Ferdinando Reale

An X-Band Radar System for Bathymetry and Wave Field Analysis in a Harbour Area

Marine X-band radar based systems are well tested to provide information about sea state and bathymetry. It is also well known that complex geometries and non-uniform bathymetries provide a much bigger challenge than offshore scenarios. In order to tackle this issue a retrieval method is proposed, based on spatial partitioning of the data and the application of the Normalized Scalar Product (NSP), which is an innovative procedure for the joint estimation of bathymetry and surface currents. The strategy is then applied to radar data acquired around a harbour entrance, and results show that the reconstructed bathymetry compares well with ground truth data obtained by an echo-sounder campaign, thus proving the reliability of the whole procedure. The spectrum thus retrieved is then analysed to show the evidence of reflected waves from the harbour jetties, as confirmed by chain of hydrodynamic models of the sea wave field. The possibility of using a land based radar to reveal sea wave reflection is entirely new and may open up new operational applications of the system.

Remote Sensing of Small-Scale Storm Variations in Coastal Seas

ABSTRACT Reale, F.; Dentale, F.; Pugliese Carratelli, E., and Torrisi, L., 2014. Remote sensing of small-scale storm variations in coastal seas. Estimating extreme values of significant wave heights (SWH) is a necessity in many branches of coastal science and engineering. Storm intensity, however, is not a smooth varying quantity, but it oscillates with random variations around a generally regular trend; the estimated value of extreme sea states is, therefore, necessarily affected by the sampling time of the available data. This is especially important when making use of synthetic data deriving from weather and wave simulation systems, which artificially smoothen the SWH record history. Active remote sensing provides valuable help to overcome this problem: the work described here very briefly recalls the available satellite SWH and wind measurements and shows how such data may help clarify and reduce a possible cause of error in wave climate evaluation, and especially so along coastal areas.

Numerical Simulation of Whitecaps and Foam Effects on Satellite Altimeter Response

The determination of wave height by active satellite remote sensing, be it Synthetic Aperture Radar (SAR) or altimeter, has been a common practice for many years and is now imbedded on many meteorological and oceanographic forecasting systems. Despite their differences, all active sensors are based on the measurement of the Normalized Radar Cross Section (NRCS) of the sea surface, i.e., of its backscattering properties, which in turn depend on the wind velocity. At small and moderate wind speeds, the main mechanism is the formation of ripples (small scale waves); at higher speeds, whitecaps appear, and foam starts playing an essential role in determining NRCS. In the past few years much research effort has gone into clarifying these effects, thus improving the general quality of the measurements. Little work, however, has been devoted so far to consider the vertical spatial variation of backscattering properties, and in particular of the floating foam, over the sea surface. As it is shown in the present paper, the shape of the backscattered electromagnetic impulse in radar altimeters depends on the spatial distribution of foam over the water height in the sea waves and therefore the performance of these instruments in determining Significant Wave Height (Hs) and Sea Surface Level (SSL) is strongly affected by this effect. This work tackles these problems by making use of specially implemented numerical algorithms to simulate both sea surface processes and radar altimeter techniques. Results show that some causes of errors can be better understood and eventually corrected: in particular, the paper deals with the reconstruction of the electromagnetic Sea State Bias (SSB), the well known altimeter ranging error due to the presence of ocean waves on the sea surface.

On the Effects of Wave-Induced Drift and Dispersion in the Deepwater Horizon Oil Spill

The objective of this work is to provide an indication of the effects of wave-induced movement of 10 oil on the sea surface in connection with the Deepwater Horizon oil spill. By making use of 11 modeled wave fields, satellite altimeter and buoy data, mean trajectories and wave-induced oil 12 spreading are computed for some of the storm events which took place during the accident. The 13 effects of mean Stokes’ drift are confirmed to be an important element in most situations, causing 14 spill movements of 30 km and more in about 5 days. The diffusion due to random wave movement 15 is also shown to be relevant at least for smaller spills; for large accidents, its effects are less 16 important, but it still has an influence on some aspects of the oil spreading. 17

Estimating Nearshore Bathymetry from X-Band Radar Data

In the last few years, a number of land- and ship-based X-band radar systems have been developed to collect information on the sea state, i.e., on the significant wave height as well as on its spectral distribution. More recently, new techniques have been developed to estimate shallow water bathymetry, by taking into account the effects of depth on the dispersion of gravity waves. This chapter is aimed at reviewing such state-of-the-art algorithms and at providing some examples of applications of the most recent technique. It is shown that the accuracy is fairly adequate for shallow water, but it decreases significantly as the depth increases; besides, because the effects of stationary or nearly stationary currents interfere with the analysis, it is necessary to make use of algorithms that can estimate both depth and currents simultaneously.

Influence of Sea State on Sea Surface Height Oscillation from Doppler Altimeter Measurements in the North Sea

This paper reports on an investigation on the influence of waves on the sea surface height error, σh, as measured by Delay Doppler satellite altimetry (DDA). CryoSat-2 altimeter sea surface height (SSH) data in the North Sea, processed in both DDA and pseudo low resolution mode (PLRM), are correlated with European Centre for Medium-Range Weather Forecasts (ECMWF) co-located sea state data. We find a small, but consistent correlation between the 1 Hz standard deviation, σh, of the 20 Hz altimeter SSH and the ECMWF total significant wave height, SWHt. The same analysis carried out between σh and the swell component of the wave spectrum shows a smaller correlation. In contrast, the correlation between the PLRM σh and any component of the SWH spectrum has not been found to be significant. To provide an explanation of these results, the aliasing effect caused by the interaction between the sea wavelength and the altimeter resolution has been considered; a simple model has, therefore, been produced to simulate the dependence of the aliasing-derived, σA, on the sea wavelength. The alias/wavelength curve obtained helps to explain why—at least for the relatively low wavelength sea data considered—the wave direction and its wavelength have little or no influence on σh.

Integration between X-Band Radar and Buoy Sea State Monitoring

ABSTRACT Ludeno, G.; Reale, F.; Raffa, F.; Dentale, F.; Soldovieri, F.; Carratelli, E.P., and Serafino, F., 2018. Integration between X-band radar and buoy sea state monitoring. This article presents the results of an integrated buoy and X-band radar sea state monitoring activity carried out on the southern coast of Sicily. The work involved the integration of buoy and radar data, as well as the simultaneous acquisition of significant wave height (SWH) values from two similar radar sets located at a slight distance from each other—a rare and fortunate circumstance that took place during two storms in the winter 2014–2015. Good consistency and repeatability was reached between the two radars, and the reliability of X-band radar as a wave monitoring system was confirmed by the comparison with the buoy wave meter. Some knowledge has also been gained on the short spatial and temporal fluctuations of the sea state: while such “gustiness” or small scale storm variation (SSSV) cannot be easily discriminated from electromagnetic effects and from algorithm artefacts, some important progress has been made toward the identification of this phenomenon. Integration of various sensors is the key to a definite improvement of sea state monitoring for most coastal applications.