A retrieval of tropospheric volcanic ash from Mt Etna has been carried out, using measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS). The NASA-MODIS satellite instrument acquires images in the 0.4 to 14 µm spectral range with a spatial resolution of 1 km at nadir. The eruption which occurred on 24 November 2006 is considered as a test case in this work. In order to derive the ash plume optical thickness, the particle effective radius and the total mass, the Brightness Temperature Difference procedure has been applied to MODIS channels 31 (centered at 11 µm) and 32 (centered at 12 µm). Channel 5 (centered at 1.24 µm) has been used to refine the cloud discrimination, exploiting the distinct reflectivity of meteorological and volcanic clouds in the near infrared spectral range. The detection of volcanic ash pixels has been significantly improved by applying an atmospheric water vapor correction to MODIS data. This procedure doubles the number of pixels identified as containing volcanic ash compared to the original method. The retrieved mean ash optical thickness at 0.55 µm, mean particle effective radius and the total ash mass in the plume are 0.4, 3.5 µm and 3620 tons, respectively. A detailed sensitivity analysis has been carried out to investigate errors in the retrieval caused by the uncertainty in various parameters: surface temperature and emissivity, plume geometry (altitude and thickness), ash type and atmospheric water vapor. Results show that the largest contributions to retrieval errors are from uncertainty in surface parameters, aerosol type and atmospheric water vapor. The total tropospheric volcanic ash retrieval errors are estimated to be 30%, 30% and 40% for mean AOT, mean effective radius and total mass retrieval, respectively.
The volcanic ash detection procedures are based on Brightness Temperature Difference (BTD) algorithm using the
thermal infrared channels centred around 11 and 12 microns of a multispectral satellite sensor. The Mie code has been is
included in the retrieval procedure to compute the ash optical properties from the ash microphysical characteristics. The
simulations has been carried out using MODTRAN radiative transfer model. The Nasa-Modis and the Noaa-Avhrr
measurements of Mt. Etna eruptive plume occurred in November 2006 have been analyzed to retrieve the plume optical
thickness, the particle effective radius and the size distribution.
The Spinning Enhanced Visible and Infra Red Imager (SEVIRI) radiometer, on board on Meteosat Second Generation
(MSG) geostationary satellite, collects, each 15 minutes, images of the underneath part of the globe in 12 spectral bands
with a spatial resolution of 3 km. In this work the Aerosol Optical Thickness (AOT) retrieval over land using SEVIRI
data is presented. AOT at 0.55 μm is estimated minimizing the difference between measured and computed radiances in
the visible channel centered at 0.6 μm by means Look-Up Tables (LUT) obtained using 6S radiative ransfer code. The
0.6 μm surface reflectance has been computed using different procedures based on SEVIRI channels 3 and 4 centered
respectively around 1.6 and 3.9 µm. For the 0.6 μm surface reflectance retrieval using the 1.6 μm procedure, the
measurements of five automatic sun-photometers of the Aerosols Robotic Network (AERONET) located in the
Mediterranean area (Avignon, Laegeren, Modena, Rome and Lecce) has been used. The procedures show encouraging
results in case of 1.6 μm procedure retrieval and the inadequacy of 3.9 μm procedure. An AOT map of the Po Valley
(Italy), obtained from an MSG image taken during a typical winter polluted day, is shown in the paper and compared
with MODIS retrieval.
Remote sensing data acquired by satellite or airborne sensor need on ground validation measurements. As concern volcanoes monitoring, important information may be retrieved by observing these targets in the InfraRed spectral range.
A portable μFTIR (Fourier Transform Infrared Interferometer) capable of making sensitive and accurate measurements of radiance and emissivity of surface in the (600-5000 cm-1) spectral range with a spectral resolution of 2 cm-1 is available at the remote sensing laboratory of INGV (Rome). These kinds of measurements are very important firstly for the validation of remote sensed data and secondly for the improvement of many gas models used in volcanology for the diagnosis of volcano inner state. On 2003 μFTIR in situ spectral emissivity measurements were made during field surveys on selected test sites on Mount Etna. This area was observed also by a Fourier interferometer (MIROR) on board on a Dornier 228 and by ASTER a satellite borne sensor. The MIROR and ASTER data have been calibrated and compared with ground measurements. The agreement suggested to organize periodic measurements on selected test sites of Italian volcanic regions e.g. Solfatara and Stromboli volcano.
The new generation of instruments to measure the atmosphere gives a large amount of information whose content is not immediately obvious. For this reason the information content (in the Shannon sense) and the number of degrees of freedom for signal related to parameters to retrieve, are crucial to carry out, for instance, the selection of microwindows or subset of channels for retrieval. This seems to be even true for the miniaturized devices like those obtained by MEOMS technologies that have high flexibility because are highly integrated. In this paper we present the methodologies to obtain the information content related to wavelengths selection from satellite with limb and nadir view with the MEOMS technology in mind.
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