Oceanographic Platform Research Articles

Sodar Observation of the ABL Structure and Waves over the Black Sea Offshore Site

Sodar investigations of the breeze circulation and vertical structure of the atmospheric boundary layer (ABL) were carried out in the coastal zone of the Black Sea for ten days in June 2015. The measurements were preformed at a stationary oceanographic platform located 450 m from the southern coast of the Crimean Peninsula. Complex measurements of the ABL vertical structure were performed using the three-axis Doppler minisodar Latan-3m. Auxiliary measurements were provided by a temperature profiler and two automatic weather stations. During the campaign, the weather was mostly fair with a pronounced daily cycle. Characteristic features of breeze circulation in the studied area, primarily determined by the adjacent mountains, were revealed. Wave structures with amplitudes of up to 100 m were regularly observed by sodar over the sea surface. Various forms of Kelvin–Helmholtz billows, observed at the interface between the sea breeze and the return flow aloft, are described.

Sea Surface Ka-Band Doppler Measurements: Analysis and Model Development

Multi-year field measurements of sea surface Ka-band dual-co-polarized (vertical transmit–receive polarization (VV) and horizontal transmit–receive polarization (HH)) radar Doppler characteristics from an oceanographic platform in the Black Sea are presented. The Doppler centroid (DC) estimated using the first moment of 5 min averaged spectrum, corrected for measured sea surface current, ranges between 0 and ≈1 m/s for incidence angles increasing from 0 to 70 ∘ . Besides the known wind-to-radar azimuth dependence, the DC can also depend on wind-to-dominant wave direction. For co-aligned wind and waves, a negative crosswind DC residual is found, ≈−0.1 m/s, at ≈20 ∘ incidence angle, becoming negligible at ≈ 60 ∘ , and raising to, ≈+0.5 m/s, at 70 ∘ . For our observations, with a rather constant dominant wave length, the DC is almost wind independent. Yet, results confirm that, besides surface currents, the DC encodes an expected wave-induced contribution. To help the interpretation, a two-scale model (KaDOP) is proposed to fit the observed DC, based on the radar modulation transfer function (MTF) previously developed for the same data set. Assuming universal spectral shape of energy containing sea surface waves, the wave-induced DC contribution is then expressed as a function of MTF, significant wave height, and wave peak frequency. The resulting KaDOP agrees well with independent DC data, except for swell-dominated cases. The swell impact is estimated using the KaDOP with a modified empirical MTF.

Modulation of Dual-Polarized X-Band Radar Backscatter Due to Long Wind Waves

Investigation of microwave scattering mechanisms is extremely important for developing methods for ocean remote sensing. Recent studies have shown that a common two-scale scattering model accounting for resonance (Bragg) scattering has some drawbacks, in particular it often overestimates the vertical-to-horizontal polarization radar return ratio and underestimates the radar Doppler shifts if the latter are assumed as associated with quasi linear resonance surface waves. It is supposed nowadays that radar backscattering at moderate incidence angles is determined not only by resonance Bragg mechanism but also contains non polarized (non Bragg) component which is associated supposedly with wave breaking but which is still insufficiently studied. Better understanding of the scattering mechanisms can be achieved when studying variations of radar return due to long wind waves. In this paper, results of experiments from an Oceanographic Platform on the Black Sea using dual co-polarized X-band scatterometers working at moderate incidence are presented and variations of Bragg and non-Bragg components (BC and NBC, respectively) and radar Doppler shifts are analysed. It is established that BC and NBC are non-uniformly distributed over profile of dominant (decametre-scale) wind waves (DWW). Variations of BC are characterized by some “background” return weakly modulated with the dominant wind wave periods, while NBC is determined mostly by rare and strong spikes occurred near the crests of the most intense individual waves in groups of DWW. We hypothesize that the spikes are due to intensification of nonlinear structures on the profile of short, decimetre-scale wind waves when the latter are amplified by intense DWW. Bragg scattering in slicks under the experimental conditions was suppressed stronger than NBC and spikes dominated in total radar return. It is obtained that radar Doppler shifts at HH-polarization are larger than at VV-polarization, particularly in slicks, the same relation is for NBC and BC Doppler shifts, thus indicating different scattering mechanisms for these components.

The Azimuthal Dependence of the Microwave Emissions of a Water Surface Based on Remote Measurements at the Black Sea

This paper presents an analysis of the experimental measurements of the azimuthal anisotropy of the microwave emissions of a rough water surface. The experimental data were obtained using microwave radiometers with operating frequencies of ~37 GHz (a wavelength of ~8 mm) on the oceanographic platform of the Black Sea Hydrophysical Experimental Facility of the Russian Academy of Sciences in the period from 2005 to 2016. This paper describes the azimuthal anisotropy effect, experimental studies, measurement methods, and data processing. A qualitative comparison of the measurement results with the model calculations and other experimental studies of anisotropy was performed. It was shown that there is a strong sensitivity of the variation in the radio emissions of a rough water surface, both to the wind speed and to its direction at different incidence observation angles.

A Regression Model of Microwave Emission of a Water Surface at 37.5 GHz

This paper presents the functional dependence of the microwave emission of a rough water surface at a frequency of 37.5 GHz (wavelength of ~8 mm). The MiROSE model (Microwave Rough Ocean Surface Emission model) is based on the experimental studies of the own thermal radio emission of the water surface, which were carried out in 2005 and 2007 on the oceanographic platform of the Black Sea Hydrophysical Experimental Facility of the Russian Academy of Sciences. This paper demonstrates the steps of the simulation for selecting the optimal functions to describe the incident angle, wind, and temperature dependences of the increment of the own water surface emission. The following parameters can be calculated on the basis of the proposed model: the increment of the brightness temperature, radio-brightness contrast, and radio-brightness temperature for horizontal and vertical polarizations of the received emission. The model is applicable to water temperatures ranging from 12.5 to 25°C, wind speeds of 3–13 m/s, and incident angles of 30°–80° measured from the nadir.

Empirical Model of Radar Scattering in the 3-cm Wavelength Range on the Sea at Wide Incidence Angles

We present the results of radar investigation of scattering of electromagnetic radiation in the 3-cm wavelength range by the sea surface in the case of horizontal polarization of the transmitted and received signals and wide incidence angles. Full-scale measurements were performed on the Oceanographic Stationary Platform in the Black Sea in the range of incidence angles from 84.0° to 87.5° at wind velocities from 4 to 19 m/s. Only the conditions of wind waves with no swelling were considered. It is shown that the specific scattering cross section is almost invariant, when the incidence angle varies, but has a strong wind dependence. It can be described by the power function with the exponents 2.8 and 3.8 at the upwind and downwind probing directions, respectively. At weak and moderate winds, the azimuthal dependence of the specific scattering cross section is characterized by the unimodal function with one maximum in the upwind direction and a minimum in the downwind direction. At wind velocities exceeding 10 m/s, the azimuthal dependence of the scattering cross section becomes bimodal and has a minimum at the azimuth being close to the direction perpendicular to the wind (crosswind). The measured values of the specific scattering cross section are approximated by a finite Fourier series with respect to the azimuth, in which the expansion coefficients depend on the wind velocity and inclination angle.

GNSS Reflectometry of the Black Sea Level in the Experiments at the Stationary Oceanographic Platform

The results of sea level estimation by GNSS reflectometry (GNSS-R) in the Black Sea in the experiments at the stationary oceanographic platform are presented. The influence of the layout of the antennas and the characteristics of the wind waves on the GNSS-R sea level estimates is studied with the use of multi-antenna GPS/GLONASS receiver and wire resistance wave gauge. For the selected geometry of the experiment, the threshold amplitudes and frequencies of the wind waves at which estimation of the sea level by GNSS reflectometry becomes impossible are determined.

Correlation Analysis of Experimental Remote-Sensing Data and Models of Microwave Rough Sea-Surface Emission

A correlation analysis of the model calculations and experimental measurements of wind-speed sensitivity of a rough sea-surface microwave emission at a frequency of 37.5 GHz are presented. The field data used in the research were collected over 3 years in the summer and autumn periods at the oceanographic platform of the Marine Hydrophysical Institute, Russian Academy of Sciences (RAS). A hypothesis about a significant correlation between the model calculations and experimentally measured sea-surface emission ability caused by wind forcing was formulated and tested to reveal this correlation. An evaluation of the discrepancy between the model and experimental data has been performed by an analysis of residuals. Our studies have shown that among the selected models not a single one adequately describes the experimental data.

A study of sea-wave spectra in a wide wavelength range from satellite and in-situ data

The results of studying sea-wave spectra in a wide wavelength range using high resolution (0.5–1.0 m) satellite optical imagery spectra and the results of measurements carried out from an oceanographic platform using string-wave recorders, stereo system, and drifting wave buoys are presented in this paper. The wave spectra retrieved from satellite imagery and sea-truth data have been compared. A comparison has shown the adequacy of the purposely developed retrieval methods. Power approximation indices for spatial spectra in the 0.04–5.0-m wavelength range have been found. It has been shown that the wave spectra measured experimentally by satellite-based and in-situ methods best approximate the Toba spectrum.

Experimental studies of thermal radiation intensity dependence on near-water wind speed for rough sea surface

The results of in situ measurements of the characteristics of intrinsic microwave radiation from the sea surface are presented. The studies were carried out on the oceanographic platform of the Experimental Department of Marine Hydrophysical Institute during the summer–fall periods for 3 years, which made it possible to accumulate a significant amount of information on the change in the radiation intensity of under different meteorological conditions. Attention is primarily focused on the construction of wind-radiation dependences and their steepness analysis based on the measurement data obtained by a radiometer-polarimeter with a central frequency of received radiation equal to 37.7 GHz. The results are compared with the previous experimental studies and the model estimates of this effect.

Underwater Glider Observations in the Oxygen Minimum Zone off Central Chile

Abstract Gliders have become an efficient and reliable oceanographic platform for measuring physical and biogeochemical properties of the seawater, and the global glider fleet is rapidly expanding. In Chile, glider observations have been carried out in very different oceanographic environments, from the mild upwelling region of subtropical northern Chile to the channels of southern Patagonia. Herein, we briefly present observations and results obtained in the oxygen minimum zone off Concepcion (∼36°30′S). Many new features have been observed in this region thanks to the relatively high resolution of the glider measurements. Future plans for the glider program include an oceanic time series off central Chile that will contribute to the regional observing system of the ocean and allow evaluations of low-frequency changes like those associated with El Niño and La Niña events.

Observation of Extreme Sea Waves in a Space–Time Ensemble

AbstractIn this paper, an observational space–time ensemble of sea surface elevations is investigated in search of the highest waves of the sea state. Wave data were gathered by means of a stereo camera system, which was installed on top of a fixed oceanographic platform located in the Adriatic Sea (Italy). Waves were measured during a mature sea state with an average wind speed of 11 m s−1. By examining the space–time ensemble, the 3D wave groups have been isolated while evolving in the 2D space and grabbed “when and where” they have been close to the apex of their development, thus exhibiting large surface displacements. The authors have selected the groups displaying maximal crest height exceeding the threshold adopted to define rogue waves in a time record, that is, 1.25 times the significant wave height (Hs). The records at the spatial positions where such large crests occurred have been analyzed to derive the empirical distributions of crest and wave heights, which have been compared against standard statistical linear and nonlinear models. Here, the maximal observed wave crests have resulted to be outliers of the standard statistics, behaving as isolated members of the sample, apparently uncorrelated with other waves of the record. However, this study has found that these unexpectedly large wave crests are better approximated by a space–time model for extreme crest heights. The space–time model performance has been improved, deriving a second-order approximation of the linear model, which has provided a fair agreement with the empirical maxima. The present investigation suggests that very large waves may be more numerous than generally expected.

Statistical characterization of short wind waves from stereo images of the sea surface

We propose a methodology to extract short‐scale statistical characteristics of the sea surface topography by means of stereo image reconstruction. The possibilities and limitations of the technique are discussed and tested on a data set acquired from an oceanographic platform at the Black Sea. A validation is made with simultaneous in situ measurements as well as results from the literature. We show that one‐ and two‐point properties of the short‐scale roughness can be well estimated without resorting to an interpolation procedure or an underlying surface model. We obtain the first cumulants of the probability distribution of small‐scale elevations and slopes as well as related structure functions. We derive an empirical parametrization for the skewness function that is of primary importance in analytical scattering models from the sea surface.

Offshore stereo measurements of gravity waves

Stereo video techniques are effective for estimating the space-time wave dynamics over an area of the ocean. Indeed, a stereo camera view allows retrieval of both spatial and temporal data whose statistical content is richer than that of time series data retrieved from point wave probes. To prove this, we consider an application of the Wave Acquisition Stereo System (WASS) for the analysis of offshore video measurements of gravity waves in the Northern Adriatic Sea. In particular, we deployed WASS at the oceanographic platform Acqua Alta, off the Venice coast, Italy. Three experimental studies were performed, and the overlapping field of view of the acquired stereo images covered an area of approximately 1100m2. Analysis of the WASS measurements show that the sea surface can be accurately estimated in space and time together, yielding associated directional spectra and wave statistics that agree well with theoretical models. From the observed wavenumber-frequency spectrum one can also predict the vertical profile of the current flow underneath the wave surface. Finally, future improvements of WASS and applications are discussed.

Effect of Interpass Temperature on Microstructure and Mechanical Properties of Weld Metal of 690 MPa HSLA Steel

690 MPa grade HSLA steel has wide application in engineering, such as mechanical equipment and oceanographic platform. Qualified joint is crucial for structure safety. Many factors, such as preheating temperature and interpass temperature, welding heat input and plate thickness, may influence the welding thermal cycle so that microstructure and mechanical properties of weld metal are changed. In this paper, the effect of interpass temperature on microstructure and mechanical properties of weld metal of 690 MPa grade steel are studied. Gas metal arc welding method was used to weld the joint. Four interpass temperatures, 80°C, 120°C, 160°C and 200°C are adopted during welding. Optical microscope, scanning electron microscope, transmission electron microscope and electron back-scattered diffraction (EBSD) were used to analyze the microstructure of weld metal. Tensile test and impact test were used to measure its mechanical properties. Research results show that the weld metal are composed of lath bainite and granular bainite. M-A constituents are found on grain boundary and inside grains. The shape and size of bainite structure and M-A constituent are detailed. The distribution and quantity of residual austenite are also detected. The relationship between microstructure and mechanical properties is discussed. Experiment results show that when interpass temperature is 80°C the yield strength and impact toughness are higher than the other three cases.

Simulating of the probability density of sea surface elevations using the Gram-Charlier series

The capabilities and limitations of the simulation of the probability density of rough sea surface elevations using the Gram-Charlier series are analyzed. The data of direct wave measurements at an oceanographic platform in the Black Sea are used in the analysis. The skewness of the sea surface elevation’s distribution depends on the mean slope formed by the dominating waves and on the degree of the wave field development. A weak trend in the variations of the peakedness during the measurements of these parameters is also observed. The estimates of the errors in the peakedness measurements during the construction of the probability density of the sea surface elevations using the first five terms of the Gram-Charlier series are presented.

Optical characteristics of coastal waters and atmosphere near the south coast of the crimea at the end of summer 2008

We present the results of measurements of optical characteristics of waters (the beam attenuation coefficient, volume scattering function, sea water reflectance, and Secchi depth) and optical characteristics of the atmosphere (aerosol optical thickness, content of vapors, and the Angstrom exponent) carried out in September 2008 on the oceanographic platform near Katsiveli. We carried out the comparative analysis of hydrooptical characteristics measured in various years. The optical type of sea waters in the period of observations is determined.

The effect of wind on the residual current velocities in the inlets of Venice lagoon

The residual flow in the inlets of Venice lagoon subject to Bora and Sirocco winds has been studied. Current velocities have been monitored since 2001 using Acoustic Doppler Current Profilers (ADCP) installed on the beds of the inlets that connect the lagoon to the Adriatic Sea; these inlets are Lido, Malamocco and Chioggia. Wind velocity data have also been continuously measured at an oceanographic platform 14.8 km offshore from the lagoon; these data were subsequently decomposed into Principal Components, which are associated with Bora and Sirocco wind directions. Analyses show that the inflow in Lido inlet is strongly related to the Bora wind. The outflow in Chioggia inlet occurs during Bora events but shows a slightly weaker correlation with the wind speed, while Malamocco inlet shows little or no influence of Bora winds on flow patterns. A net residual inflow through Lido and Malamocco inlet was found, while outflow prevails in Chioggia inlet. During Bora events, the average residual inflow increased three-fold in Lido inlet, whereas the outflow in Chioggia inlet doubled. The current velocities in Lido and Chioggia inlets are best described by an exponential function of wind velocity with exponents of −0.1187 and −0.0924, respectively. The response to Sirocco events was evident mainly in Chioggia inlet. Specifically, there was a slow down of the outflow in linear proportion to wind speed. In excess of 10 m/s a complete current reversal was observed. Lido and Malamocco inlets showed little or no response to Sirocco winds, except during rare cases when wind speeds exceeded 15 m/s.

Acoustic observations of subsurface instability.

A high‐frequency (0.6–18‐kHz), shallow water acoustic experiment (HFA2000) was conducted in Delaware Bay (15‐m depth) during December 2000. Reciprocal transmissions of chirp signals (0.345‐s duration) were radiated between three bottom mounted source‐receiver tripod stations separated by 70–353 m. Environmental data were collected at a nearby oceanographic platform; simultaneously, a shipboard ADCP and CTD were deployed. Analysis of direct path station‐to‐station arrival times (between December 18th 00:00 and December 19th 10:00, during which 126 acoustic transmissions consisting of 29 chirps were radiated) revealed significant deviation from arrival time patterns established during previous tidal cycles. Examination of the corresponding signal intensity reflected this deviation. Independent ADCP data displayed current profile distortion during the period along the direction of the dominant flow channel. The mean slope of a wave number vs geo‐time spectrum was calculated from each geo‐time’s respective ch...

Dynamics of Short Sea Wave Spectrum Estimated From Microwave Radiometric Measurements

This paper presents some results of the experiment CAPMOS'05 performed on an oceanographic research platform in the Black Sea. The platform, located 600 m offshore, was equipped with a set of contact and remote sensing instruments. Conventional contact sensors were used for direct measurements of atmosphere and sea boundary layer parameters (wind speed and direction, air temperature, water temperature and salinity profiles, etc.), whereas microwave and infrared band radiometers were used for remote sensing measurements of surface temperature and wave parameters. In particular, microwave Ka-band radiometer measurements were applied for gravity-capillary wave spectrum retrieval using the original techniques based on angular measurements. The spectrum component evolution under unstable wind conditions has been investigated. It has been demonstrated that the spectral components in the vicinity of the maximum of the wave curvature are the most sensitive to the wind-velocity variations.