A Multisensor Comparison of Ocean Wave Frequency Spectra from a Research Vessel during the Southern Ocean Gas Exchange Experiment

Obtaining accurate measurements of wave statistics from research vessels remains a challenge due to the platform motion. One principal correction is the removal of ship heave and Doppler effects from point measurements. Here, open-ocean wave measurements were collected using a laser altimeter, a Doppler radar microwave sensor, a radar-based system, and inertial measurement units. Multiple instruments were deployed to capture the low- and high-frequency sea surface displacements. Doppler and motion correction algorithms were applied to obtain a full 1D (0.035–1.3 ± 0.2 Hz) wave spectrum. The radar-based system combined with the laser altimeter provided the optimal low- and high-frequency combination, producing a frequency spectrum in the range from 0.035 to 1.2 Hz for cruising speeds ≤3 m s−1 with a spectral rolloff of f−4 Hz and noise floor of −20/−30 dB. While on station, the significant wave height estimates were comparable within 10%–15% among instrumentation. Discrepancies in the total energy and in the spectral shape between instruments arise when the ship is in motion. These differences can be quantified using the spectral behavior of the measurements, accounting for aliasing and Doppler corrections. The inertial sensors provided information on the amplitude of the ship’s modulation transfer function, which was estimated to be ~1.3 ± 0.2 while on station and increased while underway [2.1 at ship-over-ground (SOG) speed; 4.3 m s−1]. The correction scheme presented here is adequate for measurements collected at cruising speeds of 3 m s−1 or less. At speeds greater than 5 m s−1, the motion and Doppler corrections are not sufficient to correct the observed spectral degradation

Optical measurements of small deeply penetrating bubble populations generated by breaking waves in the Southern Ocean

Bubble size distributions ranging from 0.5 to 125 μm radius were measured optically during high winds of 13 m s−1 and large-scale wave breaking as part of the Southern Ocean Gas Exchange Experiment. Very small bubbles with radii less than 60 µm were measured at 6–9 m depth using optical measurements of the near-forward volume scattering function and critical scattering angle for bubbles (∼80°). The bubble size distributions generally followed a power law distribution with mean slope values ranging from 3.6 to 4.6. The steeper slopes measured here were consistent with what would be expected near the base of the bubble plume. Bubbles, likely stabilized with organic coatings, were present for time periods on the order of 10–100 s at depths of 6–9 m. Here, relatively young seas, with an inverse wave age of approximately 0.88 and shorter characteristic wave scales, produced lower bubble concentrations, shallower bubble penetration depths, and steep bubble size distribution slopes. Conversely, older seas, with an inverse wave age of 0.70 and longer characteristic wave scales, produced relatively higher bubble concentrations penetrating to 15 m depth, larger bubble sizes, and shallower bubble size distribution slopes. When extrapolated to 4 m depth using a previously published bubble size distribution, our estimates suggest that the deeply penetrating small bubbles measured in the Southern Ocean supplied ∼36% of the total void fraction and likely contributed to the transfer and supersaturation of low-solubility gases

The Wave Boundary Layer Over the Open Ocean and the Implications to Air-Sea Interaction

Wave field and atmospheric observations during the Southern Ocean Gas Exchange experiment 2008 were used to explore air-sea boundary layer dynamics. The closure of a momentum budget at the air-sea interface allows the selection and tuning of a wave growth parameter consistent with the observed conditions. An energy balance between the atmospheric energy input and the observed wind-wave spectral energy is posed based on the turbulent kinetic energy budget. The energy input is defined as the rate of work done by the wave-induced stress over the wind velocity profile. Wave induced perturbations on the airflow are modeled by an exponential decay function with a variable dimensional decay rate (A m-1). Wave-induced perturbations are incorporated into the atmospheric input term to account for the wind-wave coupling. The decay rate is tuned iteratively by minimizing the difference between the input and the wind-wave spectral energy. Under weaker forcing the model works within 40-45%. It is hypothesized, that this is due to long-wave modulation and an upward ocean–atmosphere momentum flux. Under stronger forcing (i.e. 0.4 \u3c u* \u3c 0.9 m s-1) results are within 10-20% predicting progressively slower decay rates (A ~ 0.5 ± 0.4 m-1). This suggests that longer waves support the wave-induced momentum flux, extending the depth of the wave boundary layer to an average height of 2 m inducing stronger perturbations on the airflow. Under weaker forcing the model suggests that wind and waves become uncoupled exhibiting a shallower wave boundary layer

Air–Sea Interaction in the Southern Ocean: Exploring the Height of the Wave Boundary Layer at the Air–Sea Interface

We investigate the momentum and energy exchange across the wave boundary layer (WBL). Directly at the air–sea interface, we test three wave-growth parametrizations by comparing estimates of the wave-induced momentum flux derived from wave spectra with direct covariance estimates of the momentum flux. An exponential decay is used to describe the vertical structure of the wave-induced momentum in the atmospheric WBL through use of a decay rate, a function of the dimensionless decay rate and wavenumber (A = α k). The decay rate is varied to minimize the difference between the energy extracted from the WBL and the energy flux computed from wave spectra using our preferred wave-growth parametrization. For wave ages (i.e. the peak phase speed to atmospheric friction velocity ratio) in the range 15<cp/u∗<35 we are able to balance these two estimates to within 10%. The decay rate is used to approximate the WBL height as the height to which the wave-induced flux is 0.1 of its surface value and the WBL height determined this way is found to be between 1–3 m. Finally, we define an effective phase speed with which to parametrize the energy flux for comparison with earlier work, which we ultimately attempt to parametrize as a function of wind forcing

Shipboard wave measurements in the Southern Ocean

Surface wave measurements from ships pose difficulties because of motion contamination. Cifuentes-Lorenzen et al. analyzed laser altimeter and marine X-band radar (MR) wave measurements from the Southern Ocean Gas Exchange Experiment (SOGasEx). They found that wave measurements from both sensors deteriorate precipitously at ship speeds 3 m s−1. This study demonstrates that MR can yield accurate wave frequency–direction spectra independent of ship motion. It is based on the same shipborne SOGasEx wave data but uses the MR wave retrieval method proposed by Lund et al. and a novel empirical transfer function (ETF). The ETF eliminates biases in the MR wave spectra by redistributing energy from low to high frequencies. The resulting MR wave frequency–direction spectra are shown to agree well with laser altimeter wave frequency spectra from times when the ship was near stationary and with WAVEWATCH III (WW3) model wave parameters over the full study period

Novel methods for optically measuring whitecaps under natural wave-breaking conditions in the Southern Ocean

Traditional methods for measuring whitecap coverage using digital video systems mounted to measure a large footprint can miss features that do not produce a high enough contrast to the background. Here, a method for accurately measuring the fractional coverage, intensity, and decay time of whitecaps using above-water radiometry is presented. The methodology was developed using data collected in the Southern Ocean under a wide range of wind and wave conditions. Whitecap quantities were obtained by employing a magnitude threshold based on the interquartile range of the radiance or reflectance signal from a single channel. Breaking intensity and decay time were produced from the integration of and the exponential fit to radiance or reflectance over the lifetime of the whitecap. When using the lowest magnitude threshold possible, radiometric fractional whitecap coverage retrievals were consistently higher than fractional coverage from high-resolution digital images, perhaps because the radiometer captures more of the decaying bubble plume area that is difficult to detect with photography. Radiometrically obtained whitecap measurements are presented in the context of concurrently measured meteorological (e.g., wind speed) and oceanographic (e.g., wave) data. The optimal fit of the radiometrically estimated whitecap coverage to the instantaneous wind speed, determined using robust linear least squares, showed a near-cubic dependence. Increasing the magnitude threshold for whitecap detection from 2 to 4 times the interquartile range produced a wind speed–whitecap relationship most comparable to the concurrently collected fractional coverage from digital imagery and previously published wind speed–whitecap parameterizations