Retrieval of microphysical and morphological properties of volcanic ash plumes from satellite data: Application to Mt Ruapehu, New Zealand

A quantitative analysis of the properties of several Mt Ruapehu, New Zealand, ash plumes has been performed using multispectral satellite data from the AVHRR-2 and ATSR-2 instruments. The analysis includes: identification of the plume from background clouds using the 'reverse' absorption effect in the thermal channels; modelling and retrieval of particle sizes; determination of the plume height from cloud shadows, stereoscopy and meteorological data; and estimates of the mass of fine particles (radii less than 10 mum). A new spectral technique for identifying opaque, silica-rich ash clouds is demonstrated by utilizing the near-infrared (1.6 mum) and visible (0.67 mum) channels of the ATSR-2, and the optical properties of a simple volcanic cloud are presented for use in radiative transfer studies. It is found that the Ruapehu eruption cloud contained silica-rich ash particles with radii generally less than a few micrometres. The distribution of fine particles is monomodal with a dominant mode peak of about 3 mum radius. Mass loadings of fine particles are found to be in the range approximate to 1 to approximate to7 mg m(-3), and are consistent with estimates of mass loadings of volcanic clouds from eruptions of other volcanoes. The height of the plume top, derived from radiosonde data and plume-top temperatures in the opaque regions, was found to be between 7.5 and 8.5 km, while the plume thickness was estimated to be between 1.5 and 3 km. Cloud height derived from ATSR-2 stereoscopy on a different plume gave heights in the range 5 to 8 km. The results of this study provide important information on the optical properties of nascent volcanic eruption plumes. This information may prove useful in determining the potential effects of volcanic clouds on local climate, and in assessing any hazard to aviation.