Comparison between volcanic ash satellite retrievals and FALL3D transport model

Abstract

Because the large emission of gas and solid particles into the atmosphere, the volcanic eruptions represent one of the most important source of natural pollution. Among different gases (mainly H2O, CO2, SO2 and HCl), the volcanic clouds contain a mix of silicate-bearing ash particles in the size range 0.1μm to mm size or larger. Interest in determining the properties, movement and extent of volcanic ash clouds is an important scientific, economic, and public safety issue because the effects on environment, public health and aviation. In particular the problem to track in real time and forecast the volcanic cloud transport is the key task for the aviation safety problems and for the political decision making. Several encounters of en-route aircrafts with volcanic ash clouds have demonstrated the harming effects of ash particles on modern aircrafts (loss of power, failure of high-bypass turbine engines, abrasion of turbine blades, windscreens, fuselage, and Pitot static tubes). Alongside these considerations also the economical problem induced by an airport closure must be taken into account. Both security and economical requirements make essential a great effort to realize robust and affordable ash cloud detection and trajectory forecasting, combining remote sensing and modeling. In this work a quantitative comparison between Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals of volcanic ash cloud mass and Aerosol Optical Depth (AOD) with the FALL3D ash dispersal model has been performed. MODIS, aboard the NASA-Terra and NASA-Aqua polar satellites is a multispectral instrument with 36 spectral bands in the wavelength range from Visible (VIS) to Thermal InfraRed (TIR) and spatial resolution varying between 250 and 1000 m at nadir. The channels centered around 11 and 12 micron have been used for the ash retrievals through the Brightness Temperature Difference algorithm and MODTRAN simulations. FALL3D is a 3-D time-dependent Eulerian model for the transport and deposition of volcanic particles that outputs, among other variables, cloud column mass and AOD. In this work the Mt. Etna volcano 2002 eruptive event has been considered as test case. The results indicate a general good agreement between the mean AOT retrieved and the spatial ash dispersion in the different images, while the modeled FALL3D total mass retrieved result significantly overestimated.