178 ◽  
Author(s):  
Sebastian Landwehr ◽  
Niall O’Sullivan ◽  
Brian Ward
Keyword(s):  
Wind Speed ◽  
Correction Method ◽  
New Method ◽  
Wind Vector ◽  
Mean Flow ◽  
Mobile Platforms ◽  
Flow Distortion ◽  
Scalar Fluxes ◽  
Flux Measurements ◽  
Bulk Model

AbstractShip-based measurements of wind speed and direct fluxes are affected by airflow distortion that can lead to a tilt of the wind vector as well as acceleration or deceleration of the wind speed. Direct flux measurements are additionally affected by the fluctuating velocity of the platform. The classic approach is to first correct the wind speed for angular and translational platform velocities and thereafter rotate the wind vector into the mean flow. This study finds that for ships under way, this leads to an overestimation of the vector tilt and biased flux estimates. This may explain the common observation that flux estimates from ships in transit have lower quality than measurements taken on station. Here an alternative approach is presented, where the flow-distortion-induced tilt of the wind vector is estimated from the 3D wind speed measurements and applied to the apparent wind vector. The tilt correction is carried out after correction for the fluctuating part of the platform velocity but before removing the ship’s mean translational velocity. This new method significantly improved the agreement of direct momentum flux measurements made from a ship under way with the parameterization of the COARE3.5 bulk model. The sensitivity of the eddy covariance measurements of momentum and scalar fluxes to the choice of the tilt-motion correction method is analyzed, and this study proposes that a reanalysis of previous direct flux measurements with the new method discussed here can improve researchers’ understanding of air–sea interaction.

scholarly journals Gradient flux measurements of sea-air DMS transfer during the Surface Ocean Aerosol Production (SOAP) experiment

2017 ◽  
Author(s):  
Murray J. Smith ◽  
Carolyn F. Walker ◽  
Thomas G. Bell ◽  
Mike J. Harvey ◽  
Eric S. Saltzman ◽  
...  
Keyword(s):  
Biologically Active ◽  
Physical Factors ◽  
Ionization Mass ◽  
Flow Distortion ◽  
Ocean Turbulence ◽  
Transfer Velocity ◽  
Near Surface ◽  
Surface Ocean ◽  
Flux Measurements ◽  
Good Agreement

Abstract. Direct measurements of marine DMS fluxes are sparse, particularly in the Southern Ocean. The Surface Ocean Aerosol Production (SOAP) voyage in February–March 2012 examined the distribution and flux of dimethylsulfide (DMS) in a biologically-active frontal system in the southwest Pacific Ocean. Three distinct phytoplankton blooms were studied with oceanic DMS concentrations as high as 25 nmol L−1. Measurements of DMS fluxes were made using two independent methods: the eddy covariance (EC) technique using API-CIMS chemical ionization mass spectrometry, and the gradient flux technique (GF) from an autonomous catamaran platform. Catamaran flux measurements are relatively unaffected by air flow distortion and are made close to the water surface where gas gradients are largest. Flux measurements were complemented by near-surface hydrographic measurements to elucidate physical factors influencing DMS emission. Individual DMS fluxes derived by EC showed significant scatter and, at times, consistent departures from the COARE gas exchange parameterization. A direct comparison between the two flux methods was carried out to separate instrumental effects from environmental effects, and showed good agreement with a regression slope of 0.96 (r2 = 0.89). A period of abnormal downward atmospheric heat flux enhanced near-surface ocean stratification and reduced turbulent exchange, during which GF and EC transfer velocities showed good agreement but modelled COAREG values were significantly higher. The transfer velocity derived from near surface ocean turbulence measurements on a spar buoy compared well with the COAREG model in general, but showed less variation. This first direct comparison between EC and GF fluxes of DMS provides confidence in compilation of flux estimates from both techniques, and also in the stable periods when the observations are not well-predicted by the COAREG model.

scholarly journals Quantifying the Airflow Distortion over Merchant Ships. Part I: Validation of a CFD Model

2006 ◽  
Vol 23 (3) ◽  
pp. 341-350 ◽  
Author(s):  
Bengamin I. Moat ◽  
Margaret J. Yelland ◽  
Robin W. Pascal ◽  
Anthony F. Molland
Keyword(s):  
Wind Speed ◽  
Charles Darwin ◽  
Reverse Direction ◽  
Cfd Model ◽  
Mean Flow ◽  
Flow Distortion ◽  
Flux Measurements ◽  
Computational Fluid Dynamics Cfd ◽  
The Mean

Abstract The effects of flow distortion created by the ship’s hull and superstructure bias wind speed measurements made from anemometers located on ships. Flow distortion must be taken into account if accurate air–sea flux measurements are to be achieved. Little work has been undertaken to examine the wind speed bias due to flow distortion in wind speed reports from voluntary observing ships (VOS). In this first part of a two-part paper the accuracy of the computational fluid dynamics (CFD) code VECTIS in simulating the airflow over VOS is investigated. Simulations of the airflow over a representation of the bridge of a VOS are compared to in situ wind speed measurements made from six anemometers located above the bridge of the RRS Charles Darwin. The ship’s structure was ideal for reproducing the flow over VOS when the wind is blowing onto either beam. The comparisons showed VECTIS was accurate to within 4% in predicting the wind speed over ships, except in extreme cases such as wake regions or the region close to the bridge top where the flow may be stagnant or reverse direction. The study showed that there was little change in the numerically predicted flow pattern above the bridge with change in Reynolds number between 2 × 105 and 1 × 107. The findings showed that the CFD code VECTIS can reliably be used to determine the mean flow above typical VOS.

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

2016 ◽  
Vol 33 (5) ◽  
pp. 873-890 ◽  
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.

scholarly journals Using Eddy Covariance to Measure the Dependence of Air-Sea CO<sub>2</sub> Exchange Rate on Friction Velocity

2017 ◽  
Author(s):  
Sebastian Landwehr ◽  
Scott D. Miller ◽  
Murray J. Smith ◽  
Thomas G. Bell ◽  
Eric S. Saltzman ◽  
...  
Keyword(s):  
Eddy Covariance ◽  
Friction Velocity ◽  
Air Flow ◽  
Gas Transfer ◽  
Flow Distortion ◽  
Transfer Velocity ◽  
Wind Speeds ◽  
Physically Based ◽  
Flux Measurements ◽  
Ocean Carbon

Abstract. Parameterisation of the air-sea gas transfer velocity of CO2 and other trace gases under open-ocean conditions has been a focus of air-sea interaction research and is required for accurately determining ocean carbon uptake. Ships are the most widely used platform for air-sea flux measurements but the quality of the data can be compromised by air flow distortion and sensor cross-sensitivity effects. Recent improvements in the understanding of these effects have led to enhanced corrections to the shipboard eddy covariance (EC) measurements. Here we present a revised analysis of eddy covariance measurements of air-sea CO2 and momentum fluxes from the Southern Ocean Surface Ocean Aerosol Production study (SOAP). We show that it is possible to significantly reduce the scatter in the EC data and achieve consistency between measurements taken on-station and with the ship underway. The gas transfer velocities from the EC measurements correlate better with the EC friction velocity (u*) than with mean wind speeds derived from shipboard measurements corrected with an air flow distortion model. For the observed range of wind speeds (u10N = 3–23 m s−1), the transfer velocities can be parameterised with a linear fit to u*. The SOAP data are compared to previous gas transfer parameterisations using u10N computed from the EC friction velocity with the drag coefficient from the COARE model. The SOAP results are consistent with previous gas transfer studies, but at high wind speeds they do not support the sharp increase in gas transfer associated with bubble-mediated transfer predicted by physically based models.

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

2015 ◽  
Vol 15 (18) ◽  
pp. 10619-10629 ◽  
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.

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