Geoscience Reference
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eruptions. We could more ambitiously imagine vertical soundings by transportable Doppler
radars from underneath wind-advected ash clouds to provide the vertical distribution of
both the particle concentration and fallout velocities, owing to their directive antenna and
range gating capability. Tracking the time evolution of these parameters would give access
to the internal dynamics of ash clouds and sedimentation processes. Terminal fall velocities
could in turn be related to the particle size distribution that could be compared with field
deposits and results from tephra dispersal models. Fallout measurements from ground
collectors and several continuously operating laser or microwave disdrometers would
improve understanding of the relationship between particle size and fall velocity, and the
spatial heterogeneities often observed in ground deposits.
Unfortunately, a comprehensive real-time technique that can provide the erupted mass
associated with the whole particle-size spectrum does not yet exist. This could only be
derived from a combination of complementary techniques. This argues for a synergetic
strategy for the assimilation of multiple datasets quantitatively describing the different parts
of a plume from the gas-thrust region through the convective region to the buoyant distal
cloud. As described in this chapter, the proximal region can be well quantified by radars,
and developments are expected with multiple/complementary frequencies and dual
polarimetry. The cross-correlation of data from radars and complementary passive remote-
sensing methods, particularly ground-based imagery (IR, VIS, UV), is also a potentially
powerful tool to retrieve crucial parameters like particle size distribution, gas and tephra
mass fluxes. Ground-based radars are not useful for long-term volcanic cloud tracking because
the large ash particles, that provide strong radar signals fall out soon after an eruption. C-band
radars, for instance, do not detect ash particles with diameter <1-100 m in drifting volcanic
clouds that can persist in the atmosphere for several days or more (Rose et al., 1995). Thus,
for the long-range tracking of ash clouds, satellite-based imagers (IR, VIS, UV, microwave)
bring an obvious synergetic contribution, along with MISR and lidars, more sensitive to
micron- to submicron-sized particles and aerosols, ceilometers, sun photometers, and DOAS
mainly for SO
2
. An important objective of future works should aim at comparing and
calibrating data from different instruments and/or acquired at different wavelengths,
always cross-validated with field data (e.g. Bonadonna et al., 2011, 2012; Donnadieu et al.,
2009b; Gouhier et al., 2011). Further coupling with other geophysical methods, in particular
seismic and acoustic, seems promising to investigate the eruptive behavior of a volcano
from down the conduit up through the magma-air interface where explosions occur, up to
the surface where the dynamics of the tephra emissions can be recorded. Finally, back to
figure 1, the synergetic integration of source-targeting Doppler radars, medium-range
weather radars (few tens-hundreds km), and satellite- and ground-based imagery,
combined with traditional monitoring networks and field methods, is a promising approach
to improve the assessment of ash plume-related hazards, the forecast from tephra dispersal
models and the mitigation of associated risks. In order to retrieve accurate eruption
parameters, future research should focus on joint measurements of ash plume characteristics
at different levels by means of complementary techniques. To this purpose, a good strategy
would be to (i) carry out well-targeted multi-method experiments on volcanoes showing
either recurrent activity or sudden resumption of activity, and (ii) operate long-term
observations at selected laboratory volcanoes having a well-instrumented monitoring
network.
