MT. ETNA TROPOSPHERIC ASH RETRIEVAL AND SENSITIVITY ANALYSIS USING MODERATE RESOLUTION IMAGING SPECTRORADIOMETER MEASUREMENTS

A retrieval of tropospheric volcanic ash from Mt Etna has been carried out, usingmeasurements from the Moderate Resolution Imaging Spectroradiometer (MODIS). TheNASA-MODIS satellite instrument acquires images in the 0.4 to 14 μm spectral range with aspatial resolution of 1 km at nadir. The eruption which occurred on 24 November 2006 is consideredas a test case in this work. In order to derive the ash plume optical thickness, the particleeffective radius and the total mass, the Brightness Temperature Difference procedure has beenapplied 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 distinctreflectivity of meteorological and volcanic clouds in the near infrared spectral range. The detectionof volcanic ash pixels has been significantly improved by applying an atmospheric watervapor correction to MODIS data. This procedure doubles the number of pixels identified ascontaining volcanic ash compared to the original method. The retrieved mean ash optical thicknessat 0.55 μm, mean particle effective radius and the total ash mass in the plume are 0.4, 3.5μm and 3620 tons, respectively. A detailed sensitivity analysis has been carried out to investigateerrors in the retrieval caused by the uncertainty in various parameters: surface temperatureand 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 surfaceparameters, aerosol type and atmospheric water vapor. The total tropospheric volcanic ash retrievalerrors are estimated to be 30%, 30% and 40% for mean AOT, mean effective radius andtotal mass retrieval, respectively.