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preferential directions during long periods of activity, transportable Doppler radars can be
used to track this type of activity in order to assess the state of the activity, the stability of
the lava flow or dome, and better understand the destabilization processes (Hort et al.,
2006). They could be further used to correlate the rock fall activity, linked to the lava
effusion rate, to the emissions of tephra. As both types of event could be recorded by the
radar, such a comparative quantified analysis over a representative time sequence spanning
several weeks could provide information on the eruptive behavior of the volcano and the
dynamics of its upper plumbing system.
8. Conclusion and future prospects
Illustrations provided in this chapter show the many capabilities of Doppler radars to
investigate and monitor volcanic phenomena in real-time and in all weather conditions. The
L-band portable volcano Doppler radar of the OPGC is particularly useful for monitoring
explosive activities of variable intensity from a chosen location. Owing to its 23.5 cm
wavelength, VOLDORAD is able to sound the interior of dense particle-laden volcanic jets.
By directly probing the jets near the vent, quantified eruption source parameters can be
retrieved in different volumes with a high spatiotemporal resolution.
A major challenge in the mitigation of ash plume-related risks is to determine the relative
mass flux of volcanic material propelled into the atmosphere, in particular the mass
transport rate in the cloud at the neutral buoyancy level. The mass transport rate represents
only a fraction of the total flux of magma erupted at the vent (magma mass eruption rate),
which also includes the effusive activity (lava flows, lava dome) as well as all the lava falling
back into the crater and its immediate surroundings contributing to the growth of
pyroclastic cones (e.g. ballistics). The proportion of material propelled into the atmosphere
strongly controls the hazards to humans and infrastructures, the economic costs and
environmental consequences, but also represents an essential input to volcanic ash transport
and dispersion models. Its estimation in near real-time could allow models to be constantly
refined by comparing their predictions with measurables from ground-based and satellite
remote sensing methods and ground deposit data. Model inputs from deposit observations
(e.g. thickness) to quantify eruption characteristics cannot be done in real-time as this
requires the collection of many fallout samples at remote locations. Although models allow
the tephra mass flux to be estimated from the ash column height, the latter needs to be
accurately measured and defined, in particular with regard to the strong effect of
crosswinds. This can be achieved most reliably through a combination of methods, but
radars appear particularly relevant in this case. Weather radars can provide the plume
height, within the uncertainties discussed in section 2, along with characteristics of the ash
cloud including transport speed estimates. Compact Doppler radars sounding the plume
base at a high acquisition rate should help discriminate the mass eruption rate from the
mass transport rate and link models of plume ascent to models of tephra dispersal by
providing crucial source kinetic and mass loading parameters. The radar echo power is also
related to the amount of material ejected and can be inverted to retrieve the mass. Among
the most stringent assumptions for this is the particle size distribution in the sounded
volumes. It can be inferred from direct measurements of fallout (rain radar, disdrometer etc)
or deposit sampling, in situ sampling by aircraft, or comparison with data from similar
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