Spaceborne EO and a combination of inverse and forward modelling for monitoring lava flow advance.

Kurzfassung

Spectral emissivity is a seldom measured parameter, and it is generally assumed to be a constant. However, by ignoring the variation of emissivity with temperature for spaceborne data and modelling applications, we demonstrate that substantial uncertainties in determination of land surface temperatures (LST) are introduced. Errors in computation of lava surface temperature consequently produce errors in radiant fluxes and forecasted lava flow lengths. We aim here to improve the understanding of the relationship between emissivity and temperature by carrying out a multi-stage experiment for the 2017 Mt Etna (Italy) eruption, by combining laboratory and spaceborne and numerical modelling data. Our laboratory-based Fourier Transform Infra-red (FTIR) results indicate that emissivity and temperature are inversely correlated, which supports the argument that emissivity of molten material is significantly lower than that of the same material in its solid state. Recent research suggests that emissivity of molten lava may be as low as 0.60. Our forward-modelling tests using MAGFLOW Cellular Automata suggest that a 35% emissivity variation (0.95 to 0.60) can produce up to 46% overestimation in simulated/forecasted lava flow lengths (compared to actual observed). In comparison, our simulation using a ‘two-component’ emissivity approach compares well with the actual 2017 lava flow lengths. We evaluated the influence of variable emissivity on lava surface temperatures using spaceborne data by performing several parametrically controlled assessments, using both constant (‘uniform’) and a ‘two-component’ emissivity approach (i.e. for melt and cooled lava). Computed total radiant fluxes, using the same spaceborne scene (Landsat 8), indicate a ≤15 % difference between emissivity endmembers (i.e. 0.95 and 0.60). These results further suggest a radiant flux variation/uncertainty bordering at lower boundary values of the moderate-to-high temporal resolution spaceborne data (MODIS & SEVIRI), acquired for the same target area (and the same time interval). These findings may have considerable impact on civil protection decisions made during volcanic crisis involving lava flows as they approach protected or populated areas.