Oceanic internal solitary waves (ISWs) can be generated by several mechanisms, among which by tidal flow over shallow bathymetry, atmospheric forcing, unbalanced sub-mesoscale flow without external forcing and rapid displacements of the sharp fronts separating e.g. cold ocean water masses or river plumes from the ambient seawater. In this paper, we analyze synthetic aperture radar (SAR) images acquired by the Sentinel-1 satellites over the Adventure Bank area, located on the western side of the Strait of Sicily in the years 2018-2021 showing numerous radar signatures of ISWs. This is the first time that massive presence of ISWs is reported in this area of the Mediterranean Sea. The internal wave field, observed primarily in the months from May to September, is very heterogeneous, with wavelengths ranging from a few hundred meters to a few kilometers, wave front lengths from a few km to about 40 km, and propagating in almost any direction. This suggests that the internal waves detected in the SAR images are generated by a variety of mechanisms and not by a specific one. Although in general tidal currents are weak in the Mediterranean Sea, the sum of the tidal flow and the Atlantic-Ionian Stream current can attain values up to 0.3 m s(-1) such that ISW generation by interaction with shallow underwater bottom topography becomes possible. Furthermore, cold filaments and sub-mesoscale cyclonic eddies generated by upwelling at the south coast of Sicily, feature sharp and rapidly moving fronts, from which ISWs may originate. Theoretical evidence for these two types of ISW generation mechanisms is provided by model results obtained from the 1/48 degrees resolution ENEA model and the 100 m resolution Delft3D model.

Radar signatures of rain over the ocean have a complex structure since they receive contributions from surface scattering and volume scattering and attenuation by hydrometeors in the atmosphere. These contributions overlap and are often difficult to detangle. While most of the mechanisms contributing to radar signatures of rain over the ocean are well understood, there is one remaining issue that has been discussed controversially in the literature for a long time. It is the question what scattering mechanism causes the areas of strongly enhanced radar backscatter, also called 'bright blobs' or 'bright patches', which are frequently observed on spaceborne C-band SAR images acquired over tropical and subtropical oceans in the presence of convective rain. Recently, papers have been published in which it is hypothesized that they are caused by radar backscattering at hydrometeors in the melting layer (ML). Although many observational facts seem to support this hypothesis, there exists one strong argument against this hypothesis: It is the observation that the position of the ML radar signatures (bright blob) in the SAR image is not shifted in anti-range from the position, where the rain column hits the sea surface. This absence of a shift is observed when 1) comparing Sentinel-1 SAR images on which rain cells are visible with quasi-concurrently acquired weather radar images and 2) when inter-comparing of SAR images of rain cells acquired concurrently at different frequencies and polarizations. Based on these observations, we discard the hypothesis that the bright blobs are due to volume scattering at hydrometeors in the ML and hypothesize instead that they are due to scattering at splash products at the sea surface. This hypothesis is supported by radar backscattering measurements carried out in the laboratory and from a shore-based platform, which show that, at C-and X-band, strong rain can give rise to strong radar returns also at cross-polarization.

Synthetic aperture radar (SAR) images acquired over the ocean often show radar signatures of rain, which are not easy to interpret. The scattering mechanisms causing radar signatures are usually attributed to surface scattering due to sea surface roughness variations caused by raindrops impinging onto the sea surface and/or by up- and downdraft winds. In this paper, we address another radar signature of rain, which is often observed in C-band (and also in X-band) SAR images, but whose origin has been a matter of debate in the ocean remote sensing community since long time and has not been solved yet. This radar signature consists of areas of very high radar backscatter (bright patches) at co- as well as well as at cross-polarization. This paper aims at providing evidence that it is not caused by surface scattering, but by volume scattering from wobbling, non-spherical, oblate hydrometeors within the melting layer. To this end, we first review the theory of radar backscattering from the melting layer as developed by D'Amico et al (1098) and then present historic radar backscatter data from the melting layer carried out by ground-based and airborne radars, which validate this theory. Then we show four representative Sentinel-1 SAR images acquired over the sea area close to Hong Kong and a SIR-C/X-SAR image acquired over the Gulf of Mexico, which show pronounced radar signatures of rain (bright patches) at co-polarization (VV) and cross-polarization (VH). The analysis of the SAR images yields the result that within the bright patches the ratio of the radar backscatter at cross-polarization to the one at co-polarization shows the same characteristics as the linear depolarization ratio (LDR) measured by radar meteorologist in radar backscattering from the melting layer. Furthermore, we show that radar signatures of rain due to volume scattering may interfere with co-polarization radar signatures of rain due to surface scattering. Thus, cross-polarization SAR images are better suited to detect radar backscattering from the melting layer than co-polarization SAR images, This investigation is of relevance for ocean surface wind retrieval using C-band SARs, since scattering at hydrometeors in the melting layer can cause significant errors in ocean wind retrieval. Areas with simultaneously high co- and cross-polarization NRCS values of around -10 dB and -20 dB, respectively, have to be flagged as areas where the conventional wind retrieval algorithm cannot be applied.

We present a semi-analytical model for predicting the breaking location of internal solitary waves (ISWs) over a sloping seabed. Our conceptual model is based on laboratory experiments, performed in a wave tank, that reproduce the ISW breaking mechanisms and show how the steepening of the trailing edge leads to verticalization of the wave profile during the shoaling phase. We derive the location of ISWs breaking, that is, the wave verticalization point, through two-layer, interfacial theoretical models and conservation of wave mass. We apply our model to the case of tidally forced ISWs that are generated in the Strait of Messina (Central Mediterranean Sea), where northward traveling ISWs are expected to refract and break over the frontal slope of Capo Vaticano. Our application is then assessed through numerical investigations, which allow to consider realistic field conditions in terms of water column stratification and geometrical setting. Our results, and the expected ISW-induced bed shear stress, suggest a link between the predicted breaking locations and the occurrence of sediment resuspension over that specific portion of the slope.

Breaking surface waves play a key role in the exchange of momentum, heat, and gases between the atmosphere and the ocean. Waves break at the ocean's surface at high or medium wind speeds or in the absence of wind due to shoaling of the seafloor. However, surface waves also break due to interactions with internal solitary waves (ISWs). In this paper, we revisit surface wave breaking caused by ISWs and how ISWs are manifested in synthetic aperture radar (SAR) images acquired by the TerraSAR-X and Sentinel-1 satellites and in high-resolution radar altimeter data acquired by the SAR altimeter (SRAL) onboard the Sentinel-3A satellite. X-band TerraSAR-X images acquired at low wind speeds suggest that meter-scale surface breaking waves resulting from large-scale ISWs are associated with large modulations in backscatter at HH and VV polarizations that cannot be explained by present theories. Furthermore, Sentinel-1 C-band SAR satellite images acquired at moderate to high wind speeds also exhibit large radar signatures from surface wave breaking at VV and VH cross-polarizations. Finally, new observations from the Sentinel-3 SRAL altimeter show clear evidence of significant wave height (SWH) variations along the propagation paths of ISWs. The SWH signatures are unique in showing that the surface wave energy does not return to its unperturbed level after an ISW passes, most likely because intense meter-scale wave breaking results in surface wave energy dissipation. In summary, these results show that surface wave breaking contributes significantly to radar remote sensing of ISWs.

In the wake of upwelling events, often small-scale and sub-mesoscale phenomena, like filaments, sub-mesoscale eddies and internal waves, are encountered in the upper ocean layer. Because of their small scale, they are hard to detect from space. Sometimes, under favorable weather and viewing conditions, they become detectable by high-resolution infrared and optical sensors. In this paper, we show that also synthetic aperture radars (SAR) is a useful sensor for studying these small-scale phenomena. To this end, we present some representative SAR images showing radar signatures of sub-mesoscale eddies and short-scale internal waves in the upwelling region south of Sicily.

Surface films floating on the sea surface are most often not mineral oil films, but surface films of biological origin (biogenic slicks) consisting of surface-active substances secreted by biota (e.g., plankton and fish) in the water column. They form monolayers (typically 3 nm thick) and damp short surface waves as strongly as mineral oil films, which implies that on SAR images the degree of reduction of the radar backscatter from both types of surface films are similar. Biogenic slicks are of relevance for climate studies since they reduce the transfer of greenhouse gases across the air–sea interface.

Synthetic aperture radar (SAR) is an ideal instrument to image oceanic features and atmospheric features in the marine boundary layer independent of the time of the day and of cloud conditions. The oceanic features include ocean swell, underwater bottom topography in tidal channels, internal waves, oceanic eddies, ship wakes, and oil pollution. The atmospheric features include coastal wind fields, wind fronts, atmospheric gravity waves, atmospheric boundary layer rolls, atmospheric vortices, polar lows, hurricanes/typhoons, and rain cells. Some examples of the features acquired by the SARs onboard the ERS, Envisat, and Radarsat-1 satellites are presented.

In studies of upwelling, usually data from infrared and optical sensors are used which provide information on the sea surface temperature (SST) and the chlorophyll-a (Chl-a) concentration. In this paper we show that also synthetic aperture radars (SAR) images can give valuable contribution to such studies. Upwelling regions become detectable by SAR because they are associated with a reduction of the radar backscatter due to 1) a change of the stability of the air-sea interface or/and 2) the presence of biogenic slicks.

We challenge the often-made claim that fully polarimetric synthetic aperture radars (SARs) are beneficial for discriminating between mineral oil films and biogenic slicks. We conjecture that the results obtained from previous analyses of spaceborne polarimetric SAR data, which seem to show differences in the scattering mechanism between radar scattering from mineral oil films and biogenic slicks, result from instrument noise. Measurements carried out with the Unmanned Aerial Vehicle Synthetic Aperture Radar (UAVSAR) of NASA/JPL, which has an extremely low noise floor, confirm this view and show that Bragg scattering theory applies also for scattering from mineral oil films. However, measured differences in the statistics of the radar backscattering from mineral oil films and biogenic slicks, may be real and not noise-related.

It is well known that rain leaves footprints on the sea surface that sometimes become visible on synthetic aperture radar (SAR) images. Rain cells can easily be detected on SAR images at all radar frequencies when they are associated with a downdraft pattern. But rain cells are not always associated with downdraft and rain can also occur in other forms, as stratified rain, rain bands, and squall lines. It turns out that radar signatures of rain at C-band are much more complex than at L- or X-band radar and that it is particularly difficult to identify unambiguously rain events on C-band SAR images acquired over the ocean. This is because C-band lies in the transition region where raindrops impinging onto the sea surface can increase (usually) or decrease the backscattered radar power and where volume scattering and attenuation by rain drops in the atmosphere are not always negligible (at very high rain rates). In order to get an insight into the physical mechanisms causing the C-band radar signatures of rain, we first revisit results obtained from historic laboratory and field experiments and multi-frequency/multi-polarization SAR data acquired during the SIR-C/X-SAR spaceshuttle mission in 1994. Then we analyze several C-band SAR images acquired by the European satellites Envisat and Sentinel-1A, and the Canadian satellite Radarsat-2 and compare them, whenever possible, with quasi-coincident and collocated weather radar images. The observational data show that, at low to medium rain rates, the main physical mechanism causing C-band radar signatures of rain is Bragg scattering at ring waves generated by the rain drops impinging onto the sea surface, which increase the radar backscatter. However, areas of increased radar backscatter are often accompanied by adjacent areas of decreased radar backscatter, which is due to attenuation of the Bragg waves by turbulence also generated by the impinging rain drops. Furthermore, we present a full-polarimetric Radarsat-2 SAR image of a rain cell together with a polarimetric decomposition analysis, which shows that the C-band radar signature of a rain cell is caused by surface scattering. The observation show that radar signatures of rain cells often contain segments, where the co-polarized as well the cross-polarized radar backscatter are strongly enhanced, which indicates non-Bragg scattering contributions to the scattering process. Furthermore, the polarimetric decomposition analysis shows that the C-band radar signature of a rain cell is dominated by surface scattering. Possible mechanisms, like scattering at splash products, are discussed. Whether the normalized radar cross section (NRCS) due to rain is increased or decreased depends on rain rate, wind speed, incidence angle, and history of the rain event. At low to moderate wind speeds (<10ms−1) and low to medium high rain rates (<50mmh−1), the NRCS is usually increased by up to 8dB, and at high wind speeds (>10ms−1) and low to high rain rates (but <50mmh−1), the NRCS is usually decreased by up to 3dB.

In the southeast Asian Waters, often meso-scale oceanic phenomena, like internal waves, oceanic fronts, eddies, upwelling, river plumes, and sea areas covered with mineral oil films or biogenic slicks, are encountered, which are detectable by synthetic aperture radar (SAR). SAR images are used, usually in conjunction with other remote sensing data and with numerical models, to study the dynamics of these phenomena. In particular, SAR data have been very instrumental in studies of the generation, propagation, and dissipation of internal waves in the South China Sea.

Envisat C-band SAR images acquired over the ocean have been screened for radar signatures of rain and compared with quasi-simultaneously acquired weather radar images, mostly provided by the Hong Kong Observatory. Compared to X- and L-band radar signatures of rain over the ocean, C-band radar signatures are much more complex because rain can increase or decrease the radar backscattering due to ring wave generation by the rain drops impinging onto the sea surface and due to wave damping by turbulence generated in the upper water layer by the impact of the rain drops. Which of these two mechanisms dominates, depends on rain rate, drop size distribution, wind speed, and the temporal evolution of the rain event. Stratiform rain usually causes an increase of the radar backscattering, while most of the other forms of rain cause a mixture of increased and decreased radar backscattering.

Wind fronts associated with cold-air outbreaks from the Chinese continent in the winter are often observed over the northern South China Sea and are well studied. However, wind fronts caused by another type of synoptic setting, the sudden increase or freshening of the north-east monsoon, which is caused by the merging of two anticyclonic regions over the Chinese continent, are also frequently encountered over the northern South China Sea. For the first time, such an event is investigated using multi-sensor satellite data, weather radar images, and a high-resolution atmospheric numerical model. It is shown that the wind front generated by the freshening of the north-east monsoon is quite similar to wind fronts generated by cold-air outbreaks. Furthermore, we investigate fine-scale features of the wind front that are visible on synthetic aperture radar (SAR) images through variations of the small-scale sea-surface roughness. The SAR image was acquired by the Advanced SAR of the European Envisat satellite over the South China Sea off the coast of Hong Kong and has a resolution of 150 m. It shows notches (dents) in the frontal line and also radar signatures of embedded rain cells. This (rare) SAR image, together with a quasi-simultaneously acquired weather radar image, provide excellent data with which to test the performance of the pre-operational version of the Atmospheric Integrated Rapid-cycle (AIR) forecast model system of the Hong Kong Observatory with respect to modelling rain cells at frontal boundaries. The calculations using a horizontal resolution with 3-km resolution show that the model reproduces quite well the position of the notches where rain cells are generated. The model shows further that at the position of the notches the vorticity of the airflow is increased leading to the uplift of warmer, moister air from the sea-surface to higher levels. With respect to the 10-km resolution model, the comparison of model data with the near-surface wind field derived from the SAR image shows that the AIR model overestimates the wind speed in the lee of the coastal mountains east of Hong Kong, probably due to the incorrect inclusion of the coastal topography.