Effects of Sea-Surface Waves and Ocean Spray on Air-Sea Momentum Fluxes

The effects of sea-surface waves and ocean spray on the marine atmospheric boundary layer(MABL) at different wind speeds and wave ages were investigated. An MABL model was developed that introduces a wave-induced component and spray force to the total surface stress. The theoretical model solution was determined assuming the eddy viscosity coefficient varied linearly with height above the sea surface. The wave-induced component was evaluated using a directional wave spectrum and growth rate. Spray force was described using interactions between ocean-spray droplets and wind-velocity shear. Wind profiles and sea-surface drag coefficients were calculated for low to high wind speeds for wind-generated sea at different wave ages to examine surface-wave and ocean-spray effects on MABL momentum distribution. The theoretical solutions were compared with model solutions neglecting wave-induced stress and/or spray stress. Surface waves strongly affected near-surface wind profiles and sea-surface drag coefficients at low to moderate wind speeds. Drag coefficients and near-surface wind speeds were lower for young than for old waves. At high wind speeds, ocean-spray droplets produced by wind-tearing breaking-wave crests affected the MABL strongly in comparison with surface waves, implying that wave age affects the MABL only negligibly. Low drag coefficients at high wind caused by ocean-spray production increased turbulent stress in the sea-spray generation layer, accelerating near-sea-surface wind. Comparing the analytical drag coefficient values with laboratory measurements and field observations indicated that surface waves and ocean spray significantly affect the MABL at different wind speeds and wave ages.     

Impact of Sea-Spray on the Atmospheric Surface Layer

The feedback effects of sea-spray on the heat and momentum fluxes under equilibrium conditions associated with winds of tropical cyclones are investigated using a one-dimensional coupled sea-spray and atmospheric surface-layer (ASL) model. This model is capable of simulating the microphysical aspects of the evaporation of saline water droplets of various sizes and their dynamic and thermal interaction with the turbulence mixing that is simulated by the Mellor–Yamada 1.5-order closure scheme. Sea-spray droplet generation is described by a state-of-the-art parametrization that predicts the size spectrum of sea-spray droplets for a given surface forcing. The results from a series of simulations indicate the way in which evaporating droplets of various sizes modify the turbulence mixing near the surface, which in turn affects further droplet evaporation. All these results are direct consequences of the effects of sea-spray on the balance of turbulent kinetic energy in the spray-filled surface layer. In particular, the overall impact of sea-spray droplets on the mean wind depends on the wind speed at the level of sea-spray generation. When the wind speed is below 40 m s⁻¹, the droplets are small in size and tend to evaporate substantially and thus cool the spray-filled layer, while for wind speeds above 50 m s⁻¹, the size of the droplets is so large that they do not have enough time to evaporate much before falling back into the sea. The sensible heat carried by the droplets is released to the ambient air, increasing the buoyancy of the surface layer and enhancing the turbulent mixing. The suspension of sea-spray droplets reduces the buoyancy and makes the surface layer more stable, decreasing the friction velocity and the downward turbulent mixing of momentum. The results from the numerical experiments also suggest that, in order not to violate the constant flux assumption critical to the Monin–Obukhov similarity theory, a displacement equal to the mean wave height should be included in the logarithmic profiles of the wind and thermal fields.     

Water vapor flux at the sea surface

Methods and instrumentation for determining the rate of evaporation at the sea surface are reviewed. At experimental sites free of local influences, there is a consensus that the evaporation coefficient in neutral conditions C _EN = 1.2 × 10^–3 at low and moderate wind speeds. Combining this with Businger-Dyer flux-gradient formulas, a parameterization scheme is proposed. Evaporation of spray droplets from breaking waves is expected to cause C _EN to increase at high wind speeds, but no direct observations of this are found. Recently it has become possible to estimate water vapor flux in tropical regions from satellite data, opening the possiblity of studying large-scale evaporative events as a function of both time and space.Visiting scholar, Department of Atmospheric Sciences AK-40, University of Washington, Seattle, Washington, 98195, U.S.A. (until June, 1989).     

Effect of Ocean Spray on Vertical Momentum Transport Under High-Wind Conditions

Two mathematical models are proposed detailing the influence of ocean spray on vertical momentum transport under high-wind conditions associated with a hurricane or severe storm. The first model is based on a turbulent kinetic energy (TKE) equation and accounts for the so-called lubrication effect due to the reduction of turbulence intensity. The second model is based on Monin–Obukhov similarity (MOS) and uses available experimental data. It is demonstrated that the flow acceleration is negligible for wind speeds below a certain critical value due to the fact that the spray volume concentration is low for such speeds. For wind speeds higher than the critical value, the spray concentration rapidly increases, which results in significant flow acceleration. Both models produce qualitatively similar results for all turbulent flow parameters considered. It was found that the MOS-based model tends to predict a noticeably stronger lubrication effect than the TKE-based model, especially for lower wind speeds. The results of model calculations are in very good agreement with available experimental data for the spray production values near the upper bound. It is also shown that neither the value of the turbulent Schmidt number in the TKE-based model nor the choice of a stability profile function affects the spray-laden flow dynamics significantly.     

The Signature of Sea Spray in the Hexos Turbulent Heat Flux Data

The role of sea spray intransferring heat and moisture across the air-sea interface has remained elusive. Some studies have reported that sea spray does not affect the turbulent air-sea heat fluxes for 10-m wind speeds up to at least 25 m s^-1, while others have reported important spray contributions for wind speeds as low as 12 m s^-1. One goal of the HEXOS (Humidity Exchange over the Sea) program was to quantify spray's contribution to the turbulent air-sea heat fluxes, but original analyses of the HEXOS flux data found the spray signal to be too small to be reliably identified amid the scatter in the data. We look at the HEXOS data again in the context of the TOGA-COARE bulk flux algorithm and a sophisticated microphysical spray model. This combination of quality data andstate-of-the-art modelling reveals a distinct spray signature in virtually all HEXOS turbulent heat flux data collected in winds of 15 m s^-1 and higher. Spray effects are most evident in the latent heat flux data, where spray contributes roughly 10% of the total turbulent flux in winds of 10 m s^-1 and between 10 and 40% in winds of 15–18 m s^-1. The spray contribution to the total sensible heat flux is also at least 10% in winds above 15 m s^-1. These results lead to a new, unified parameterization for the turbulent air-sea heat fluxes that should be especially useful in high winds because it acknowledges both the interfacial and spray routes by which the sea exchanges heat and moisture with the atmosphere.     

Comparing Estimates of Turbulence Based on Near-Surface Measurements in the Nocturnal Stable Boundary Layer

Tethered Lifting System (TLS) estimates of the dissipation rate of turbulent kinetic energy (e){(\varepsilon)} are reasonably well correlated with concurrent measurements of vertical velocity variance (s_w²){(\sigma_{w}^{2})} obtained from sonic anemometers located on a nearby 60-m tower during the CASES-99 field experiment. Additional results in the first 100 m of the nocturnal stable boundary layer confirm our earlier claim that the presence of weak but persistent background turbulence exists even during the most stable atmospheric conditions, where e{\varepsilon} can exhibit values as low as 10⁻⁷ m² s⁻³. We also present a set of empirical equations that incorporates TLS measurements of temperature, horizontal wind speed, and e{\varepsilon} to provide a proxy measurement for s_w²{\sigma_{w}^{2}} at altitudes higher than tower heights.     

An analysis of the turbulent structure in the unstable surface layer nearby a shelter belt

An analysis was performed of the turbulent data obtained from Yucheng experimental station in the Shandong Province in 1984. [t is shown that at variant wind speed, the spectra of streamwise velocity remain similar and the intensity of wind fluctuations is proportional to wind speed in the downwind area of shelter belt. Therefore, we may decide the similarity of wind fluctuations by a speed scale and a length scale which is not correlated with stability, σu /V0 = F(X / H). The -5/3 power range of temperature spectra extends to lower frequency. The variation of ratio σ0 /T. with stability becomes σ0 / T . = C(X / H)( - Z / L)-1/3 . There is not such an extension of -5 / 3 power range in the humidity spectra.     

Estimation of the Exchange Coefficient of Heat During Low Wind Convective Conditions

For the first time, the exchange coefficient of heat C_H has been estimated from eddy correlation of velocity and virtual temperature fluctuations using sonic anemometer measurements made at low wind speeds over the monsoon land atJodhpur (26°18' N, 73°04' E), a semi arid station. It shows strong dependence on wind speed, increasing rapidly with decreasing wind speed, and scales according to a power law C_H = 0.025U₁₀ ^-0.7 (where U₁₀ is the mean wind speed at 10-m height). A similar but more rapid increase in the drag coefficient C_Dhas already been reported in an earlier study. Low winds (<4 m s^-1) are associated with both near neutral and strong unstable situations. It is noted that C_H increases with increasing instability. The present observations best describe a low wind convective regime as revealed in the scaling behaviour of drag, sensible heat flux and the non-dimensional temperature gradient. Neutral drag and heat cofficients,corrected using Monin–Obukhov (M–O) theory, show a more uniform behaviour at low wind speeds in convective conditions, when compared with the observed coefficients discussed in a coming paper.At low wind convective conditions, M-O theory is unable to capture the observed linear dependence of drag on wind speed, unlike during forced convections. The non-dimensional shear inferred from the present data shows noticeable deviations from Businger's formulation, a forced convection similarity. Heat flux is insensitive to drag associated with weak winds superposed on true free convection. With heat flux as the primary variable, definition of new velocity scales leads to a new drag parameterization scheme at low wind speeds during convective conditionsdiscussed in a coming paper.     

Numerical Investigations of Mean Winds Within Canopies of Regularly Arrayed Cubical Buildings Under Neutral Stability Conditions

Recently, several attempts have been made to model the wind velocity in an urban canopy in order to accurately predict the mixing and transport of momentum, heat, and pollutants within and above the canopy on an urban scale. For this purpose, unverified assumptions made by Macdonald (Boundary-Layer Meteorol 97:25–45, 2000) to develop a model for the profile of the mean wind velocity within an urban canopy have been used. In the present study, in order to provide foundations for improving the urban canopy models, the properties of the spatially-averaged mean quantities used to make these assumptions have been investigated by performing large-eddy simulations (LES) of the airflow around square and staggered arrays of cubical blocks with the following plan area densities: λ _p = 0.05, 0.11, 0.16, 0.20, 0.25, and 0.33. The LES results confirm that the discrepancy between the spatial average of wind velocity and Macdonald’s five-point average of wind velocity can be large in both types of arrays for large λ _p . It is also confirmed that Prandtl’s mixing length varies significantly with height within the canopy, contrary to Macdonald’s assumption for both types of arrays and for both small and large λ _p . On the other hand, in accordance with Macdonald’s assumption, the sectional drag coefficient is found to be almost constant with height except in the case of staggered arrays with high λ _p .     

Characteristic Study of the Boundary Layer Parameters over the Arabian Sea and the Bay of Bengal Using the QuikSCAT Dataset

The marine atmospheric boundary layer （MABL） plays a vital role in the transport of momentum and heat from the surface of the ocean into the atmosphere. A detailed study on the MABL characteristics was carried out using high-resolution surface-wind data as measured by the QuikSCAT （Quick scatterometer） satellite. Spatial variations in the surface wind, frictional velocity, roughness parameter and drag coefficient for the different seasons were studied. The surface wind was strong during the southwest monsoon season due to the modulation induced by the Low Level Jetstream. The drag coefficient was larger during this season, due to the strong winds and was lower during the winter months. The spatial variations in the frictional velocity over the seas was small during the post-monsoon season （-0.2 m s^-1）. The maximum spatial variation in the frictional velocity was found over the south Arabian Sea （0.3 to 0.5 m s^-1） during the southwest monsoon period, followed by the pre-monsoon over the Bay of Bengal （0.1 to 0.25 m s^-1）. The mean wind-stress curl during the winter was positive over the equatorial region, with a maximum value of 1.5×10^-7 N m^-3, but on either side of the equatorial belt, a negative wind-stress curl dominated. The area average of the frictional velocity and drag coefficient over the Arabian Sea and Bay of Bengal were also studied. The values of frictional velocity shows a variability that is similar to the intraseasonal oscillation （ISO） and this was confirmed via wavelet analysis. In the case of the drag coefficient, the prominent oscillations were ISO and quasi-biweekly mode （QBM）. The interrelationship between the drag coefficient and the frictional velocity with wind speed in both the Arabian Sea and the Bay of Bengal was also studied.     

Air–Sea \mathrm{CO}_{2} Gas Transfer Velocity in a Shallow Estuary

The air–sea transfer velocity of $\mathrm{CO}_{2}\, (k_{\mathrm{CO}_{2}})$ was investigated in a shallow estuary in March to July 2012, using eddy-covariance measurements of $\mathrm{CO}_{2}$ fluxes and measured air–sea $\mathrm{CO}_{2}$ partial-pressure differences. A data evaluation method that eliminates data by nine rejection criteria in order to heighten parametrization certainty is proposed. We tested the data evaluation method by comparing two datasets: one derived using quality criteria related solely to the eddy-covariance method, and the other derived using quality criteria based on both eddy-covariance and cospectral peak methods. The best parametrization of transfer velocity normalized to a Schmidt number of 600 $(k_{600})$ was determined to be: $k_{600} = 0.3\,{U_{10}}^{2.5}$ where $U_{10}$ is the wind speed in m $\mathrm{s}^{-1}$ at 10 m; $k_{600}$ is based on $\mathrm{CO}_{2}$ fluxes calculated by the eddy-covariance method and including the cospectral peak method criteria. At low wind speeds, the transfer velocity in the shallow water estuary was lower than in other coastal waters, possibly a symptom of low tidal amplitude leading to low intensity water turbulence. High transfer velocities were recorded above wind speeds of 5 m $\mathrm{s}^{-1}$ , believed to be caused by early-breaking waves and the large fetch (6.5 km) of the estuary. These findings indicate that turbulence in both air and water influences the transfer velocity.     

On the impact of thermal stability on some rough flow effects over mobile surfaces

Calculations are made of the effects of thermal stability under a range of conditions, over the sea and land, on the physical factors (including the critical wind speed) affecting dust-storm generation, snow drift, and rough sea conditions. The computational procedure involves the surface friction velocity, u _*, and its relation with the aerodynamic roughness over aerodynamically rough, mobile surfaces. The results indicated that even at relatively high wind speeds, thermal effects under extreme advection situations may be significant, particularly for those properties of the agitated surface dependent on u _* ³ and u _* ⁴.     

Airflow patterns in a small subalpine basin

Summary A study of mean wind speeds and directions has been completed in the Snowy Range of Southern Wyoming, U.S.A. It was conducted in a subalpine ecosystem at an altitude of 3 200 m to 3 400 m above sea level during the summers of 1988 and 1989. Indexes of deformation and axes of asymmetry due to wind shaping of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) are related to wind speeds and directions on a 100 m × 100 m grid spacing over the 300 ha research site. Isotach and airflow patterns are drawn to represent climatological near-ground-level winds.A statistical analysis of the wind data and deformation indexes indicates that the indexes estimated independently by three of the authors were not significantly different at the F_0.025 level. Two methods of calculating wind speeds were applied. At lower mean wind speeds in Engelmann spruce, results from the Wade-Hewson method were not significantly different from the Griggs-Putnam method at the F_0.025 level. In slightly higher wind speeds in subalpine fir, the Wade-Hewson method produced significantly lower wind speeds than the Griggs-Putnam method.With 3 Figures     

The effect of idealized water waves on the turbulence structure and kinetic energy budgets in the overlying airflow

The influence of an idealized moving wavy surface on the overlying airflow is investigated using direct numerical simulations (DNS). In the present simulations, the bulk Reynolds number is Re = 8000 (; where U₀ is the forcing velocity of the flow, h the height of the domain and v the kinematic viscosity) and the phase speed of the imposed waves relative to the friction velocity, i.e., the wave age varies from very slow to fast waves. The wave signal is clearly present in the airflow up to at least 0.15λ (where λ is the wave length) and is present up to higher levels for faster waves. In the kinetic energy budgets, pressure transport is mainly of importance for slow waves. For fast waves, viscous transport and turbulent transport dominate near the surface. Kinetic energy budgets for the wave and turbulent perturbations show a non-negligible transport of turbulent kinetic energy directed from turbulence to the wave perturbation in the airflow. The wave-turbulent energy transport depends on the size, tilt, and phase of the wave-induced part of the turbulent Reynolds stresses.According to the DNS data, slow waves are more efficient in generating isotropic turbulence than fast waves.Despite the differences in wave-shape as well as in Reynolds number between the idealized direct numerical simulations and the atmosphere, there are intriguing similarities in the turbulence structure. Important information about the turbulence above waves in the atmosphere can be obtained from DNS—the data must, however, be interpreted with care.     

A Note on the Drag of the Sea Surface at Hurricane Winds

Based on the solution of the turbulent kinetic energy balance equation for the airflow in the regime of limited saturation by suspended sea-spray droplets, some experimental evidence, and simple arguments, a resistance law of the sea surface at hurricane winds is derived. It predicts the reduction of the drag coefficient for the wind speed exceeding hurricane values of 30–40 m s ^-1 in agreement with field data.     

Investigation of Turbulence Parametrization Schemes with Reference to the Atmospheric Boundary Layer Over the Aegean Sea During Etesian Winds

The spatial structure of the marine atmospheric boundary layer (MABL) over the Aegean Sea is investigated using the Weather Research and Forecasting (WRF) mesoscale model. Two ‘first-order’ non-local and five ‘1.5-order’ local planetary boundary-layer (PBL) parametrization schemes are used. The predictions from the WRF model are evaluated against airborne observations obtained by the UK Facility for Airborne Atmospheric Measurements BAe-14 research aircraft during the Aegean-GAME field campaign. Statistical analysis shows good agreement between measurements and simulations especially at low altitude. Despite the differences between the predicted and measured wind speeds, they reach an agreement index of 0.76. The simulated wind-speed fields close to the surface differ substantially among the schemes (maximum values range from 13 to \(18\hbox { m s}^{-1}\) at 150-m height), but the differences become marginal at higher levels. In contrast, all schemes show similar spatial variation patterns in potential temperature fields. A warmer (1–2 K) and drier (2–3\(\hbox { g kg}^{-1})\) layer than is observed, is predicted by almost all schemes under stable conditions (eastern Aegean Sea), whereas a cooler (up to 2 K) and moister (1–2\(\hbox { g kg}^{-1})\) layer is simulated under near-neutral to nearly unstable conditions (western Aegean Sea). Almost all schemes reproduce the vertical structure of the PBL and the shallow MABL (up to 300 m) well, including the low-level jet in the eastern Aegean Sea, with non-local schemes being closer to observations. The simulated PBL depths diverge (up to 500 m) due to the different criteria applied by the schemes for their calculation. Under stable conditions, the observed MABL depth corresponds to the height above the sea surface where the simulated eddy viscosity reaches a minimum; under neutral to slightly unstable conditions this is close to the top of the simulated entrainment layer. The observed sensible heat fluxes vary from ?40 to \(25\hbox { W m}^{-2}\), while the simulated fluxes range from ?40 to \(40\hbox { W m}^{-2}\); however, all of the schemes’ predictions are close to the observations under unstable conditions. Finally, all schemes overestimate the friction velocity, although the simulated range (from 0.2 to \(0.5\hbox { m s}^{-1})\) is narrower than that observed (from 0.1 to \(0.7\hbox { m s}^{-1})\).     

浪花和飞沫水滴在海面大风边界层中的垂直传输

对我们在南海海域建立的大气边界层观测站的资料进行分析表明,在冷涌和热带气旋（包括强台风）过境的大风期间,在边界层底层10 min平均的水平风速u基本不随高度而变,甚至大都伴随有明显的上升气流w。而且风场脉动中含有强相干性的阵风扰动（ v _g,频率位于1/60~1/600 Hz频段）,以及近于随机性的高频湍流脉动（ v _t,频率大于1/60 Hz）,它们的特性以及w都可以很好地用水平风速u来参数化表示。取实测的（u,w）和脉动 v '= v _g+ v _t,或取实测的u与参数化的 v _g、 v _t和w,应用拉格朗日随机模式作数值模拟,结果表明:由破头浪发射出来的浪花和飞沫水滴（半径r_p为10~500 μm）有相当大的一部分可以飞离大气底层而进入100 m高以上的大气中,继而对进入大气中的海盐气溶胶通量有重要贡献,不可以被忽略。在水滴的垂直传输过程中,阵风扰动起了极重要的作用,而在w>0且较显著时w更起重要作用。我们对上扬率（可上升至100 m以上高度的水滴数与由海面发射出的水滴数之比）作出了初步的参数化公式,有很高的精度,主要的参量是无量纲量u2/（r_qg）,其中r_p和g分别是水滴半径和重力加速度。     

Generation and atmospheric heat exchange of coastal polynyas in the Weddell Sea

The forcing mechanisms for Antarctic coastal polynyas and the thermodynamic effects of existing polynyas are studied by means of an air-sea-ice interaction experiment in the Weddell Sea in October and November 1986.Coastal polynyas develop in close relationship to the ice motion and form most rapidly with offshore ice motion. Narrow polynyas occur frequently on the lee side of headlands and with strong curvature of the coastline. From the momentum balance of drifting sea ice, a forcing diagram is constructed, which relates ice motion to the surface-layer wind vector v _z and to the geostrophic ocean current vector c _g . In agreement with the data, wind forcing dominates when the wind speed at a height of 3 m exceeds the geostrophic current velocity by a factor of at least 33. This condition within the ocean regime of the Antarctic coastal current usually is fulfilled for wind speeds above 5 m/s at a height of 3 m.Based on a nonlinear parameter estimation technique, optimum parameters for free ice drift are calculated. Including a drift dependent geostrophic current in the ice/water drag yields a maximum of explained variance (91%) of ice velocity.The turbulent heat exchange between sea ice and polynya surfaces is derived from surface-layer wind and temperature data, from temperature changes of the air mass along its trajectory and from an application of the resistance laws for the atmospheric PBL. The turbulent heat flux averaged over all randomly distributed observations in coastal polynyas is 143 W/m². This value is significantly different over pack ice and shelf ice surfaces, where downward fluxes prevail. The large variances of turbulent fluxes can be explained by variable wind speeds and air temperatures. The heat fluxes are also affected by cloud feedback processes and vary in time due to the formation of new ice at the polynya surface.Maximum turbulent fluxes of more than 400 W/m² result from strong winds and low air temperatures. The heat exchange is similarly intense in a narrow zone close to the ice front, when under weak wind conditions, a local circulation develops and cold air associated with strong surface inversions over the shelf ice is heated above the open water.     

A numerical study of the marine stratocumulus cloud layer

A one-dimensional grid-level model including longwave radiative transfer and a level-4 second-order turbulent transfer closure which contains prognostic equations for turbulent quantities, is used to study the physics and dynamics of inversion-capped marine stratocumulus clouds.A set of numerical experiments had been performed to examined the role of sea surface temperature, large-scale vertical velocity, wind speed, and vertical wind shear in the formation and the structure of low-level clouds. For a given sea surface and geostrophic wind speed, stratocumulus clouds can grow higher with smaller large-scale subsidence as less dry air entrains into the cloud. Clouds grow higher with higher sea surface temperature for a given geostrophic wind speed and large-scale subsidence as a result of enhanced moist convection. In high wind speeds, the entire cloud deck is lifted up because of larger surface energy flux. In the budget studies of the turbulent kinetic energy (TKE), the buoyancy term is a major source term when the wind speed and the vertical shear are small across the inversion top. When the wind speed and the vertical wind shear across the inversion top become large, the mixed layer is decoupled into a cloud and a subcloud layer. In the TKE budget studies, the shear generation term becomes an important term in the budgets of the TKE and the variance of vertical velocity.     

Tilt errors and precipitation effects in trivane measurements of turbulent fluxes over open water

For 390 ten-minute samples of turbulent flux, made with a trivane above a lake, the vertical alignment is determined within 0.1 ° through azimuth-dependent averaging. One degree of instrumental misalignment is found to produce an average tilt error of 9 ± 4% for momentum flux, and 4 ± 2% for heat flux. The tilt error in the vertical momentum flux depends mainly ons _u/u_*, and cannot be much diminished with impunity by high-pass pre-filtering of the turbulence signals. The effects of rain on trivane measurements of vertical velocity are shown to be negligible at high wind speeds, and adaptable to correction in any case.The normalized vertical velocity variance,s _w/u_*, appears to be proportional to the square root ofz/L for unstable stratification. For a wind speed range of 2 to 15 m s^–1, the eddy correlation stresses measured at 4- and 8-m heights can be reasonably well estimated by using a constant drag coefficientC _d=1.3 X 10^-3, while cup anemometer profile measurements give an overestimate of eddy stress at high wind speeds. A good stress estimate is also obtained from the elevation variance; it is suggested that trivane measurement of this variance might be made from a mobile platform, e.g., a moderately stabilized spar buoy.