We show that if ocean surface waves of the order of a few meters long are frequently steep enough to generate bound centimetric waves, then composite surface scattering theory can account for many anomalous properties of microwave backscatter from the sea at high incidence angles. The model proposed here postulates that these intermediate waves are made sufficiently steep to generate bound centimetric waves because of their modulation by longer, dominant ocean surface waves. The bound centimetric waves have a nonzero mean tilt because they are located on the steep forward face of the intermediate waves, and they move at the speed of the intermediate waves. Applying composite surface scattering theory to this sea surface model, we show that much of the apparently anomalous behavior of microwave sea return measured at incidence angles between 50° and 80° during the Synthetic Aperture Radar and X Band Ocean Nonlinarities-Forschungsplatform Nordsee (SAXON-FPN) experiment can be explained using reasonable parameters to characterize the surface waves. In the SAXON-FPN measurements the mean values of the first moments of microwave Doppler spectra for horizontally polarized backscatter differ from those for vertically polarized backscatter by an amount which varies with the incidence angle and with the azimuthal angle between the radar look direction and the direction of the dominant wave. The modulation of this first moment by surface waves tens of meters in length is the same for the two polarizations at low to moderate incidence angles and can be interpreted in terms of the advection of free centimetric waves by the long waves. At higher incidence angles, however, this modulation is different for the two polarizations and cannot be explained by simple advection of free waves. Finally, microwave cross sections measured at high incidence angles using horizontal polarization are much larger than can be explained by a composite surface theory that includes only freely propagating centimetric waves. Most of these effects can be explained by the composite surface model presented here, which includes Bragg scattering from both free and bound, tilted waves.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JC01225

A model for microwave Doppler sea return at high incidence angles: Bragg scattering from bound, tilted waves

Breaking without air entrainment of very short wind-forced waves, or microscale wave breaking, is undoubtedly widespread over the oceans and may prove to be a significant mechanism for enhancing the transfer of heat and gas across the air-sea interface. However, quantifying the effects of microscale wave breaking has been difficult because the phenomenon lacks the visible manifestation of whitecapping. In this brief report we present limited but promising laboratory measurements which show that microscale wave breaking associated with evolving wind waves disturbs the thermal boundary layer at the air-water interface, producing signatures that can be detected with infrared imagery. Simultaneous video and infrared observations show that the infrared signature itself may serve as a practical means of defining and characterizing the microscale breaking process. The infrared imagery is used to quantify microscale breaking waves in terms of the frequency of occurrence and the areal coverage, which is substantial under the moderate wind speed conditions investigated. The results imply that ”bursting“ phenomena observed beneath laboratory wind waves are likely produced by microscale breaking waves but that not all microscale breaking waves produce bursts. Oceanic measurements show the ability to quantify microscale wave breaking in the field. Our results demonstrate that infrared techniques can provide the information necessary to quantify the breaking process for inclusion in models of air-sea heat and gas fluxes, as well as unprecedented details on the origin and evolution of microscale wave breaking.

/pdf/defining-and-quantifying-microscale-wave-breaking-with-21tuzc9641.pdf

Defining and quantifying microscale wave breaking with infrared imagery

Brewster angle damping and local multipath effects are considered as sources of polarization differences in low grazing angle sea scatter characteristics. The authors show that at least five observed polarization differences can be explained by local multipath interference effects that occur due to the illumination of discrete nonlinear ocean surface features, such as bores and small scale breaking waves. The illumination gain factor (IGF) is defined at a point in space, as the total power at that point relative to the power in the incident plane wave. The IGF resulting from local multipath from the sea surface forward of a discrete scatterer produces strong interference patterns that can vary both with grazing angle and scatterer height. As a result, IGF values up to a factor of 16 (12 dB) can occur for horizontal polarization (HH) when the interference is constructive; a corresponding strong cancellation occurs for destructive interference. These extreme variations can cause strong HH NRCS amplitude modulations due either to a change of local wave slope or a change of scatterer shape with time. However, Brewster angle damping of the forward scatter path for grazing angles below 20/spl deg/ occurs for vertical (VV) polarization, and reduces the VV IGF in magnitude and dynamic range, eliminating such strong modulations. This effect scales with radar wavelength, and higher wave features are required to produce equivalent effects for radar frequencies far below 10 GHz. As an illustration, six radar bands are compared: L (1.4 GHz), S (3.5 GHz), C (5 GHz), X (10 GHz), K/sub u/ (15 GHz), and K/sub a/ (35 GHz), for a sea water dielectric. X-band results indicate that 12-dB IGFs can occur for water surface features just a centimeter above the mean surface. As an application of these results, the influence of these HH and VV IGF patterns is modeled for discrete scatterers distributed uniformly along an ocean gravity wave. The dynamic range of the HH IGF for a distribution of bore scatterers up to 5 cm high is found to he significantly larger than for VV at all locations on the long wave. Moreover, the IGF HHVV polarization ratio forward of the crest, where the largest number of small scale breaking wave scatterers occurs, is larger than at all other regions of the long wave, of the order of 20 dB. These results suggest that HH polarization may be sensitive to small scale breaking features on the ocean surface at low grazing angles, and thus may be a sensitive measure of air-sea fluxes.

A model for Brewster angle damping and multipath effects on the microwave radar sea echo at low grazing angles

[1] An empirical inversion method is presented for determination of time series of ocean surface elevation maps from nautical radar-image sequences. The method is based on the determination of the surface tilt angle in antenna look direction at each pixel of the radar images. Thereby in situ sensors are not required. An external calibration is not necessary. A conventional nautical X-band radar, operating at grazing incidence and horizontal polarization in transmit and receive, is used as a sensor. Radar-image sequences, with their high spatial resolution and large coverage, offer a unique opportunity to derive and study individual waves and wave fields in space and time and therefore allow the measurement of individual wave parameters and wave groups. For validation of the inversion scheme, the significant wave heights derived from the inverted radar data sets and from colocated wave records are compared. It is shown that the accuracy of the radar-retrieved significant wave height is within the accuracy of the in situ sensors. Furthermore, a wave elevation time series is directly compared to a buoy record to show the capabilities of the proposed method.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JC002130

Ocean surface determination from X‐band radar‐image sequences

In this paper, we study the retrieval of wind information from nautical X-band radar data. In contrast to previous studies, where data from stationary research platforms were used, this study focuses on data from a moving platform, encountering a larger variety of conditions than a platform at a fixed location. Compared to traditional in situ sensors, wind data derived from nautical radar images are much less susceptible to air flow distortion by the platform, since the images cover a large area around the ship. Images collected with a standard nautical HH-polarized X-band radar operating at grazing incidence exhibit a single intensity peak in upwind direction. The wind retrieval method developed here uses a harmonic function that is least-squares fitted to the radar backscatter intensity as a function of antenna look direction. The upwind direction is given by the direction that corresponds to the peak of the fitted function. An empirical model function is derived to retrieve the wind speed from the average radar backscatter intensity. Contrary to wind retrieval methods that have been proposed before, this approach is well suited for data acquired from a moving platform, as it functions well even if the radar field of view is partially shadowed and does not require ship motion correction. Here, we focus on data that were collected during two storms, using the first storm to derive and the second to test the empirical model functions. The method is validated using measurements from two ship-based anemometers.

Wind Retrieval From Shipborne Nautical X-Band Radar Data

Reports on experiments featuring horizontally (HH) and vertically (VV) polarized X-band marine radar sea scatter imagery in the coastal environments of Bermuda and La Jolla, under light to moderate winds and the absence of long gravity waves. These conditions allowed the study of fundamental scattering mechanisms from small scale roughness and short waves of a few meters wavelength or less, shorter than the radar pulse. While a large fraction of radar data collected revealed the presence of slick bands, the authors analyze the radar echo of ambient background outside of the slicks. Sea scatter data were digitally recorded for 360/spl deg/ azimuthal coverage for grazing angles between 1-3.5/spl deg/, and were converted to normalized radar cross section (NRCS or /spl sigma//spl deg/) images. The HH and VV polarizations show quite different spatial texture, with HH exhibiting a discrete character and VV being more nearly spatially homogeneous. Grazing angle dependencies are different for HH and VV: upwind-downwind differences of mean NRCS show ratios of just 4-6 dB for VV, but are equal or greater than the 16-dB noise-imposed limit for HH for the low wind conditions. HH NRCS grazing angle characteristics are shown to correlate with biological activity indicators of the coastal waters, with a fourth power dependence in relatively unproductive waters, to a nearly quadratic in productive waters. Arguments are presented that suggest different scattering mechanisms for the two polarizations: evenly distributed Bragg scatter patches for VV and scatter from small asymmetric bore features for HH. A multipath illumination model for small bore features is outlined, and scale sizes for the small scale breakers inferred from the experimental results presented are between 2 and 4 cm in height, with crest widths between 24 and 48 cm.

D.J. Carlson

Papers

Studies of dual polarized low grazing angle radar sea scatter in nearshore regions