Volcanic ash cloud detection from space: a comparison between RSTASH technique and water vapour corrected BTD procedure

摘要

Volcanic eruptions can inject large amounts (Tg) of gas and particles into thetroposphere and, sometimes, into the stratosphere. Besides the main gases (H2O,CO2, SO2 and HCl), volcanic clouds contain a mix of silicate ash particles in thesize range from 0.1 mm to 1 mm or larger. The interest in volcanic ash detection ishigh, particularly because it represents a serious hazard for air traffic. Particleswith dimensions of several millimetres can damage the aircraft structure(windows, wings, ailerons), while particles less than 10 mm may be extremelydangerous for the jet engines and are undetectable by the pilots during night or inlow visibility conditions. Furthermore, ash detection represents a critical steptowards quantitative retrievals of plume parameters. In this paper two differentsatellite techniques for volcanic cloud detection and tracking are compared,namely a water vapour corrected version of the brightness temperature difference(BTD-WVC) procedure and an implementation of the robust satellite technique,specifically configured for volcanic ash (RSTASH). The BTD method identifiesvolcanic ash clouds on the basis of the brightness temperature differencemeasured in two infrared spectral bands at around 11 and 12 mm. To account forthe atmospheric water vapour differential absorption in the 11–12 mm spectralrange, which tends to reduce (and in some cases completely mask) the BTDsignal, a water vapour correction procedure has been developed (BTD-WVC),based on measured or synthetic atmospheric profiles. RSTASH instead, is based onthe analysis of a time series of satellite records, aimed at identifying signalanomalies through an automatic unsupervised change detection step. To assessthe performance of the BTD-WVC and RSTASH methods in detecting volcanicash clouds, some eruptive events of Mt Etna, observed by the Advanced VeryHigh Resolution Radiometer (AVHRR) sensor, have been analysed. Theobtained results show a good agreement between the BTD-WVC and RSTASHtechniques for all the considered images, in terms of pixels detected as ‘ashaffected’ (i.e. the ash cloud area). In particular, compared to the traditional BTD procedure, the BTD-WVC and RSTASH techniques significantly improve volcanicash cloud detection, both in daytime and night-time data, especially in the case oflow ash loading.