Design of a tri-axial thrust anemometer for measurement of atmospheric turbulence over water

The BIO Mark 8 thrust anemometer measures the drag of the wind on a perforated table tennis ball mounted on a vertical beam. The tri-axial displacement of the beam is sensed by eddy current proximity sensors. This anemometer has a flat frequency response from 0 to 10 Hz and can measure wind from 0 to 28 m s⁻¹ at temperature from − 19 to + 28°C. It is designed for remote operation for extended periods of time such as on stable towers at sea.     

The statistical distribution of nearbed wave orbital velocity in intermediate coastal water depth

The statistical distribution of wave orbital velocity in intermediate coastal water depth has been quantitatively determined from the comprehensive field velocity data collected near the seabed in this study. Two ocean ADV current meters, which were mounted at 0.5 m above the seabed on two separate stainless steel tripods sitting on the seabed, were used to measure instantaneous water particle velocities at a 2 Hz sampling rate for 17.07 min every hour in two coastal water depths of 11 m and 23 m in nine field deployments over a period of 2 years. The zero-crossing method is applied to analyse the field velocity data collected in each field deployment to obtain a large sample of wave orbital velocity amplitudes of individual waves. Based on the collected field velocity data, it is found that the histogram of instantaneous wave orbital velocities perfectly follows the Gaussian distribution as commonly assumed, while the histogram of wave orbital velocity amplitudes is less accurately described by the Rayleigh distribution than the modified Rayleigh and the Weibull distribution. It is also found that large orbital velocity amplitudes are generally overestimated by the Rayleigh distribution, but well predicted by the modified Rayleigh and the Weibull distribution. The expected value of maximum orbital velocity in a velocity record of finite size is also derived from the three distributions and found to agree well with the present field data.     

HF radar observations of Arctic pack-ice breakup

This paper describes the first reported high-resolution remote measurements of sea-ice velocities during the summer Arctic pack-ice breakup, made with a high-frequency (HF) radar system (CODAR, for Coastal Ocean Dynamics Applications Radar) located on Cross Island, Alaska. Each 36-min observation also gives the positions of the ice edge, the moving ice, and the open water, with an azimuthal and distance resolution of5degand 1.2 km, respectively, to a range of 15 km. The statistical uncertainties in speed are typically 2-4 cm/s. The ice breakup was observed over a two-day period starting with low ice velocity and no open water and ending with ice and current velocities of approximately 40 cm/s. The position of the ice edge is verified by a simultaneous synthetic aperture radar (SAR) image. To compare the ice, current, and wind velocities, a uniform velocity model was fitted to the measurements of radial velocity. The speed of both ice and current under free drift conditions was found to lie between 2 and 5 percent of the wind speed and the direction within20degof the wind direction.     

沿海市区近地面大气层风、温、湿湍流特征的观测和分析

用三向风速仪和高灵敏温湿度脉动观测装置,测量沿海城市上空大气表面层风速、温度和湿度脉动。并用涡动相关法计算了大气表面层动量、热量和水汽湍流通量,给出了复杂地形上各物理量的谱特征,讨论了各湍流通量的特性。结果指出,风速、温度和湿度谱密度分布基本上都服从—5/3律。由于建筑物的影响,在f≥0.2Hz频段内拢动能量明显偏大。动量和热量输送主要由f<0.2Hz的扰动完成的,高频扰动的贡献可以忽略不计。     

Average velocity field of the air flow over the water surface in a laboratory modeling of storm and hurricane conditions in the ocean

Laboratory experiments on studying the structure of the turbulent air boundary layer over waves were carried out at the Wind-Wave Channel of the Institute of Applied Physics, Russian Academy of Sciences (IAP RAS), in conditions modeling the near-water boundary layer of the atmosphere under strong and hurricane winds and the equivalent wind velocities from 10 to 48 m/s at the standard height of 10 m. A modified technique of Particle Image Velocimetry (PIV) was used to obtain turbulent pulsation averaged velocity fields of the air flow over the water surface curved by a wave and average profiles of the wind velocity. The measurements showed that the logarithmic part of the velocity profile of the air flow in the channel was observed in the immediate vicinity from the water surface (at a distance of 30 mm) and could be detected only using remote methods (PIV). According to the measured velocity profiles, dependences of aerodynamic drag factors of the water surface on the wind velocity at a height of 10 m were retrieved; they were compared with results of contact measurements carried out earlier on the same setup. It is shown that they agree with an accuracy of up to 20%; at moderate and strong wind velocities the coincidence falls within the experimental accuracy.     

Measurements of the atmospheric turbulence in the coastal zone of the sea during weak wind from a mountainous coast

Results of measurements of the atmospheric turbulence in the layer between 1.5 and 21 m above sea level and the drag coefficient of the sea surface as the wind blows from a 4-km-long mountainous slope with a mean inclination of 11° are presented. The measurements of wind-speed profiles and its fluctuations at several levels, waves, and the main meteorological parameters were carried out in autumn 2005 and 2008 from a stationary platform located in the Black Sea at a distance of approximately 1 km from the southern coast of Crimea. It is shown that during weak synoptic wind a low-level wind jet develops at night over the sea with a maximum velocity up to 5–6 m/s at a level of approximately 6 m over the sea induced by the katabatic wind over the coastal slope. According to the approximate estimates, the horizontal scale of the low-level jet can reach a few tens of kilometers. This flow is characterized by the dissipation rate of the turbulence energy independent of height and low-frequency velocity fluctuations related to the gravity waves and advection of turbulence from the coast. It is shown that the lower part of the boundary layer (up to a height of 3 m) is adjusted to the sea-surface roughness. The dependencies of the drag coefficient on the wind speed or wave age are steadier than in the data for the open sea. However, the age of the waves is not a universal parameter at long and short fetches.     

Measurements of the water-following capability of holey-sock and TRISTAR drifters

Since 1985, a number of measurements have been made in deep water to determine the water-following characteristics of mixed layer drifters with both holey-sock and TRISTAR drogues at 15 m depth. The measurements were done by attaching two neutrally buoyant vector measuring current meters (VMCMs) to the top and the bottom of the drogues and deploying the drifters in different wind and upper ocean shear conditions for periods of 2–4 h. The average velocity of the VMCM records was taken to be a quantitative measure of the slip of the drogue through the water, observed to be 0.5-3.5 cm s⁻¹. The most important hydrodynamic design parameter which influenced the slip of the drogue was the ratio of the drag area of the drogue to the sum of the drag areas of the tether and surface floats: the drag area ratio R. The most important environmental parameters which affected the slip were the wind and the measured velocity difference across the vertical extent of the drogue. A model of the vector slip as a function of R, vector wind and velocity difference across the drogue was developed and a least squares fit accounts for 85% of the variance of the slip measurements. These measurements indicated that to reduce the wind produced slip below 1 cm s⁻¹ in 10 m s⁻¹ wind speed, R > 40. Conversely, if the daily average wind is known to 5 m s⁻¹ accuracy, the displacement of the R = 40 drifter can be corrected to an accuracy of 0.5 km day⁻¹.     

A dual sensor device to estimate fluid flow velocity at diffuse hydrothermal vents

Numerous attempts have been made over the last thirty years to estimate fluid flow rates at hydrothermal vents, either at the exit of black smoker chimneys or within diffuse flow areas. In this study, we combine two methods to accurately estimate fluid flow velocities at diffuse flow areas. While the first method uses a hot film anemometer that performs high-frequency measurements, the second allows a relatively rapid assessment of fluid flow velocity through video imagery and provides in situ data to calibrate the sensor. Measurements of flow velocities on hydrothermal diffuse flow areas were obtained on the Mid-Atlantic Ridge (MAR). They range from 1.1 to 4.9 mm/s at the substratum level, in low-temperature (4.5–16.4 °C) diffuse flow areas from the Tour Eiffel sulfide edifice. A strong correlation was observed between fluid flow velocities and temperature, supporting the possible use of temperature as a proxy to estimate the flow rates in diffuse flow areas where such a simple linear flow/temperature relation is shown to dominate.     

Comparing satellite and meteorological data on wind velocity over the Black Sea

Wind-velocity data obtained from in situ measurements at the Golitsyno-4 marine stationary platform have been compared with QuikSCAT scatterometer data; NCEP, MERRA, and ERA-Interim global reanalyses and MM5 regional atmospheric reanalysis. In order to adjust wind velocity measured at a height of 37 m above the sea surface to a standard height of 10 m with stratification taken into account, the Monin–Obukhov theory and regional atmospheric reanalysis data are used. Data obtained with the QuikSCAT scatterometer most adequately describe the real variability of wind over the Black Sea. Errors in reanalysis data are not high either: the regression coefficient varies from 0.98 to 1.06, the rms deviation of the velocity amplitude varies from 1.90 to 2.24 m/s, and the rms deviation of the direction angle varies from 26° to 36°. Errors in determining the velocity and direction of wind depend on its amplitude: under weak winds (<3 m/s), the velocity of wind is overestimated and errors significantly increase in determining its direction; under strong winds (>12 m/s), its velocity is underestimated. The influence of these errors on both spatial and temporal estimates of the characteristics of wind over the Black Sea is briefly considered.     

Three-dimensional circulation in the NW Africa coastal transition zone

High-resolution data collected southeast of the Canary Islands during late winter 2006 are analyzed to describe the hydrography and three-dimensional circulation in the coastal transition zone off NW Africa. The data are optimally interpolated over a regular grid, the geostrophic velocity field is calculated and the Q-vector formulation of the omega equation is used to compute the quasi-geostrophic (QG) mesoscale vertical velocity. The coastal transition zone is divided into upwelling, frontal and offshore regions with distinct physical and dynamic characteristics. The upwelling region is characterized by cold and weakly stratified waters flowing towards the equator, with a poleward undercurrent of approximately 0.05 m s⁻¹ over the continental slope. The frontal region exhibits a southwestward baroclinic jet associated with cross-shore raising isopycnals; the jet transport is close to 1 Sv, with maximum velocities of 0.18 m s⁻¹ at surface decreasing to 0.05 m s⁻¹ at 300 db. Vertical sections across the frontal region show the presence of deep eddies probably generated by the topographic blocking of the islands to the southward current, as well as much shallower eddies that likely have arisen as instabilities of the baroclinic upwelling jet. The QG mesoscale vertical velocity field is patchy, estimated to range from −18 to 12 m day⁻¹, with the largest absolute values corresponding to an anticyclonic eddy located south of Fuerteventura Island. These values are significantly larger than estimates for other vertical velocities: diapycnal vertical velocities associated with mixing in the frontal region (a few meters per day), and wind-induced vertical velocities (non-linear Ekman pumping arising from the interaction between the wind stress and the background vorticity, maximum values of a few meters per day; linear Ekman pumping due to the divergence of Ekman transport, a fraction of a meter per day; or the coastal constraint in the upwelling region, about 0.7 m day⁻¹). However, the patchiness in both the QG mesoscale vertical velocity and the non-linear Ekman pumping velocity cause their integrated vertical transports to be one order of magnitude smaller than either coastal Ekman transport (0.08 Sv), integrated linear Ekman pumping (−0.05 Sv) or diapycnal transfer (about 0.1–0.2 Sv). The pattern of the near-surface fluorescence field is a good indicator of these different contributions, with large homogeneous values in the coastal upwelling region and a patchy structure associated with the offshore mesoscale structures.     

A new approach for the determination of the drag coefficient from the upper ocean response to a tropical cyclone: a feasibility study

We seek to determine if a small number of measurements of upper ocean temperature and currents can be used to make estimates of the drag coefficient that have a smaller range of uncertainty than previously found. We adopt a numerical approach using forward models of the ocean’s response to a tropical cyclone, whereby the probability density function of drag coefficient values as a function of wind speed that results from adding realistic levels of noise to the simulated ocean response variables is sought. Allowing the drag coefficient two parameters of freedom, namely the values at 35 and at 45 m/s, we found that the uncertainty in the optimal value is about 20% for levels of instrument noise up to 1 K for a misfit function based on temperature, or 1.0 m/s for a misfit function based on 15 m velocity components. This is within tolerable limits considering the spread of measurement-based drag coefficient estimates. The results are robust for several different instrument arrays; the noise levels do not decrease by much for arrays with more than 40 sensors when the sensor positions are random. Our results suggest that for an ideal case, having a small number of sensors (20–40) in a data assimilation problem would provide sufficient accuracy in the estimated drag coefficient.     

Turbulent structure in water under laboratory wind waves

The structure of the turbulent boundary layer underneath laboratory wind waves was studied by using a combination of a high-sensitivity thermometer array with a two-component sonic flowmeter. The temperature fluctuations are used to detect movements of water parcels, with temperature as a passive quantity. The turbulence energy was dominant in the frequency range (0.01 0.1 Hz), which was much smaller than the wind-wave frequency (2 5 Hz), and in which the turbulence was anisotropic. There was a frequency range (0.2 2 Hz for velocity, 0.2 5 Hz for temperature fluctuation) where the turbulence was isotropic and had a –5/3 slope in the energy spectrum. These points are the same as those in previous works. However, by analyses of the time series by using a variable-interval time-averaging technique (VITA), it has been found that conspicuous events in this main turbulence energy band are the downward bursting from the vicinity of the water surface. Thus the structure of the water layer underneath the wind waves has characters which are similar to the familiar turbulent boundary layer over a rough solid wall, as already conceived. It has been found that, at the same time, the turbulence energy can be related to quantities of the wind waves (the root mean squared water level fluctuation and the wave peak frequency), for different wind and wave conditions. That is, the turbulence underneath the wind waves develops under a close coupling with the wind waves.     

Settling velocity spectra and the ballast ratio hypothesis

Recently it has been observed that a strong quantitative relationship exists between asymptotic fluxes of particulate organic carbon (POC) to the deep ocean and asymptotic fluxes of “ballast” minerals (opal; calcium carbonate; dust). It has further been suggested that this relationship might provide a mechanistic basis for improved representations of remineralization in ocean carbon models. Since the depth scale of remineralization z* is the ratio k/v of a remineralization rate k and a settling velocity (SV) v, a mechanistic understanding of settling velocities will be crucial in developing such models.Historically, there have been two approaches to estimating the speed with which POC is transported to the deep ocean. First, settling speeds of single particles have been observed directly in both field and laboratory settings; estimates of fecal pellet sinking velocities tend to be higher and more variable than those of aggregates. Second, estimates have been made of the velocity at which temporal patterns in flux propagate between pairs of sediment traps separated in depth (the “benchmark approach”); recent studies have shown these results to be variable and to depend on mineral ballasting. Here we present SV estimates using a relatively new technology: indented rotating sphere (IRS) sediment traps run in settling velocity (SV) mode. In this approach, particles are separated into SV classes during settling to collection cups. In MedFlux, SV data were collected concurrently with time-series (TS) data; the latter were used to construct benchmark estimates for comparison to the SV estimates. From the SV data, the range of modal settling velocities (sinking velocities having the largest time-averaged mass flux densities on a logarithmic scale of SV) in the fast-sinking fraction was estimated to be 287–503 m/d; the average of these modal values is 353 m/d, with standard deviation 76 m/d. In contrast, mean settling velocities of the fast-sinking fraction depend on the range of settling velocity classes included in the estimate. If only SV classes settling at >50 m/d are included, the range of SVs at MedFlux was 214–298 m/d, with average mean value 242 m/d and standard deviation 31 m/d. These mean-velocity results are in excellent agreement with benchmark estimates of signal propagation velocities at Medflux (220±65 m/d); they are also well within the range of other recent benchmark studies. The agreement between the benchmark estimates and mean settling velocity estimates at MedFlux, but not with modal velocities, argues that the benchmark method estimates mean settling velocities.     

Comparison of wind-stress algorithms and their influence on wind-stress curl using buoy measurements over the shelf off Bodega Bay, California

The main objectives of this study were to compare three wind-stress algorithms of varying intricacy and estimate the extent to which each method altered computed wind-stress curl. The algorithms included (1) a simple bulk formula for neutral conditions that is dependent only on wind velocity components; (2) a formula that in addition to dependence on wind components includes a simplified effect of thermal stability through differences in air and sea temperatures; and (3) an algorithm that includes full treatment of dynamics and atmospheric stability. Data for the analysis were from a field program that used a special buoy network off Bodega Bay during 28 June–4 August 2001.A diamond-shaped setup of five closely separated buoys in Bodega Bay allowed for one of the first attempts to compute wind-stress curl over the ocean using buoy measurements. Based on an analysis of the available dataset, the marine layer over Bodega Bay is characterized by positive wind-stress curl with a median value around 0.2 Pa (100 km)⁻¹ and maximum values reaching 2.5 Pa (100 km)⁻¹. Positive wind-stress curl was observed for all wind speed conditions, whereas negative wind-stress curl episodes were associated mostly with low-wind conditions.Comparison of wind-stress curl computed using the three algorithms showed that differences among them can be significant. The first and third algorithms indicated similar stress curl (difference around 10%), but the differences between these two and the second algorithm were much higher (approximately 40%). The reason for the difference is the stability correction, which in the third algorithm strongly decreases with an increase in wind speeds, but stays at a similar level for all wind speeds in the second algorithm. Consequently, for higher wind speeds the variability of wind stress calculated using the second algorithm is greater than for the other two algorithms, causing significant differences in computed wind-stress curl (root mean-square error equal to 0.19 Pa (100 km)⁻¹).Despite the apparent biases in computed wind stress and wind-stress curl among the algorithms, all of them show a significant trend of decreasing sea-surface temperature (SST) with increasing wind-stress curl. The bootstrapping analysis has revealed that both the along-shore wind stress and wind-stress curl have noticeable correlation with the changes in the sea-surface temperature as an indirect indication of the upwelling. An additional analysis, based on the low-pass filtered data, showed also significant agreement between the measured divergence in the cross-shore surface transport and the wind-stress curl computed for all three algorithms.     

On the variability of the high-frequency part of the spectrum of wind-forced sea waves

Data on the temporal variability of sea wave spectral components in the frequency range 1–8 Hz, collected by a drifting vessel in the Pacific ocean (wind speed 1–10 m/s), are discussed in this paper. For the frequency range 3–6 Hz (wind speed 5–8 m/s), a weak variability of the ripples is observed, synchronous with long waves; in the remaining part of the spectral range studied the fluctuations are fortuitous. It is concluded that the wind plays a crucial role in forming the ripples' fluctuation characteristics in the high-frequency part of the spectrum.Translated by Vladimir A. Puchkin.     

Reconstructing tsunami run-up from the characteristics of tsunami deposits on the Thai Andaman Coast

Tsunamis can leave deposits on the land surface they inundate. The characteristics of tsunami deposits can be used to calculate tsunami run-up height and velocity. This paper presents a reconstruction of tsunami run-up from tsunami deposit characteristics in a simple mathematical model. The model is modified and applied to reconstruct tsunami run-ups at Ao Kheuy beach and Khuk Khak beach, Phangnga province, Thailand. The input parameters are grain-size and maximum run-up distance of the sediment. The reconstructed run-up heights are 4.16–4.91 m at Ao Kheuy beach and 5.43–9.46 m at Khuk Khak beach. The estimated run-up velocities (maximum velocity) at the still water level are 12.78–19.21 m/s. In the area located 70–140 m inland to the end of run-up inundation, estimated mean run-up velocities decrease from approximately 1.93 m/s to 0 m/s. Reasonably good agreements are found between reconstructed and observed run-up heights. The tsunami run-up height and velocity can be used for risk assessment and coastal development programs in the tsunami affected area. The results show that the area from 0 to 140 m inland was flooded by high velocity run-ups and those run-up energies were dissipated mainly in this area. The area should be designated as either an area where settlement is not permitted or an area where effective protection is provided, for example with flood barriers or forest.     

Currents and turbulent diffusion in the bottom boundary layer of the Barents Sea

The numerical analysis of the stationary field of current velocity on the upper boundary of the bottom boundary layer in the Barents Sea is performed on the basis of a simplified model taking into account the fields of wind velocity and density of water for the principal periods of the seasonal cycle and the bottom topography. The analysis is based on the climatic BarKode database and the data on the wind velocity over the Barents Sea for the last 50 yr. The numerical results demonstrate that the field of bottom currents is fairly nonuniform and the current velocities vary from several fractions of 1 cm/sec to 5 cm/sec in the zones with noticeable slopes of the bottom. The estimates of the thickness of the bottom boundary layer are obtained for the constant coefficient of bottom friction C _f = 0.04. In the major part of the water area of the Barents Sea, the thickness of the bottom boundary layer is close to 1 m. In the regions with significant slopes of the bottom, it increases to 2–2.5 m and, in the two zones of intensification of the bottom currents, becomes as large as 5 m. The maximum estimate of the coefficient of turbulent viscosity is close to 5 cm²/sec. The mean value of the coefficient of vertical density diffusion K _S is equal to 2.34 cm²/sec and its standard deviation is equal to 1.52 cm²/sec. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 4, pp. 31–49, September–October, 2007.     

Simulation of surge phenomena and transformation of the admixture field in the Sea of Azov in the presence of stationary currents

Using a three-dimensional nonlinear mathematical model, we study the processes of transfer and diffusion of contaminants in the Sea of Azov in the presence of stationary currents. Changes in the sea level, surge phenomena, and the direction and velocity of stationary currents caused by winds with different maximum velocities are analyzed. We estimate the region of applicability of the linear approximation and the choice of the value of integration steps over space and time coordinates. It is shown that the growth of the maximum wind velocity increases the contaminated domains and the time of complete dispersion of the admixture. Solutions obtained in the linear approximation differ slightly from those obtained by using the nonlinear model for wind velocities up to 5 m/sec. Translated from Morskoi Gidrofizicheskii Zhurnal, No. 4, pp. 52–68, July–August, 2008.     

Large-scale laboratory measurements of solitary wave inundation on a 1:20 slope

The run-up flow and related pressure of solitary waves breaking on a 1:20 plane beach were investigated experimentally in a super tank (300 m × 5 m × 5.2 m). Swash flow measurements of flow velocities are compared with an existing analytical solution. By incorporating an analytical solution, the hydrodynamic pressure for a quasi-steady flow state is determined and compared with laboratory data. Concerning the evident extra pressure exerted by the impact of swash flow, an empirical drag coefficient for a circular plate is also suggested in the present study.     

2012年夏季楚科奇海中部温盐与海流特征分析

During the summer of 2012, the fifth CHINARE Arctic Expedition was carried out, and a submersible mooring system was deployed in M5 station located at(69°30.155'N,169°00.654'W) and recovered 50 d later. A set of temperature, salinity and current profile records was acquired. The characteristics of these observations are analyzed in this paper. Some main results are achieved as below.(1) Temperature generally decreases while salinity generally increases with increasing depth. The average values of all records are 2.98°C and 32.21 psu.(2)Salinity and temperature are well negatively correlated, and the correlation coefficient between them is –0.84.However, they did not always vary synchronously. Their co-variation featured different characters during different significant periods.(3) The average velocity for the whole water column is 141 mm/s with directional angle of347.1°. The statistical distribution curve of velocity record number gets narrower with increasing depth. More than85% of the recorded velocities are northward, and the mean magnitudes of dominated northward velocities are100–150 mm/s.(4) Rotary spectrum analysis shows that motions with low frequency take a majority of energy in all layers. The most significant energy peaks for all layers are around 0.012 cph(about 3.5 d period), while the tidal motion in mooring area is nonsignificant.(5) Velocities in all layers feature similar and synchronous temporal variations, except for the slight decrease in magnitude and leftward twist from top to bottom. The directions of velocity correspond well to those of surface wind. The average northward volume transport per square meter is0.1–0.2 m3/s under southerly wind, but about –0.2 m3/s during northerly wind burst.