Systematic analysis of volcanic ashclouds over a large range of scales using integrated satellite sensors

Volcanic ashclouds pose increasing hazards to air traffic. To enable accurate detection, tracking, and ultimately nowcasting (ie. near-real-time tracking and short-term forecasting), it is becoming increasingly important to be able to retrieve, in addition to information on SO2 (and aerosols) and ash (ice) mass, and ash (ice) size, key information such as cloud peak height, average height and height of the leading cloud edge, intensity changes, lateral and downwind spreading, ash/gas separation, evidence of microphysical changes (eg. mixed-phase aggregation), duration and cloud thickness. Here, we have systematically examined radar data and satellite data from a variety of sensors in combination with field and meteorological information (eg. balloon soundings) for a number of ashclouds covering a large range of spatial and temporal scales, as well as a large range of eruption intensity and meteorological conditions. We illustrate that the analysis of detailed surface features such as the overshoot geometry, cloud top gravity waves and cloud top or cloud edge Kelvin-Helmholz (K-H) shear billows can supply key additional information, compared to previous studies. We document that the overshoot size allows extraction of an independent estimate of cloud-top average height, that the number and characteristics of gravity waves allow to derive another independent estimate of column height as well as information on the height difference between peak and neutral buoyancy heights (Ht-Hb). These estimates are compared with estimates of cloud edge height extracted using the shadow method and also comparisons with wind profiles data, and also lead to the identification of ash/gas separation in some cases. The gravity waves can be understood by analogy with fluid mechanics modelling of mixed region collapse in a stratified fluid. Conditions for the development of K-H billows are also considered and we show how they can be used to identify the transition from the gravity-advection flow phase and the advection-diffusion phase of spreading. We also discuss their potential use in deriving ashcloud thickness from the quasi-2D satellite images. This, together with the information from ash-gas separation and gravity waves, enables to extract 3D information from quasi-2D satellite images. Implications of this work for generic understanding of processes in ashclouds and for aircraft safety are briefly discussed.