Volcanic eruption plume top topography and heights as determined from photoclinometric analysis of satellite data

Photoclinometry, a shape-from-shading technique, is applied to satellite da ta to determine the three-dimensional height and morphology of the upper op tical surface of a plume from the eruption of Redoubt Volcano, Alaska. The technique can be applied to visible images of volcanic plumes that have uni form scattering properties and is most effective on images with moderate in cident Sun angles and plume transport perpendicular to the azimuth of the S un. The two most significant sources of error are the finite resolution of the image data and the choice of image data number (DN) value for a flat pl ume element. Provided the plume spans at least 10 image pixels, useful resu lts can be obtained for the central portion of the plume. The closest appro ximation to a flat plume element is found to be the visible DN value corres ponding to the coldest pixel along a transect. Errors in the element altitu de due to uncertainty in the DN value for a flat pixel amount to similar to 10-20 m per pixel and are cumulative along the plume transect. For Redoubt this results in an uncertainty of approximately 300 m at the highest point s. The method indicates that the Redoubt plume rose to an altitude of appro ximately 3 km while traveling a distance of 150 km from the vent and that t he surface topography of the plume exhibits influences of the ambient wind through simple wave structures. This technique will have a tremendous impac t on studies of plume spreading dynamics and the time-integrated evolution of plumes.