V. Wismann

An improved composite surface model for the radar backscattering cross section of the ocean surface 1. Theory of the model and optimization/validation by scatterometer data

An improved composite surface model for the calculation of the normalized radar backscattering cross section (NRCS) of the ocean surface at moderate incidence angles is presented. The model is based on Bragg scattering theory. A Taylor expansion of the NRCS in the two-dimensional surface slope yields nonzero second-order terms which represent a first approximation for the effect of the geometric and hydrodynamic modulation of the Bragg scattering facets by all waves that are long compared to these facets. The corresponding expectation value of the NRCS varies with the wave height spectral density of all these waves, and it depends in a well-defined way on frequency, polarization, incidence angle, and azimuthal look direction of the radar. We show that measured NRCS values at frequencies ranging from 1 GHz (L band) through 34 GHz (Ka band) and wind speeds between 2 and 20 m/s can be well reproduced by the proposed model after some reasonable tuning of the input ocean wave spectrum. Also, polarization effects and upwind/downwind differences of the NRCS appear to be relatively well represented. The model can thus be considered as an advanced wind scatterometer model which is based on physical principles rather than on empirical relationships. The most promising field of application, however, will be the calculation of NRCS variations associated with local distortions of the wave spectrum by surface current gradients or wind effects.

On the Reduction of the Radar Backscatter by Oceanic Surface Films: Scatterometer Measurements and Their Theoretical Interpretation

Abstract During the two SIR-C/X-SAR missions in 1994, surface film experiments were performed in the North Sea with a 5-frequency/multipolarization scatterometer flown on a helicopter, in order to investigate the reduction of the radar backscatter in the presence of quasibiogenic and anthropogenic sea surface films, particularly, at different wind speeds. Under all wind conditions encountered in this study, the measured damping ratio (i.e., the ratio of the radar backscatter from a slick-free and a slick-covered water surface) increases with increasing Bragg wavenumber. It is shown that not only Marangoni damping theory, but also wind-induced effects, primarily the energy input by the wind into the wave spectrum, also have to be taken into account. The reductions measured at low to moderate wind speeds (3.5–4 m/s and 5 m/s) are qualitatively explained by means of a comparison of the different source terms of the action balance equation. For the case of high wind speed (12 m/s) a theoretical model for the damping ratios is developed. Using this model, the experimental data can well be reproduced, and the absence of the Marangoni damping maximum at intermediate Bragg wavenumbers (approximately 100 rad/m) can be interpreted. Furthermore, the model can explain the similarities between the radar backscatter reductions measured over quasibiogenic and anthropogenic surface films under high wind conditions.

Radar signatures of marine mineral oil spills measured by an airborne multi-frequency radar

Radar signatures of mineral oil spills consisting of heavy and light fuel were measured by an airborne five-frequency ( L - S - C - X - and K -band) multi u polarization microwave scatterometer flown on a helicopter during a controlled oil spill experiment in the North Sea. The damping ratio, defined as the ratio of the backscattered radar power from an oil-free and an oil-covered sea surface, was measured at different radar frequencies and incidence angles such that the Bragg wavenumbers, k, between 20 radm 1 and 500 radm 1 were covered. The B following results were obtained: for the five oil spills deployed in the experiment the damping ratio, in general, increases monotonically from k 20 radm 1 to B k 500 radm 1 . At S - C - X - and K -band, the damping ratio is larger for heavy B u fuel than for light fuel spills, while at L- band it is almost the same. For heavy fuel, the damping ratio increases with increasing thickness of the oil layer. Furthermore, for wind speeds between 6 m s 1 and 10 m s 1 the ...

Classification of sea slicks by multifrequency radar techniques: New chemical insights and their geophysical implications

Sea slick experiments with an airborne five-frequency radar scatterometer were performed in the presence of surface active substances that represent different fractions of biogenic slicks (fats, amines, sugar derivatives, and fatty acids). Measurements at water temperatures of 282.2 K (9.0°C) and 290.6 K (17.4°C) showed that temperature effects appear to play a secondary role for slick-induced water wave damping, at least in the temperature range encountered during the present experiments. Different procedures of slick generation, with and without application of a spreading solvent, indicated that the wave-damping effect in the short-gravity/capillary wave range, and thus the modification of backscattered radar signals, is not only dependent on the chemical structure but also on the arrangement and distribution (morphology; formation of domains) of the surface-active compounds. Thus far this aspect, which appears to be of particular importance for biogenic sea slicks, has been completely ignored. External infrared reflection-absorption spectroscopy laboratory measurements with infrared radiation in the wavelength range between 3.3 μm and 7 μm enabled us allowed to form a link between some important elements of the morphological structure of the monolayers and their viscoelastic characteristics, which are closely related to the wave-damping effect of surface active compounds and to the compounds influence on remote sensing signals. Furthermore, the IRAS measurements supplied detailed insight into the relaxation process that occurs during the generation of a sea slick and on a slick-covered undulating water surface. In particular, strong hydration/dehydration effects appear to play an important role.

Simultaneous measurements of the ocean wave-radar modulation transfer function at L, C, and X bands from the research platform Nordsee

Radar backscatter measurements were performed from the German Forschungsplattform Nordsee (FPN) in the North Sea in order to determine the ocean wave-radar modulation transfer function (MTF), which relates the backseat t ered radar power to the long surface waves. The radar operated quasi-simultaneously at 1.0 GHz (L band), 5.3 GHz (C band), and 10.0 GHz (X band) at HH and VV polarization by using a single antenna. MTFs obtained at these radar frequencies and polarizations are compared. Our measurements of the dependence of the MTF on wind speed and long wave frequency are in agreement with earlier measurements. It is shown that the dependence of the coherence between the backscattered radar power and the long ocean wave height is a strongly decreasing function of radar frequency. This implies that a real aperture radar operating at a low radar frequency, e.g., at L band, is best suited for imaging ocean waves. Residual MTFs, Mres, are calculated by subtracting the theoretical tilt and range MTFs from the measured total MTFs. According to conventional ocean wave radar modulation theory, Mres should be identical to the hydrodynamic MTF and therefore be independent of polarization. However, the experimental data show a strong dependence of the modulus and phase of Mres on polarization. We find larger values of |Mres| for HH than for VV polarization at C and X bands. In principle, a difference between Mres for HH and VV polarization can be explained by a three-scale composite surface model which takes into account also the modulation of the Bragg waves by intermediate-scale waves (i.e., waves with wavelengths between the long waves and the Bragg waves). However, the differences observed in this experiment are found to be much larger than expected from this theory.

Radar signatures of mineral oil spills measured by an airborne multi-frequency radar and the ERS-1 SAR

Radar signatures of different mineral oil spills were measured by an airborne five-frequency (L-, S-, C-, X-, and K/sub u/-band) microwave scatterometer during a controlled oil spill experiment in the North Sea. Furthermore, signatures of oil spills on C-band SAR images obtained by the First European Remote Sensing Satellite (ERS-1) were analysed. The radar contrast or damping ratio, defined as the ratio of the backscattered radar power from an oil-free and an oil-covered sea surface, increases monotonically for Bragg wavenumbers from k=20 m/sup -1/ to k=500 m/sup -1/. The radar contrast depends on the oil type and the thickness of the oil layer but was found to be independent of the polarization of the radar and the look direction of the radar relative to the wind for wind speeds between 6 m/s and 10 m/s.<<ETX>>

Radar signatures of mineral oil spills measured by an airborne multi-frequency multi-polarization microwave scatterometer

Radar signatures of different mineral oil spills were measured by an airborne five-frequency, (L-, S-, C-, X-, and K/sub u/-band) four-polarization (HH, HV, VV and VH), microwave scatterometer during a controlled oil spill experiment in the North Sea. The damping ratio, defined as the ratio of the backscattered radar power from an oil-free and an oil-covered sea surface, increases monotonically for Bragg wavenumbers from k/sub B/=20 m/sup -1/ to k/sub B/=500 m/sup -1/. The damping ratio depends on the oil type and the thickness of the oil layer, but was found to be independent of the polarization and the look direction of the radar relative to the wind direction for wind speeds between 6 m/s and 10 m/s. These experimental results are interpreted in terms of a theoretical model on the damping of short gravity-capillary waves by surface films.<<ETX>>

Monitoring ecological dynamics in Africa with the ERS-1 scatterometer

Radar backscatter measurements obtained over Africa by the C-band scatterometer aboard the First European Remote Sensing Satellite (ERS-1) are analyzed for the period from November 1991 to December 1994. The predominant signal in this time series is the annual variation of the normalized radar cross section (NRCS) in the savanna regions north and south of the tropical rain forest. The change in the incidence angle dependence of the NRCS as well as the increase in radar cross section correlates well with the precipitation rate and indicates an instantaneous onset of vegetation growth during the rainy season. In particular the exceptional high rain rates in 1994 as well as the drought in 1991/92 in southern Africa are well reflected in the NRCS data.

Global land surface monitoring using the ERS-1 scatterometer

More than two years of global radar backscatter measurements from the C-band scatterometer aboard the First European Remote Sensing Satellite (ERS-1) are used to investigate radar signatures of land surfaces and their seasonal variation. These scatterometer measurements are independent of cloud coverage and illumination by the Sun and, therefore, superior to measurements by optical systems. Compared to the synthetic aperture radar (SAR) aboard ERS-1, the scatterometer delivers a manageable amount of data albeit the global within 3 to 4 days and a geometric resolution which is reasonable for many applications. The authors illustrate the potential of the scatterometer data for global land surface monitoring, especially when exploiting the multi-temporal and multi-incidence angle capabilities of the ERS-1 scatterometer.<<ETX>>

Large scale radar signatures of the land surface measured by the ERS-1 scatterometer

A C-band scatterometer is presently flying aboard the First European Remote Sensing Satellite (ERS-1) which is capable of measuring the normalized radar cross section (NRCS) of the Earths surface globally. The scatterometer data are primarily used for determining the surface wind over the ocean. Up to now, minor attention was given to the scatterometer data over land. In this investigation ERS-1 scatterometer data from one year were used to investigate large scale signatures of land surfaces and their seasonal variations. This paper illustrates that the ERS-1 C-band scatterometer is a very useful instrument for global land monitoring.<<ETX>>