Martin Gade

Imaging of biogenic and anthropogenic ocean surface films by the multifrequency/multipolarization SIR-C/X-SAR

Results from the analyses of several spaceborne imaging radar-C/X-band synthetic aperture radar (SIR-C/X-SAR) images are presented, which were acquired during the two SIR-C/X-SAR missions in April and October 1994 by the L-, C-, and X-band multipolarization SAR aboard the space shuttle Endeavour. The images showing natural (biogenic) surface slicks as well as man-made (anthropogenic) mineral oil spills were analyzed with the aim to study whether or not active radar techniques can be applied to discriminating between these two kinds of surface films. Controlled slick experiments were carried out during both shuttle missions in the German Bight of the North Sea as well as in the northern part of the Sea of Japan and the Kuroshio Stream region, where surface films of different viscoelastic properties were deployed within the swath of the shuttle radars. The results show that the damping behavior of the same substance is strongly dependent on wind speed. At high wind speed (8–12 m/s) the ratio of the radar backscatter from a slick-free and a slick-covered water surface (damping ratio) is smaller than at low to moderate wind speeds (4–7 m/s). At 12 m/s, only slight differences in the damping behavior of different substances were measured by SIR-C/X-SAR. Furthermore, several SAR scenes from various parts of the worlds oceans showing radar signatures of biogenic as well as anthropogenic surface films at low to moderate wind speeds are analyzed. The damping behavior of these different kinds of oceanic surface films varies particularly at Lband where the biogenic surface films exhibit larger damping characteristics. Results of polarimetric studies from multipolarization SAR images showing various surface films are presented. It can be delineated from these results that Bragg scattering as well as specular reflection contribute to the backscattered radar signal at low incidence angles (up to 30°). It is concluded that at low to moderate wind speeds, multifrequency radar techniques seem to be capable of discriminating between the different surface films, whereas at high wind conditions a discrimination seems to be difficult.

Investigation of multifrequency/multipolarization radar signatures of rain cells over the ocean using SIR‐C/X‐SAR data

Radar signatures of rain cells are investigated using multifrequency/multipolarization synthetic aperture radar (SAR) images acquired from the space shuttle Endeavour during the spaceborne imaging radar-C/X-band SAR (SIR-C/X-SAR) missions in April and October 1994. In SIR-C/X-SAR images, radar signatures of rain cells over the ocean usually consist of irregularly shaped bright and dark patches that strongly depend on radar frequency and polarization. The radar signatures of rain cells observed in SIR-C/X-SAR imagery of the ocean originate from (1) the scattering and attenuation of the microwaves by raindrops and ice particles in the atmosphere and (2) the modification of the sea surface roughness induced by the impact of raindrops and by wind gusts associated with rain cells. Raindrops impinging on the sea surface generate ring waves, which enhance the sea surface roughness, but they also generate turbulence in the upper water layer, which reduces the sea surface roughness. Depending on the radar wavelength, ocean areas struck by rain can have higher or lower normalized radar cross section (NRCS) than the surrounding rain-free area; in ocean areas where heavy rain is impinging on the sea surface, the X- and C-band NRCS is usually enhanced, and the L-band NRCS is reduced. From the phase difference between the horizontally and vertically copolarized signals, estimates of the rain rate are obtained. The present analysis shows further that the presently used wind speed retrieval algorithms for the scatterometers aboard the ERS and ADEOS satellites may yield biased wind fields if several rain cells lie within a scatterometer resolution cell.

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 ...

Simultaneous observations of rain cells over the ocean by the synthetic aperture radar aboard the ERS satellites and by surface-based weather radars

Radar images acquired over the ocean by the C band synthetic aperture radar (SAR) aboard the European Remote Sensing satellites ERS 1 and ERS 2 often show sea surface manifestations of rain cells. We have searched the archives of several weather stations for weather radar data acquired concurrently with ERS SAR data and have found four concurrent data pairs: one in the South China Sea, two in the Baltic Sea and one in the North Sea. The comparison of ERS 1/2 SAR images showing radar signatures of rain cells with weather radar images reveals that the radar signatures of rain cells on ERS SAR images vary considerably, which makes it often difficult to distinguish them from radar signatures of other mesoscale or submesoscale atmospheric and oceanic phenomena. The present analysis, together with results obtained from previous analyses of spaceborne multifrequency SAR data and laboratory data as well as results obtained from theoretical models on radar backscattering at the sea surface suggest the following: C band radar signatures of rain cells with rain rates below 50 mm/h are mainly caused by a modification of the sea surface roughness induced by (1) the raindrops impinging on the sea surface and thus modifying the sea surface roughness and by (2) local wind field variations associated with rain cells (spreading downdrafts). Raindrops impinging on the sea surface generate ring waves as well as turbulence in the upper water layer. Depending on rain rate, drop size distribution, wind speed, and temporal evolution of the rain event, the net effect can be an increase or a reduction of the amplitude of the C band Bragg waves and thus of the backscattered radar power. Thus ocean areas struck by rain can show up on ERS SAR images as areas with higher or lower image brightness than the surroundings.

Wind‐wave tank measurements of bound and freely propagating short gravity‐capillary waves

Measurements of the surface elevation and slope and of the backscattered radar power at X and Ka band were carried out in a wind-wave tank with mechanically generated gravity waves as well as with wind waves on slick-free and slick-covered water surfaces. The measured radar Doppler shifts show that on a slick-free water surface, bound gravity-capillary (X and Ka band Bragg) waves are generated at the crests of steep gravity waves with frequencies between 3 and 5 Hz. However, steep gravity waves with a frequency of 2Hz do not generate bound Ka band Bragg waves, and the Ka band backscattering from these waves is associated with wave breaking. In the wind speed range from 1.5 to 10m/s, bound gravity-capillary waves contribute to the X and Ka band backscatter from slick-free water surfaces. The fraction of bound to freely propagating Bragg waves depends on, among other things, radar frequency, wind speed, wave amplitude of the dominant water wave, and slick coverage. In particular, the strong damping of the gravity waves by the slick at wind speeds of approximately 8 m/s leads to the disappearance of the bound Bragg waves and therefore to a reduction of the X and Ka band Doppler shifts to values corresponding to freely propagating Bragg waves. It is concluded that the study is pertinent to the understanding of former results of radar backscattering measurements in the presence of oceanic surface films.

Marine pollution in European coastal waters monitored by the ERS-2 SAR: a comprehensive statistical analysis

The oil pollution of the southern Baltic Sea, the North Sea, and the northwestern Mediterranean Sea has been studied within a two-year period from December 1996 until November 1998. In total, the authors have analyzed more than 700 synthetic aperture radar (SAR) images, which have been acquired over the three test areas by the Second European Remote Sensing Satellite (ERS-2). The authors present the results of their statistical analyses. Moreover, the authors introduce advanced image processing techniques for the classification of the observed radar signatures, namely the calculation of the fractal dimension, and discuss the possibility of identifying different types of oceanic phenomena.

The effect of artificial rain on wave spectra and multi-polarisation X band radar backscatter

We have carried out wind-wave tank measurements using wave-height and wave-slope gauges and a coherent 9.8 GHz (X band) scatterometer, when the water surface was agitated by heavy rain (160 mm h- 1 to 300 mm h -1 ) and by wind (2 ms- 1 to 12 ms -1 ). The upwind-looking scatterometer was operating at co- (VV- and HH-) and cross- (HV-) polarisation at a steep incidence angle of 28°. In the presence of rain, the power spectral density of the wind-wave spectra is enhanced at frequencies above about 5 Hz and it is reduced at lower frequencies. This is the net effect of surface roughness production by the rain-induced splash products and of wave damping by the rain-induced turbulence. We measured isotropic (rain-dominated) wave spectra at low wind speeds and anisotropic (wind-dominated) wave spectra at high wind speeds, with a transition wind speed that increases with rain rate. The radar backscattering at co-polarisation at low wind speeds is mainly caused by rain-induced ring waves, whereas at cross-polarisation, at all wind speeds, other rain-induced splash products, like crowns, stalks, and cavities, are the dominant scatterers. We have found a rain-induced increase of the radar backscatter at co-polarisation at wind speeds of up to 9 ms- 1 and at cross-polarisation at all wind speeds. At cross-polarisation the radar backscatter slightly depends on rain rate. Using our results an analysis of spaceborne synthetic aperture radar (SAR) images of tropical rain cells was performed.

The observation of seiches in the Baltic Sea using a multi data set of water levels

Abstract This investigation addresses the difficult problem of using a multi data set of water levels to study seich oscillations in the Baltic Sea. The Baltic Sea is a semi-enclosed sea area and is connected with the North Atlantic Ocean through the North Sea. The applied method is based on the analysis of environmentally corrected radar altimeter (RA) data in conjunction with simulated water levels obtained by an operational circulation model of the Federal Maritime and Hydrographic Agency (Bundesamt fur Seeschiffahrt und Hydrographie, BSH) and tide gauge data. RA data of the First European Remote Sensing Satellite (ERS-1) have been analysed. The RA data were collected during the three-day repeat cycle from August 12 to December 9, 1991. An advantage of the analysis is the coverage and the geographical location of one and the same frequently repeated descending sub-satellite pass over the Bothnian Bay and the central Baltic Sea. Due to the assumed linear dynamics of this sea area the observation of sea level changes in the sense of a standing wave between the Bothnian Bay and the Baltic Proper is investigated. Amplitudes of water level changes up to 1 m have been recorded at tide gauge stations in the northern and southern parts of the Baltic Sea, respectively, during August and December 1991. Water level changes of less than 20 cm were calculated from the corrected altimeter data during the overflights of ERS-1. Tide gauge data and water level differences of tide gauge data measured at Kemi (Bothnian Bay), Finland, and Kolobrzeg at the Polish coast were used for frequency analyses. The analysed time series have been limited to five months due to the duration of the three-day repeat cycle of ERS-1. For frequency analysis the given sampling interval of 4 h is not appropriate, but a period of 12.4 h was clearly identified. This period corresponds to the fourth mode of theoretical estimated ones. The differences of water levels of the simulated data between Kemi and Kolobrzeg of about 25 cm agree fairly well with the results obtained by the RA data. Due to these results and the synoptic view of the along-track subsatellite passes it turned out that seiches can be observed by the ERS-1 radar altimeter.

Wind wave tank measurements of wave damping and radar cross sections in the presence of monomolecular surface films

Measurements of the damping of small gravity and gravity-capillary water surface waves covered with monomolecular organic films of different viscoelastic properties were performed in the wind wave tank facility of the University of Hamburg. The wind speed dependence of the radar cross sections for X and Ka band was measured with upwind looking microwave antennas. It is shown that Marangoni damping theory, which describes the damping of water surface waves by viscoelastic surface films, is not the only damping mechanism in wind wave tank experiments where the wind sea is not fully developed. The other source terms of the action balance equation, i.e., the energy input into the water waves from the wind, the nonlinear wave-wave interaction, and the dissipation by wave breaking, are affected differently by the various substances. It is hypothesized that this difference is caused by the different viscoelastic properties of the substances, i.e., by the different intermolecular interactions of the film molecules. A slight dip in the wind dependence of the radar cross section at Ka band at wind speeds of 8-9 m/s was measured, which corresponds to comparable reductions of the mean squared wave height and wave slope. Polarization ratios (i.e., the ratios of the radar backscatter at vertical and horizontal polarization) higher than those predicted by simple Bragg scattering theory for X band at low wind speeds and different incidence angles are explained within a (three-scale) composite-surface model. At higher wind speeds, where the polarization ratio decreases rapidly, breaking by wedges and spilling breakers is hypothesized to become more dominant. The dependence of the polarization ratio on the coverage of the water surface with a slick is explained qualitatively by means of the composite-surface model. Finally, it is stated that wind wave tank measurements in the presence of monomolecular surface films are useful for the verification of theories concerning radar backscattering, wave damping, and wind-wave and wave-wave interactions.