Giacomo De Carolis

On the Estimation of Thickness of Marine Oil Slicks From Sun-Glittered, Near-Infrared MERIS and MODIS Imagery: The Lebanon Oil Spill Case Study

The detection of marine oil slicks using satellite sun-glittered optical imagery has been recently assessed. As the nature of the imaging mechanism involves the altered features of the wind-roughened oil-covered sea surface, it is expected that the radiation reflected from the oil-water system carries information about the physical properties of the floating oil layer. In this paper, we report an investigation on the capability to retrieve the average thickness of thin marine oil slicks by using the sun-glittered component of the solar radiation in the near-infrared (NIR) bands of MEdium Resolution Imaging Spectrometer Instrument (MERIS) and MODerate Resolution Imaging Spectroradiometer (MODIS) images. The developed procedure exploits the Cox and Munk model to compute sun glint reflectance at the sea surface level for both clean and oil polluted sea surface as well. It is assumed that the Fresnel reflection coefficient of the oil-water system carries the relevant optical dependence on oil layer thickness and oil type. The expected oil-water system reflectance is computed by taking into account the non-uniform spatial distribution of the oil volume. This is achieved by considering a pdf of oil thicknesses that matches the observations on controlled oil slicks already reported in the scientific literature. MERIS and MODIS images gathered during the Lebanon oil spill occurred on July and August 2006 were selected as case study. When available, co-located SAR imagery was also considered to corroborate NIR-detected oil slicks.

Combined use of SAR and Modis imagery to detect marine oil spills

SAR spaceborne capability to detect marine oil spills through damping of wind-generated short gravity-capillary waves has been extensively demonstrated during past years. In contrast, it has not yet been found the optimal use of optical/NIR imaging sensors for detection and monitoring of polluted areas. We propose the use of Modis images acquired in sun glint conditions to reveal smoothed regions such as those affected by oil pollution. The underlying physical mechanism is based on the modification of the surface slopes distribution composing the roughened sea due to the action of mineral oils. The methodology is demonstrated for selected case studies in the Mediterranean Sea and North Atlantic where spills were detected by ERS SAR imaging. The corresponding Modis images acquired within a few hours were under sun glint conditions according to satellite imaging geometry and wind field distribution over the selected areas. Results of a detailed study about the effective applicability of the method is discussed. The importance of these results are based on the possible extensive exploitation of combined Modis and SAR data in view of the high repetitive coverage (about two times a day).

On the combined use of sun glint Modis and Meris signatures and SAR data to detect oil slicks

SAR spaceborne capability to detect marine oil spills through damping of short gravity-capillary waves has been extensively demonstrated during past years. In contrast, it has not yet been found the optimal use of VIS/NIR imaging sensors for detection and monitoring of oil polluted areas. We propose the use of Modis images acquired in sun glint conditions to reveal smoothed regions such as those affected by oil pollution. According to Cox and Munk model, the physical mechanism that allows detection of oil slicks under sun glint imaging of clear sea surface is based on the modification of the wind-generated wave slopes distribution due the action of mineral oils. The methodology is demonstrated for a number of case studies occurred in the Mediterranean Sea and North Atlantic from 2001 to 2004. For each case, the oil slicks were detected by ERS SAR imaging and the corresponding Modis images were acquired within a few hours the SAR passage under sun glint conditions. The implemented procedure compares the water-leaving Cox and Munk sun glint reflectance with the reflectance measured by Modis at the top of the atmosphere (TOA). To accomplish the task, the Modis imaging parameters and an estimate of the wind vector are provided as input. The ECMWF analysis wind fields are considered for the purpose. It was found that the ratio between the TOA reflectance and the C&M reflectance enhances the capability to detect oil slicks. Moreover, an extensive analysis of the atmospheric effects on oil slick detection has been carried out by performing simulations using the 6S code. Preliminary results show that atmosphere contribution to the reflectance has little impact on oil slick detection, so that implementation of a surveillance procedure could be envisaged.

On the synergistic use of SAR and optical imagery to monitor cyanobacteria blooms: the Curonian Lagoon case study

Abstract Multi-sensor satellite data are used to assess cyanobacteria blooms in the Curonian Lagoon. The exploitation of SAR, in combination with optical data, is investigated to take full advantage from the all-weather, night/day SAR imaging capability. A dataset of images has been analyzed to: 1) study the effect of cyanobacteria on microwave signals; 2) assess the daily evolution of cyanobacteria bloom from multi-sensors data; and 3) evaluate the dependence of dynamics of blooms on winds. The results show a significant correlation (R2 > 0.8, p<0.001) between the X-and C-band Normalized Radar Cross Section (NRCS) attenuation and the NIR-Red band ratio Index, with the latter considered as a proxy for the presence of cyanobacteria blooms. A combined use of microwave and optical observations can improve the detection of cyanobacteria blooms and their dependency on wind action.

LG-Mod: A Modified Local Gradient (LG) Method to Retrieve SAR Sea Surface Wind Directions in Marine Coastal Areas

This paper describes a novel SAR wind direction estimation method based on the computation of local gradients over quasi-linear and quasi-periodic structures detected by SAR imagery. The method relies upon the standard LG method for the part relevant to the computation of the local gradients. The novelty is that the dominant local wind direction and related accuracy are estimated using results derived from the Directional Statistics. The LG-Mod is validated against in situ coastal wind measurements provided by instrumented buoys with 63 ENVISAT ASAR images. Results show an overall agreement with RMSE values obtained for off-shore areas, but residual effects due to the complex phenomena occurring in the proximity of shoreline may degrade the performance when running in automated mode.

Retrieval of ocean wave spectra from ERS synthetic aperture radar image cross spectra

The retrieval of ocean wave spectra from ERS SAR image cross spectra is addressed in order to assess their potential to estimate the thickness of thin sea ice such as frazil and pancake ice. The inversion procedure based on the gradient descent algorithm, already demonstrated for airborne SAR data, is exploited and the capability of this method when applied to satellite SAR sensors is investigated. In fact the major differences between the two imaging situations lie in the illumination geometry and azimuth integration time. The SAR- ERS SLC image acquired on April 10, 1993 over the Greenland Sea was selected as test image. A couple of windows that include open sea only and sea ice cover, respectively, were selected. The inversions were carried out using different guess wave spectra taken from SAR image cross spectra. Moreover, care was taken to properly handle negative values eventually occurring during the inversion runs. This results in a modification of the gradient descending technique that is required if a non-negative solution of the wave spectrum is searched for. Results are discussed in view of the possibility of SAR data to detect ocean wave dispersion as a means for the retrieval of ice thickness.

Synergic use of SAR imagery and high-resolution atmospheric model to estimate wind vector over the Mediterranean Sea

An experiment whose aim is the retrieval of surface wind fields from SAR imagery coupled to a high resolution mesoscale numerical atmospheric model in semi-enclosed sea basin, is presented. A sea region belonging to North-Western Mediterranean Sea, which spans in W-E direction from Corsica (8.8 E) to Italian coast (10.5 E) and in N-S direction from Lygurian Sea (44.0 N) to North Tyrrhenian Sea (42.2 N), was selected as test area. Two consecutive ERS-2 SAR frames from the pass of March 30, 2000, along with a set of NOAA/AVHRR and MODIS images acquired on the same day were used for the analysis. SAR wind speeds and directions at 10 m above the sea surface were retrieved from predictions of the semi-empirical backscatter models CMOD4 and CMOD-IFREMER, which describe the dependency of the normalized radar cross section (NRCS) on wind vector and ERS-2 SAR image geometry. Surface wind vectors predicted by the meteorological ETA model were exploited as guess input to SAR wind inversion procedure to describe atmospheric conditions in the area, according a Bayesian approach recently proposed in literature. ETA is a three-dimensional, primitive equation, grid-point operational model running at the National Centers for Environmental Prediction of the U.S. National Weather Service. The model was adapted to run on selected regions of the Mediterranean basin with a nested very high, up to about 4.0 Km, resolution. The latter feature makes ETA model particularly suitble for its use in combination with SAR images. Besides, to simulate and predict several specific atmospheric weather phenomena, ETA outputs also include the vertical distributions of physical parameters such as air pressure, temperature, moisture up to about 25 Km. Apart some discrepancies in sparse and small areas, an overall agreement between SAR inversion results and ETA predictions was found. More importantly, it was found that the inversion methodology was not able to resolve wind speed modulations due to the manifestation of an atmospheric gravity wave, which occurred in the analyzed area as a result of the terrain disturbance to the air flow imposed by the peninsula located North of Corsica. Temporal evolution of the wave propagation phenomenon was allowed by inspection of NOAA/AVHRR and MODIS images through the detection of a cloud band associated to the atmospheric wave. A wave propagation model describing waves in the atmosphere owing to the disturbing action on the primary air flux by terrain features was thus used to account for the observed surface wind speed modulation on SAR image. Synergy with ETA model outputs was further exploited as atmospheric parameters up-wind the atmospheric wave were considered as input to the wave propagation model.

Preliminary study for a spaceborne ultrawideband microwave radiometer for the monitoring of cryosphere elements: The cryorad project

A new space-borne mission based on a multi-channel microwave radiometer in the frequency range 0.5–2 GHz as been recently approved by the Italian Space Agency for a preliminary concept study. The innovative instrument will be devoted to the study of the Cryosphere and in particular on sea ice volume, ice sheet temperature and soil status. Scientific objectives and first mission concept are presented here and in depth discussed at the conference.

Quantitative analysis of Bora events in the Adriatic Sea by means of SAR-based techniques and the ETA model

Bora is the north-eastern wind which, blowing in the Adriatic Sea from NE and interacting with the orography of the Croatian Dinaric Alps, is characterized by multiple surface intense wind jets [1]. The exceptional Bora events of the beginning of February 2012 over the Gulf of Trieste have been studied by means of two ASAR Wide Swath (WS) images, acquired on the 2nd and 5th of February. Two different methods to extract wind direction from SAR images have been exploited to catch the spatial dynamic of this extreme phenomenon. In particular, SAR sea surface wind directions obtained with Local Gradient (LG) method [2] and the relatively novel technique based on the use of 2D continuous wavelet transform (CWT2) [3] have been analyzed. The retrieved wind directions extracted by the above mentioned methods have been used to estimate the wind speeds with a classical inversion procedure applied to the forward semi-empirical backscatter model CMOD-5 [4]. Results will be compared with wind fields simulated by the atmospheric ETA model [5].

Estimation of wave and wind fields from spaceborne SAR imagery in the Mediterranean Sea

This paper is focused on the analysis of SAR imagery of the Mediterranean Sea to estimate the directional wave spectrum and the wind vector. It is discussed the potential of using fetch–limited wave spectral parameterisation, which is currently used to represent wind generated waves, in the case of SAR imaging of swells. As the interest is also focused on the role of the SAR–estimated wind vector to get reliable estimates of the wave spectra, the spectral form used in this study is that due to Donelan et al.. ERS–1 SAR imagery, which is co–located with instrumented directional buoys belonging to the Italian Buoy Network (RON, Rete Ondametrica Nazionale), was exploited. The main purpose is to study SAR capability to discriminate between locally generated wind waves and old wave systems. Three case studies were selected and analysed. They are devoted to compare wave spectral properties typical of swell systems observed at buoy stations located off–shore La Spezia, Ponza island and Alghero with SAR inverted directional wave spectra.This paper is focused on the analysis of SAR imagery of the Mediterranean Sea to estimate the directional wave spectrum and the wind vector. It is discussed the potential of using fetch-limited wave spectral parameterisation, which is currently used to represent wind generated waves, in the case of SAR imaging of swells. As the interest is also focused on the role of the SAR-estimated wind vector to get reliable estimates of the wave spectra, the spectral form used in this study is that due to Donelan et al.. ERS-1 SAR imagery, which is co-located with instrumented directional buoys belonging to the Italian Buoy Network (RON, Rete Ondametrica Nazionale), was exploited. The main purpose is to study SAR capability to discriminate between locally generated wind waves and old wave systems. Three case studies were selected and analysed. They are devoted to compare wave spectral properties typical of swell systems observed at buoy stations located off-shore La Spezia, Ponza island and Alghero with SAR inverted directional wave spectra.