Assessing RAT (Robust AVHRR Technique) performances for volcanic ash cloud detection and monitoring in near real-time: the 2002 eruption of Mt. Etna (Italy).

Performances of the recently proposed RAT (Robust AVHRR Techniques) approach in detecting and monitoring eruptive ash clouds by satellite have been assessed. The Mt. Etna (Sicily, Italy) eruption of 2002, producing intense ash emissions has been used as a study case. Potentialities and limitations of two different RAT configuration schemes (i.e. two and three channel based) have been investigated, both in terms of reliability (i.e. accurate detection) and sensitivity (detailed ash cloud properties description) and tested for several days after the eruption onset, also in different observational conditions. The main outcomes of this assessment study are: i) the more complete, three-channel scheme seems to offer accurate ash detection and discrimination from meteorological clouds with a high level of reliability, better than traditional satellite schemes; ii) on the other hand, the simplified RAT configuration performs well in ash cloud monitoring and tracking, providing a better description of cloud characteristics and properties (e.g. size, shape, direction, etc.). Consequently, a specific protocol has been suggested and experimented, with both the two configuration schemes applied in cascade; it provides a first, accurate and reliable ash detection and discrimination and, afterwards, a more detailed description of the previously identified eruptive cloud, also in terms of its internal structure and density. Preliminary, qualitative validations of the derived ash cloud ‘structure’, performed by comparison with some independent satellite observations, have also been briefly presented.