A semiempirical model of the normalized radar cross section of the sea surface, 2. Radar modulation transfer function

Abstract

[1] Multiscale composite models based on the Bragg theory are widely used to study the normalized radar cross section (NRCS) over the sea surface. However, these models are not able to correctly reproduce the NRCS in all configurations. In particular, even if they may provide consistent results for vertical transmit and receive (VV) polarization, they fail in horizontal transmit and receive (HH) polarization. In addition, there are still important discrepancies between model and observations of the radar modulation transfer function (MTF), which relates the modulations of the NRCS to the long waves. In this context, we have developed a physical model that takes into account not only the Bragg mechanism but also the non-Bragg scattering associated with radio wave scattering from breaking waves. The same model was built to explain both the background NRCS and its modulation by long surface wave (wave radar MTF problem). In part 1, the background NRCS model was presented and assessed through comparisons with observations. In this part 2, we extend the model to include a third underlying scale associated with longer waves (wavelength $10-300 m) to explain the modulation of the NRCS. Two contributions are distinguished in the model, corresponding to the so-called tilt and hydrodynamic MTF. Results are compared to observations (already published in the literature or derived from the FETCH experiment). As found, taking into account modulation of wave breaking (responsible for the non-Bragg mechanism) helps to bring the model predictions in closer agreement with observations. In particular, the large MTF amplitudes for HH polarization (much larger than for VV polarization) and MTF phases are better interpreted using the present model.

Numerical simulations of SAR microwave imaging of the Brazil current surface front

“…We also suggest comparison studies using outputs from the Kudryavtsev SAR imaging simulation model (KUDRYAVTSEV et al, 2003a;2003b;, which may help to assess the effects of wave breaking, as well as from surfactant accumulations at the front. Such an approach may help to increase the correlations between real and simulated modulations, especially in cases where convergence zones are developed at the front.…”

“…Regarding the direct models, some publications address their development (ALPERS; HENNINGS, 1984;ALPERS,1997;KUDRYAVTSEV et al, 2003a;KUDRYAVTSEV et al, 2003b andKUDRYAVTSEV et al, 2005) and others their use for SAR imaging simulations to study the dynamics of ocean surface features and their effects on SAR signatures (BRANDT et al, 1999;ROMEISER et al, 2004;JOHANNESSEN et al, 2005). Recent models, such as the M4S and the Radar Imaging Model (RIM) are now able to provide very realistic simulation results.…”

“…This model is mostly based on the Bragg backscattering mechanism and the Weak Interaction theories (LONGUET-HIGGINS; STEWART, 1964;WHITHAM, 1965;BRETHERTON, 1971); it is also able to represent the geometric modulations caused by the tilt of illuminated facets and range variations, the hydrodynamic modulation of Bragg waves caused by longer waves, and it also includes the "velocity bunching" mechanism . Details of the RIM model may be found in KUDRYAVTSEV et al (2003a;2003b;, and in JOHANNESSEN et al (2005). The RIM model also accounts for the modulation caused by surfactant accumulations and wave breaking at the frontal zone.…”

An Improved Spectrum Model for Sea Surface Radar Backscattering at L-Band

“…These spectra can simulate the sea surface morphology in the specific wind speed and wave number range and find successful applications for the EM scattering models [21][22][23][24][25][26]. Some early wave spectra are empirical formulations which are statistical descriptions of wind-generated surface waves.…”

“…Kudryavtsev et al derived a physical model of the short wind wave spectrum by analyzing the input and dissipation of sea wave energy based on the propagation theory of the spectral energy density [29]. Including scattering theory and radar data, the precise retrieval of short-wave spectral properties was implemented with active microwave observations [22,25,26,30]. Based on the technique of stereophotography and image brightness contrast processing, the directional short wind wave characteristics were measured and modeled [28,31,32].…”