Volcanological Applications of Doppler Radars: A Review and Examples from a Transportable Pulse Radar in L-Band
Doppler Radar Observations - Weather Radar, Wind Profiler, Ionospheric Radar, and Other Advanced Applications
411 Ash plume monitoring Long-range trajectory tracking of ash clouds is achieved primarily by means of satellite imagery. Although Delene et al. (1996) showed the utility of a satellite-based microwave imager passively measuring radiations (19-85 GHz) of millimetric volcanic particles from an ash cloud of Mount Spurr in 1992, satellite visible-infrared radiometric observations from geostationary platforms are usually exploited (e.g., Rose et al., 2000) . The evolution of the ash cloud spatial
... istribution, in particular, can be imaged at intervals of 15-30 min. Important parameters can be further retrieved like the approximate plume height assuming thermal equilibrium with the atmosphere (non unicity of solutions for altitudes above the tropopause), and the concentration and size of distal particles (< 20 microns) transported in the atmosphere, assuming particle sphericity and vertically homogeneous concentration. Using these assumptions, the mass of SO 2 and ash can be integrated on successive images (e.g. Wen & Rose, 1994) . Scollo et al. (2010) also showed the potential of Multiangle Imaging SpectroRadiometer (MISR) working in four wavelengths in visible and near-infrared bands, for the 3-D reconstruction of ash plume shape, and for the retrieval of column height, optical depth, type and shape of the finest particles, among the most sensitive inputs for ash dispersal modeling. Yet, the exploitation of satellite images for monitoring purposes is limited by (1) the presence of clouds at higher levels, (2) an insufficient acquisition rate for event onset detection, (3) a relatively poor spatial resolution, (4) errors of the "split-window" method (brightness temperature difference) when the volcanic plume lies over a very cold surface or when the plume lies above a clear land surface at night where strong surface temperature and moisture inversions exist (Prata et al., 2001) . In addition, low ash content and/or small ash plumes might not be clearly observed and near-source emissions are obscured by the emitted tephra. For these reasons, ground-based radar systems represent an optimal complementary solution for real-time monitoring of these phenomena, by providing higher spatial resolution and data acquisition rates, as well as the ability to make observations at night and under any weather conditions. Real-time monitoring of ash plumes is crucial, in particular for the initialization of dispersion models. In this respect, essential input parameters such as plume height, mass flux, and particle concentration can be assessed quantitatively from radar data and directly contribute to improve ash dispersion forecasts. 2.2.