Search

Field-tower-based observations were used to estimate the Doppler velocity of deep water plunging breaking waves. About 1000 breaking wave events observed by a synchronized video camera and dual-polarization Doppler continuous-wave Ka-band radar at incidence angles varying from 25 to 55 degrees and various azimuths were analyzed using computer vision methods. Doppler velocities (DVs) associated with breaking waves were, for the first time, directly compared to whitecap optical velocities measured as the line-of-sight projection of the whitecap velocity vector (LOV). The DV and LOV were found correlated; however, the DV was systematically less than the LOV with the ratio dependent on the incidence angle and azimuth. The largest DVs observed at up-wave and down-wave directions were accompanied by an increase of the cross-section polarization ratio, HH/VV, up to 1, indicating a non-polarized backscattering mechanism. The observed DV was qualitatively reproduced in terms of a combination of fast specular (coherent) and slow non-specular (incoherent) returns from two planar sides of an asymmetric wedge-shaped breaker. The difference in roughness and tilt between breaker sides (the front face was rougher than the rear face) explained the observed DV asymmetry and was consistent with previously reported mean sea surface Doppler centroid data and normalized radar cross-section measurements.

Global joint measurements of sea surface winds and currents are planned using satellite-based Doppler scatterometers operating in the Ka-band to achieve improved spatial resolution and retrieval accuracy. Still, the knowledge of sea surface Ka-band backscatter properties is poor, particularly, at high winds (>20 m s−1). Sea surface radar cross-section in the Ka-band, in contrast to that in the lower frequency Ku-/X-/C-/L-bands, is likely more sensitive to sea spray, small-scale particles typically present at high winds. In this paper, tower-based field data collected by a continuous dual-co-polarized Ka-band radar during a strong offshore wind event (with wind speed reaching 33 m s−1) are analyzed. This katabatic wind event (≈12 h long) was also recorded by supplementary wave, wind, and current sensors. At the wave fetch of ≈1 km, the maximum wavelength of observed offshore waves was ≈10 m. For such extremely young wind–sea conditions, an apparent sea spray generation was observed during wind gusts. Radar measurements were performed at 20∘ and 45∘ incidence angles, mostly for cross- and up-wind azimuth look geometry. Based on these high wind measurements, the previously developed Ka-band empirical model is tested and compared with other published geophysical model functions. Dual-co-polarized measurements are used to infer resonant Bragg and non-Bragg scattering components and assess the short wind wave spectrum, which shows a clear tendency for saturation at high winds. The presence of sea spray signatures is apparent in the high-frequency tails of radar Doppler spectra, but their overall contribution to the Doppler centroid frequency is weak. Hence, the standard modulation transfer function approach developed for moderate winds is still applicable at high winds for interpreting the wave-induced Doppler velocity and inferring sea surface currents. These results can also be useful for understanding Doppler scatterometry measurements in tropical cyclones.

The effective normalized radar cross section (NRCS) of breaking waves, σwb, is empirically derived based on joint synchronized Ka-band radar and video records of the sea surface from a research tower. The σwb is a key parameter that, along with the breaker footprint fraction, Q, defines the contribution of non-polarized backscattering, NP =σwbQ, to the total sea surface NRCS. Combined with the right representation of the regular Bragg and specular backscattering components, the NP component is fundamental to model and interpret sea surface radar measurements. As the first step, the difference between NRCS values for breaking and non-breaking conditions is scaled with the optically-observed Q and compared with the geometric optics model of breaker backscattering. Optically-derived Q might not be optimal to represent the effect of breaking waves on the radar measurements. Alternatively, we rely on the breaking crest length that is firmly detected by the video technique and the empirically estimated breaker decay (inverse wavelength) scale in the direction of breaking wave propagation. A simplified model of breaker NRCS is then proposed using the geometric optics approach. This semi-analytical model parameterizes the along-wave breaker decay from reported breaker roughness spectra, obtained in laboratory experiments with mechanically-generated breakers. These proposed empirical breaker NRCS estimates agree satisfactorily with observations.

The sea surface normalized radar backscatter cross-section (NRCS) and Doppler velocity (DV) exhibit energy at low frequencies (LF) below the surface wave peak. These NRCS and DV variations are coherent and thus may produce a bias in the DV averaged over large footprints, which is important for interpretation of Doppler scatterometer measurements. To understand the origin of LF variations, the platform-borne Ka-band radar measurements with well-pronounced LF variations at frequencies below wave peak (0.19 Hz) are analyzed. These data show that the LF NRCS is coherent with wind speed at 21 m height while the LF DV is not. The NRCS-wind correlation is significant only at frequencies below 0.01 Hz indicating either differences between near-surface wind (affecting radar signal) and 21-m height wind (actually measured) or contributions of other mechanisms of LF radar signal variations. It is shown that non-linearity in NRCS-wave slope Modulation Transfer Function (MTF) and inherent averaging within radar footprint account for NRCS and DV LF variance, with the exception of VV NRCS for which almost half of the LF variance is unexplainable by these mechanisms and perhaps attributable to wind fluctuations. Although the distribution of radar DV is quasi-Gaussian, suggesting virtually little impact of non-linearity, the LF DV variations arise due to footprint averaging of correlated local DV and non-linear NRCS. Numerical simulations demonstrate that MTF non-linearity weakly affects traditional linear MTF estimate (less than 10% for typical MTF magnitudes less than 20). Thus the linear MTF is a good approximation to evaluate the DV averaged over large footprints typical of satellite observations

This paper presents dual copolarized (VV and HH) Ka-band radar measurements of joint modulation of normalized radar crass section (NRCS) and Doppler velocity (DV) performed from a sea research platform. NRCS and DV modulations are well correlated. NRCS modulations exhibit a spiky structure. HH modulations are stronger than VV ones leading to modulations of the polarization ratio. This suggests that an important portion of NRCS modulations is produced by nonpolarized radar returns from modulated wave breaking facets. DV modulations reveal that at incidence angles <50 degrees, NRCS spikes are attributable to rather slow moving facets, which may he interpreted as short wave breaking disturbances embedded in the water at crests of modulating waves. Using the DV as a proxy for wave gauge, a modulation transfer function (MTF) is estimated for both polarizations. The hydrodynamics component of the total MTF, hydro-MTF, combines NRCS modulations supported by Bragg waves and wave breaking. The contribution of each type of facets to the hydro-MTF is weighted by its partial contribution to the NRCS, and thus, hydro-MTF becomes dependent on radar polarization. Using hydro-MTF for HH and VV, Bragg wave and wave breaking modulations are separated. Wave breaking modulations are significant, with MTF amplitude varying from 5 to about 30 depending on wind speed. Ka-band Bragg waves are strongly modulated at low winds, but their modulation almost vanishes at moderate winds. Finally, we propose an empirical MTF parameterization based on polynomial fitting as a function of observation geometry and wind.

This paper presents dual copolarized (PP) (VV and HH) Ka-band sea surface backscattering measurements taken from the Black Sea research platform at incidence angles ranging from 25 degrees to 65 degrees and in the wind speed range from 3 to 18 m/s. These measurements are corrected for radar antenna pattern and geometry of observations. The resulting normalized radar cross section (NRCS) is parameterized in a form of truncated azimuthal Fourier series with coefficients dependent on the incidence angle and wind speed. This dual PP empirical model (KaDPMod) is consistent with the Ku-band NSCAT-4 model. However, some remarkable differences are revealed. They are apparent when analyzed using a decomposition of VV and HH measurements into polarized Bragg backscattering (polarization difference, PD = VV-HH) and nonpolarized (NP) backscattering from breaking waves. The polarization difference (PD) has strong azimuth and wind dependencies, with the wind exponent ranging from 2.5 to 3. The saturation wave spectra derived from multifrequency PD (based on KaDPMod and Ku- and C-band empirical models) have a notable peak in the capillary-gravity range. The relative contribution of NP radar return to the Ka-band NRCS is significant. In the upwind direction, it reaches up to 60%-80% and 25%-50% for HH and VV, respectively. It is found that the NP wind exponent is lower than that for Bragg backscattering. Therefore, the relative contribution of the NP to Ka-band NRCS decreases with increasing wind speed at both polarizations. Such a behavior is the opposite of that observed in the Ku-band.

Wind-generated waves in the Kara, Laptev, and East-Siberian Seas are investigated using altimeter data from Envisat RA-2 and SARAL-AltiKa. Only isolated ice-free zones had been selected for analysis. Wind seas can be treated as pure wind-generated waves without any contamination by ambient swell. Such zones were identified using ice concentration data from microwave radiometers. Altimeter data, both significant wave height (SWH) and wind speed, for these areas were further obtained for the period 2002-2012 using Envisat RA-2 measurements, and for 2013 using SARAL-AltiKa. Dependencies of dimensionless SWH and wavelength on dimensionless wave generation spatial scale are compared to known empirical dependencies for fetch-limited wind wave development. We further check sensitivity of Ka- and Ku-band and discuss new possibilities that AltiKa's higher resolution can open.

An effective methodology using satellite high-resolution polarized information to interpret and quantitatively assess various surface ocean phenomena is suggested. Using a sample RADARSAT-2 quad-polarization ocean synthetic aperture radar (SAR) scene, the dual co-polarization (VV and HH) radar data are combined into polarization difference, polarization ratio, and nonpolarized components. As demonstrated, these field quantities provide means to distinguish Bragg scattering mechanism and radar returns from breaking waves. As shown, quantitative characteristics of the surface manifestation of ocean currents, slicks, and wind field features in these dual co-polarization properties are very different and may be effectively used in the development of new SAR detection and discrimination algorithms.

At its seasonal peak the Amazon/Orinoco plume covers a region of 10^6 km2 in the western tropical Atlantic with more than 1m of extra freshwater, creating a near-surface barrier layer (BL) that inhibits mixing and warms the sea surface temperature (SST) to >29oC. Here new sea surface salinity (SSS) observations from the Aquarius/SACD and SMOS satellites help elucidate the ocean response to hurricane Katia, which crossed the plume in early fall, 2011. Its passage left a 1.5psu high haline wake covering >10^5 km2 (in its impact on density, the equivalent of a 3.5oC cooling) due to mixing of the shallow BL. Destruction of this BL apparently decreased SST cooling in the plume, and thus preserved higher SST and evaporation than outside. Combined with SST, the new satellite SSS data provide a new and better tool to monitor the plume extent and quantify tropical cyclone upper ocean responses with important implications for forecasting.

The main goal of the proposed research is to study theoretically the role of the breaking wind waves and the sea spray, generated by them, under strongly forced situations in the airflow dynamics in particular in energy, momentum, heat and moisture transfers through the sea surface. Through this to increase the knowledge of the air-sea interaction and to apply this knowledge for developing improved, physics-based parameterizations of the fluxes (momentum, energy, heat and moisture) in the wavecoupled atmospheric boundary layer. The proposed study is essential to quantify the wave breaking effects on surface transfers, especially in the case of strong winds, where the sea spray generated by breaking waves is believed to play the dominant role in the airflow dynamics. The improved surface forcing (parameterizations of fluxes) is aimed at improvement of the performance of the high-resolution wave and coupled atmosphere-ocean models. The principle innovation of this study is that the airflow and wind waves are considered as a self-consistent interacting coupled system, where the properties of the sea surface (shape of the wave spectrum, wave breaking statistics, etc.) and turbulent characteristics of the atmospheric wave boundary layer are interrelated with each other.

The main goal of the proposed research is to study theoretically the role of the breaking wind waves and the sea spray, generated by them, under strongly forced situations in the airflow dynamics in particular in energy, momentum, heat and moisture transfers through the sea surface. Through this to increase the knowledge of the air-sea interaction and to apply this knowledge for developing improved, physics-based parameterizations of the fluxes (momentum, energy, heat and moisture) in the wave-coupled atmospheric boundary layer. The proposed study is essential to quantify the wave breaking effects on surface transfers, especially in the case of strong winds, where the sea spray generated by breaking waves is believed to play the dominant role in the airflow dynamics. The improved surface forcing (parameterizations of fluxes) is aimed at improvement of the performance of the high-resolution wave and coupled atmosphere-ocean models. The principle innovation of this study is that the airflow and wind waves are considered as a self-consistent interacting coupled system, where the properties of the sea surface (shape of the wave spectrum, wave breaking statistics, etc.) and turbulent characteristics of the atmospheric wave boundary layer are interrelated with each other., Performed in cooperation with the Russian State Hydrometeorological University (RSHMU), St. Petersburg, Russia.

The main goal of the proposed research is to study theoretically the role of the breaking wind waves and the sea spray, generated by them, under strongly forced situations in the airflow dynamics in particular in energy, momentum, heat and moisture transfers through the sea surface. Through this to increase the knowledge of the air-sea interaction and to apply this knowledge for developing improved, physics-based parameterizations of the fluxes (momentum, energy, heat and moisture) in the wave coupled atmospheric boundary layer. The proposed study is essential to quantify the wave breaking effects on surface transfers, especially in the case of strong winds, where the sea spray generated by breaking waves is believed to play the dominant role in the airflow dynamics. The improved surface forcing (parameterizations of fluxes) is aimed at improvement of the performance of the high resolution wave and coupled atmosphere-ocean models. The principle innovation of this study is that the airflow and wind waves are considered as a self-consistent interacting coupled system, where the properties of the sea surface (shape of the wave spectrum, wave breaking statistics, etc.) and turbulent characteristics of the atmospheric wave boundary layer are interrelated with each other., Prepared in collaboration with the Russian State Hydrometeorological University.

Long-term goals are: (1) To develop an advanced physical model of the wind wave spectrum based on the Donelan and Pierson (1987) approach and accounting for the nonlinear processes in the capillary-gravity range consistent with existing analytic models of high-wavenumber spectra. (2) To construct a new version of the ABL model coupled with wind waves using an advanced wind wave spectrum model and the approach developed by Makin et al. (1995). (3) To build an unified model of the short wave modulations by long surface waves. This model describes the reaction of the coupled system `wind waves - ABL' in the presence of surface disturbances caused by long waves. (4) To verify the model results on the available radar data and the new data obtained from the MHI Black Sea Research platform., See also ADM002252. Prepared in collaboration with the Ukraine Academy of Sciences, the Royal Netherlands Meteorological Institute, and the University of Arizona.

Further refinement of the rate of wave amplification by wind has resulted from laboratory measurements and numerical calculations. Calculations of the nonlinear interactions in the capillary-gravity range have been extended. We developed a new comfortable method for description of the nonlinear gravity waves interactions on deep water. A Boltzmann's collision term in the Hasselmann's equation was replaced by a nonlinear diffusive operator. The model equation has the same constants of motion as the exact equation and gives correct expressions for weak turbulent Kolmogorov spectra. A numerical simulation of the new model demostrates a pretty good coincidence with the results of solution of the Hasselmann equation. The new model can be efficiently used in the study of physical mechanisms of air-sea interaction. We studied numerically the weak turbulence of capillary waves on deep water. By a direct solution of the Euler equation in approximation of small angles we found that stationary spectra of capillary waves obey the Kolmogorov law KA(.19/4) which is exact solution of kinetic equation for waves. In situation when the turbulence is realized in finite-size tank there is completely new effect of "frozen" turbulence which could be realized at very low levels of the excitations of capillary waves. At "frozen" turbulence regime there is no energy flux from low wavenumbers of pumping toward high wavenumbers of damping., Prepared in cooperation with University of Miami, School of Marine and Atmospheric Science, Miami, FL, Ukraine Academy of Sciences, Savastpool, Ukraine and Royal Netherlands Meteorological Institute.