The synthetic aperture radar (SAR) Doppler centroid has been used to estimate the scatter line-of-sight radar velocity. In weak to moderate ocean surface current environment, the SAR Doppler centroid is dominated by the directionality and strength of wave-induced ocean surface displacements. In this paper, we show how this sea state signature can be used to improve surface wind retrieval from SAR. Doppler shifts of C-band radar return signals from the ocean are thoroughly investigated by colocating wind measurements from the ASCAT scatterometer with Doppler centroid anomalies retrieved from Envisat ASAR. An empirical geophysical model function (CDOP) is derived, predicting Doppler shifts at both VV and HH polarization as function of wind speed, radar incidence angle, and wind direction with respect to radar look direction. This function is used into a Bayesian inversion scheme in combination with wind from a priori forecast model and the normalized radar cross section (NRCS). The benefit of Doppler for SAR wind retrieval is shown in complex meteorological situations such as atmospheric fronts or low pressure systems. Using in situ information, validation reveals that this method helps to improve the wind direction retrieval. Uncertainty of the calibration of Doppler shift from Envisat ASAR hampers the inversion scheme in cases where NRCS and model wind are accurate and in close agreement. The method is however very promising with respect of future SAR missions, in particular Sentinel-1, where the Doppler centroid anomaly will be more robustly retrieved.Index Terms-Doppler, surface wind, synthetic aperture radar (SAR).
A radar imaging model including a Doppler shift module is presented for quantitative studies of radar observations of wave-current interaction in a strong tidal current regime. The model partitions the Doppler shift into the relative contribution arising from the motion of the backscattering facets including Bragg waves, specular points, and breaking waves that are advected by and interact with the underlying surface current. Simulated and observed normalized radar cross sections and Doppler shifts for different environmental conditions and radar parameters are compared and discussed.Statistical properties of the sea surface result from a solution of the energy balance equation 9 (e.g. Hughes (1978); Thompson (1988); Lyzenga and Bennett (1988)) where wind forcing, 10 viscous and wave breaking dissipation, wave-wave interactions, and generation of shorter 11 waves by breaking waves of longer scales are accounted for. The latter mechanism is described 12 by Kudryavtsev and Johannessen (2004), and although it does not significantly alter the 13 background spectrum, it plays a crucial role in the context of wave modulations by surface 14 current (Kudryavtsev et al., 2005). The RIM thus consists of a particular decomposition 15 of the sea surface into a regular wavy surface and a number of breaking zones. Radar 16 scattering from the regular surface is described within the frame of the composite model 17 combining specular reflection and resonant (Bragg) scattering waves with local tilting effects 18 due to longer underlying waves (e.g. Plant (1986); Donelan and Pierson (1987); Romeiser 19 et al. (1994); Romeiser and Alpers (1997)). The contribution from breaking waves can 20 be described as specular reflections from very rough wave breaking patterns and is taken 21 proportional to the fraction of the sea surface covered by breaking zones based on wave 22 breaking statistics proposed by Phillips (1985). 23 Using Envisat Advanced SAR (ASAR) observations, Chapron et al. (2005) demonstrated 24 the capability to use the Doppler centroid information embedded in the radar signal to 25 map surface velocity, including wind-generated waves and current, from SAR images. The 26 difference between a predicted Doppler shift based on precise knowledge of the satellite 27 orbit and attitude, and the Doppler centroid frequency estimate in this case represents the 28 geophysical Doppler shift experienced from the moving ocean surface. This geophysical 29 Doppler shift in turn reflects the line-of-sight velocity of the scatterers, weighted by their 30 contribution to the backscattered power (Romeiser and Thompson, 2000). The retrieval and 31 subsequent error correction of the geophysical Doppler shift from the ASAR Wide Swath 32 Medium resolution image (WSM) product is presented in Hansen et al. (2011a) where the 33 accuracy of the geophysical Doppler shift is found to be about 5 Hz. This corresponds to 34 a horizontal surface velocity of 20 cm/s at an incidence angle of 40 • , and 40 cm/s at an 35 incidence angle of 20 • . As such,...
Radarsat-2 C-band quad-polarization SAR observations of crude oil, emulsion, and plant oil slicks acquired in the wind speed range from 4 to 8 m/s and incidence angles from 30 degrees to 50 degrees are analyzed to yield new insights into the attenuation of short waves and breaking waves by surface slicks in real conditions. To provide a direct quantitative assessment of the surface wave damping, the measurements are decomposed into polarization difference (PD), polarization ratio (PR), nonpolarized (NP), and cross-polarized (CP) components. The PD signals relate to the extent of Bragg damping in the slick areas, over which the PR is systematically higher than over the ambient sea surface. Attenuation of the breaking waves is revealed to affect both the CP and the NP signals, with distinct but weaker contrasts compared to that of the PD. A revised physical model description is proposed to provide consistent interpretation of the polarized and NP signals. The results suggest that the different slick types and look-alikes can be efficiently discriminated and classified.
Abstract. We present optical, near-infrared, and X-ray observations of the optical afterglow (OA) of the X-ray rich, longduration gamma-ray burst GRB 011211. Hubble Space Telescope (HST) data obtained 14, 26, 32, and 59 days after the burst, show the host galaxy to have a morphology that is fairly typical of blue galaxies at high redshift. We measure its magnitude to be R = 24.95 ± 0.11. We detect a break in the OA R-band light curve which is naturally accounted for by a collimated outflow geometry. By fitting a broken power-law to the data we find a best fit with a break 1.56 ± 0.02 days after the burst, a pre-break slope of α 1 = −0.95 ± 0.02, and a post-break slope of α 2 = −2.11 ± 0.07. The UV-optical spectral energy distribution (SED) around 14 hours after the burst is best fit with a power-law with index β = −0.56 ± 0.19 reddened by an SMC-like extinction law with a modest A V = 0.08 ± 0.08 mag. By comparison, from the XMM-Newton X-ray data at around the same time, we find a decay index of α X = −1.62 ± 0.36 and a spectral index of β X = −1.21 +0.10 −0.15 . Interpolating between the UV-optical and X-ray implies that the cooling frequency is located close to ∼1016 Hz in the observer frame at the time of the observations. We argue, using the various temporal and spectral indices above, that the most likely afterglow model is that of a jet expanding into an external environment that has a constant mean density rather than a wind-fed density structure. We estimate the electron energy index for this burst to be p ∼ 2.3.
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