Volcanic Ash Cloud Properties: Comparison Between MODIS Satellite Retrievals and FALL3D Transport Model

The moderate Resolution Imaging Spectroradiometer (MODIS) is a multispectral satellite instrument operating from the visible to thermal infrared spectral range. FALL3D is a 3-D time-dependent Eulerian model for the transport and deposition of volcanic particles. In this letter, quantitative comparison between the volcanic cloud ash mass and optical depth retrieved by MODIS and modeled by FALL3D has been performed. Three MODIS images collected on October 28, 29, and 30 on Mt. Etna volcano during the 2002 eruption have been considered as test cases. The results show a general good agreement between the retrieved and the modeled volcanic clouds in the first 300 km from the vents. Even if the modeled volcanic cloud area is systematically wider than the retrieved area, the ash total mass is comparable and varies between 35 and 60 kt and between 20 and 42 kt for FALL3D and MODIS, respectively. The mean aerosol optical depth (AOD) values are in good agreement and approximately equal to 0.8. When the whole volcanic clouds are considered the ash areas, then the total ash masses, computed by FALL3D model, are significantly greater than the same parameters retrieved from the MODIS data, while the mean AOD values remain in very good agreement and equal to about 0.6. The volcanic cloud direction in its distal part is not coincident for the October 29 and 30, 2002 images due to the difference between the real and the modeled local wind fields. Finally, the MODIS maps show regions of high mass and AOD due to volcanic puffs not modeled by FALL3D.