Abstract
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.
Highlights
Obtaining direct eddy covariance estimates of turbulent air– sea fluxes from ship-mounted sensors is extremely challenging
Computational fluid dynamics (CFD) modelling of the air flow over the James Clark Ross was initially undertaken by Yelland et al (2002) but only for flow on to the bow; we have extended the CFD study for a much wider range of relative wind directions and the results were used to determine direction-dependent corrections to the mean (30 min averaged) relative wind speed and measurement height
Interpolation requires selection of appropriate frequencies to interpolate between, in this case, 0.04 and 0.4 Hz (0.06 and 0.59 in the non-dimensionalised frequency shown in Fig. 2), and is not dependent on a physical variable related to the presumed source of the error
Summary
Obtaining direct eddy covariance estimates of turbulent air– sea fluxes from ship-mounted sensors is extremely challenging. These biases cause distortions of the cospectra between the vertical wind component and gas concentration (Edson et al, 2011) apparent in the cospectra at frequencies associated with the platform motion, and several recent studies have applied motion decorrelation algorithms to remove this signal (Miller et al, 2010; Edson et al, 2011; Blomquist et al, 2014) Such an approach can correct the apparent motionscale bias in the momentum flux but is controversial since, as discussed above, there are circumstances in which a real wave-correlated signal may be expected in the turbulence measurements. CFD modelling of the air flow over the James Clark Ross was initially undertaken by Yelland et al (2002) but only for flow on to the bow; we have extended the CFD study for a much wider range of relative wind directions and the results were used to determine direction-dependent corrections to the mean (30 min averaged) relative wind speed and measurement height. Flux estimates were rejected from the analysis where there was excessive ship manoeuvring, where flux quality control criteria were failed (Foken and Wichura, 1996; Vickers and Mahrt, 1997) and when the air temperature www.atmos-chem-phys.net/15/10619/2015/
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