F. Parmiggiani

SAR imaging of wave dispersion in Antarctic pancake ice and its use in measuring ice thickness

[1]xa0A synthetic aperture radar (SAR) image of the advancing winter marginal ice zone (MIZ) in the Antarctic, composed of frazil-pancake ice, has been analysed in a new way in order to test the predictions of a recently developed theory of wave dispersion in pancake ice which treats the ice as a viscous layer. In the image, obtained in April 2000, the structure of the wave spectrum in the MIZ and its change from the open-water spectrum are consistent with a pancake layer 24 cm thick. Intensive in situ measurements of the pancake ice in the MIZ 280 km W of the image location were made from FS Polarstern during a period covering the satellite imaging, and also yielded a mean ice thickness of 24 cm. We conclude that this technique gives realistic results for ice thickness, whereas earlier work based on a different dispersion theory (mass loading) tended to over-estimate thickness. After further validation, it is therefore possible that the SAR wave technique can become an accepted method for monitoring ice thickness in pancake icefields.

The Use of SAR to Measure Ocean Wave Dispersion in Frazil–Pancake Icefields

Abstract Icefields composed of frazil and pancake ice play important roles in both polar regions in winter. During the early to midwinter period pancake ice is a major component of the Antarctic sea ice cover, while in the Arctic the Odden ice tongue in the Greenland Sea, associated with deep convection, is composed mainly of pancake ice. The retrieval of sea ice thickness by remote sensing is, in general, a very difficult task. In this paper the change in dispersion of ocean waves as they penetrate into pancake ice is considered so as to gain insight into its possible relationship with thickness. Spectral analysis of subscenes from ERS-2 (second European Remote Sensing Satellite) synthetic aperture radar (SAR) images yields the wavelength and direction of the principal spectral component both outside and inside the ice cover. There is a decrease of wavelength at the ice edge, as well as refraction toward the normal and a loss of amplitude. The analysis is complex because the true wave spectrum must be re...

Mapping the Thickness of Pancake Ice Using Ocean Wave Dispersion in SAR Imagery

During the early to midwinter period pancake ice is a major component of the Antarctic sea ice cover, occupying a belt extending 200-300 km from the outer ice edge with an area of about 6 million km2. Experience in the Arctic suggests that the sea ice thickness in this region can be mapped by monitoring the penetration of ocean waves into it. Spectral analysis of subscenes from ERS-2 synthetic aperture radar (SAR) images yields the wavelength and direction of the principal spectral component both outside and inside the ice cover. There is a change of wavelength at the ice edge associated with a change in the wave dispersion, which can be quantitatively related to the thickness of the ice. The analysis is complex because the true wave spectrum must be retrieved from the SAR spectrum, which involves an inversion technique requiring a “first-guess” spectrum. The analysis technique is described, and the wave theory which predicts the change in wavelength. Experience in the Chukchi Sea and especially in the Odden ice tongue in the Greenland Sea is reviewed. Preliminary data from the Ross Sea have been analysed, yielding principal wave components, but the ambient wavelength was too long for the effect of ice to be detectable. The next application will be to the outer ice edge, when midwinter data from this region are available at the end of the current season.

Wave dispersion by frazil-pancake ice from SAR imagery

On the basis of a theory of wave propagation from open sea in frazil-pancake ice developed in Wadhams (1986), a project was carried out, during 1994-96 under the EU Environment Programme, to map ice thickness from SAR imagery in the Odden Ice Tongue, Greenland Sea. Ground truth data (ice thickness and composition, wind stress and local wave spectrum, etc.) were available from a field experiment carried out in the Odden during April 1993. Twenty six high-resolution SAR images for the same period and covering the same area were acquired and processed. A first, simple, procedure was developed to perform spectral analysis in order to obtain the change of wavelength and angle of refraction of the dominant wave entering the ice field. Ice thickness values derived from the theory were larger than those measured during the field experiment. The inversion scheme presented by Hasselmann et al. (1996) was then utilized to retrieve the ocean spectrum from the SAR spectrum. This was done for three scenes (April 3 and 10, 1993) for which wave spectra from buoy data were available. Variations of wavelength and propagation direction of the dominant wave systems are analyzed before and after they enter ice zones in order to: (i) evaluate the optimal conditions occurring for spectral changes detection with the ERS SAR system; and (ii) gain more insight into applicability limits of the inversion algorithm when applied to sea-ice scenes.

Eta Model Simulations and AMSR Images to Study an Event of Polynya at Terra Nova Bay, Antarctica

In the Terra Nova Bay (TNB) region, near-surface winds are persistently strong, in particular during the winter season and blow offshore with a high degree of directional constancy. This region is also known as a preferential zone of coastal polynyas. Polynyas are recurring areas of open water/thin ice surrounded by an ice-covered sea. Coastal polynyas form along ice-bound coasts; they are believed to be due to strong and persistent offshore winds and/or ocean currents which drive the sea ice away. As the ice is removed from the region of origin, open water is exposed and refrozen and the new ice is pushed away, so that coastal polynyas provide a source of new ice production. As the ice forms, much of the salt content in the freezing water is rejected, forming dense salty water, which tends to sink, eventually contributing to the deep ocean circulation. The horizontal surface temperature differences among the land ice, water, and sea ice are strong because the open water is close to the freezing point (−1.8°C for typical salt water). The energy exchange between the ocean and the atmosphere in the Antarctic marginal sea-ice zone is strongly influenced by the extent of sea-ice cover. While the sea ice acts as insulation, a direct contact between water and air is established in areas of open water, and intense energy exchanges occur due to the large difference of temperature between the water and the air above it. As a result, the polynya areas have an important impact on polar meteorology/climate.

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.

Spatial vegetation variation patterns in southern Italy as detected by AVHRR and MODIS observations

Spatial variations in Southern Italy vegetation production were analyzed between 1995 and 2005. The analysis was carried out using Normalized Difference Vegetation Index (NDVI) time series derived from NOAA AVHRR images, retrieved from DLR/EOWEB archives and corrected using an ad-hoc method. The correction method exploits more accurate MODIS NDVI maps which only became available on NASA archives in the last three years. The changes occurred is biomass production in the last decade were analyzed by only considering vegetation behaviour during the growing seasons. This approach improves the results by excluding from the analysis the winter months, during which vegetation exhibits a stationary behaviour, NDVI data are less significant and the biomass production is poor. The correlation between vegetation relative change patterns and terrain altitude, as derived from SRTM topographic data, was also investigated. This analysis shows the presence of different changing patterns such as: i) a vegetation production degradation in several areas with variations up to -1% ; ii) a systematic vegetation production increase over the Apennines up to 6% . These results, in agreement with temperature trends for the winter months in the last years highlight climate change processes occurring in the Mediterranean areas: temperature mitigation facilitates robust vegetation, like conifer stands, deciduous stands and in general Mediterranean maquis, i.e, the kind of vegetation which can be found in upland and mountain zones. On the contrary, particularly in the plains, plant foliation in the autumn-winter period is strongly affected by sudden low temperature peaks and by human activities.

NDVI fluctuations from 1995 to 2006 in South Italy and North Africa: a search for a climate change indicator

Seasonal and interannual vegetation trends in the last eleven years were analyzed for two macro-regions, South Italy and North Africa, in search for evidence of climate changes and associated desertification processes. The South Italy macro-region comprises Apulia, Campania, Basilicata, Calabria and Sicily regions, while the North Africa one covers the northern part of Lybia. Vegetation index data for the whole Europe and North Africa can be retrieved from the DLR archive of thematic maps in the form of monthly composite Normalized Difference Vegetation Index (NDVI) maps. The DLR archive dates back to 1995, thus the analysis could only be carried out for the last eleven years. The analysis of temporal vegetation variations was performed by implementing specific routines which provide objective measurements of vegetation trends and anomaly. Rainfall data for the same periods and geographic areas, were also analyzed in order to investigate the correlation between the two phenomena. Results for the two selected macro-regions, from 1995 to 2006, are presented and discussed. In a successive phase, this study will focus on distinguishing vegetation variations at regional level, in order to compare different local trends.

Analysis of polynya size fluctuations by means of active and passive microwave radiometers

Two novel microwave instruments, the Advanced Synthetic Aperture Radar (ASAR) on the Envisat satellite and the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) on EOS satellites, have recently become available to the scientific community. They offer new possibilites to study polynya size fluctuations: high-resolution ASAR images give a perfect view of polynya conditions but only a limited number of scenes is available in the ESA archive over a small area (circa 2 scenes per month); low-resolution AMSR-E images allow the detection of polynya extent on a day by day basis but with a much coarser accuracy. This study, while confirming the strong correlation between wind intensity and polynya area, suggests the feasibility of using ASAR images to calibrate the open water threshold in AMSR-E images for polynya size detection on a daly basis.

Wave and wind field extraction from ERS SAR imagery

This paper deals with the analysis of SAR imagery of the Mediterranean Sea to estimate the directional wave spectrum and the wind vector. As case study an ERS-2 SAR acquired on 13 November 1997 (orbit 13417, frame 2889) which includes Lampedusa Island in the Sicily Channel was selected. Lampedusa was chosen as test site because of its privileged location in the centre of the Mediterranean and because it hosts a fully equipped meteorological station. Besides, the selected SAR image shows a striking feature from which the wind direction can be reliably estimated.Wave field and wind vector from SAR image were compared with predictions from the WAM wave model and the wind output of the ECMWF atmospheric model, respectively. The retrieval of directional two-dimensional wave spectrum from SAR image was carried out by means of the classical Hasselmann & Hasselmann inversion scheme and the SAR image cross-spectrum methodology, respectively. Assuming the wind direction is known independently, SAR data was then analysed to retrieve the wind speed by using the predictions from empirical backscatter models, such as CMOD4 and CMOD-IFREMER. Wind vector retrieval results were validated against in situ measurements provided by the Lampedusa airport anemometer.