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rate. From the polarimetric X-band dataset of this eruption, Hannesen and Weipert (2011)
quantified ash concentrations of up to 100 g/m
3
and ash fall rates of up to 100 kg/m
2
/h at a
height of 4.5 km from all polarimetric observables. They emphasize, however, the limits of
ash quantification, the ambiguity in the separation of precipitation and ash that makes
automatic detection still difficult, and the signal weakness from distant ash that prevents
radar observations. Vulpiani et al. (2011) explored the benefits of the mobile dual
polarization X band radar (DPX 4) operated by the Department of Civil Protection at the
airport of Catania Fontanarossa (30 km to the South) to monitor Etna and offer support to
the decisions of the authorities that regulate and control air traffic. In an ash plume fed from
a lava fountain, maximum reflectivities of 35 dBZ were measured at medium distances of
10-40 km from the volcano. Estimated mass concentrations vary up to a few g/m
3
, although
most are below 1 g/m
3
. The instrumental monitoring network of Etna operated by the
Istituto Nazionale di Geofisica I Vulcanologia (INGV) also comprises, since 2009, and this is
unique, a permanent ground-based L-band Doppler radar of the Observatoire de Physique
du Globe de Clermont-Ferrand (OPGC, France) targeting the summit craters (Donnadieu et
al., 2009a, 2012). Named VOLDORAD 2B, this radar is similar to the transportable volcano
Doppler radar (VOLDORAD) successfully applied in several volcanic contexts (Dubosclard
et al., 1999, 2004; Donnadieu et al., 2003, 2005), as illustrated later in this chapter (cf. section
7.1, fig. 16). The permanent radar at Etna should complement observations from the INGV
monitoring network to constrain the inputs of the tephra dispersal models run automatically
to perform tephra dispersal forecast (Scollo et al., 2009).
2.2.1.3 Fallout measurements
A compact X-band continuous wave, low power (10 mW) Doppler Radar (PLUDIX, 9.5 GHz
frequency of operation), originally designed as a rain gauge disdrometer, was utilized to
measure the terminal settling velocities and infer sizes of plume fallout at Mount Etna in
2002 (Scollo et al., 2005) and Eyjafjallajökull in 2010 (Bonadonna et al., 2011). PLUDIX-
derived particle size distributions agree reasonably well with sieve-derived grain size
distributions, but only for diameter range above 500 microns, and so should be used within
a few kilometers from the source. Such measurements, along with deposit sampling and
other methods shown in figure 1, can usefully complement other radar observations of the
ash plume/cloud (Fig. 1) by providing the particle size distribution necessary to accurately
retrieve the loading parameters (total mass, mass concentrations, mass flux of tephra).
2.2.1.4 Compact portable Doppler radars for near-source measurements
The growing need to get insight into the dynamics of explosive eruptions and to measure
eruptive parameters at the source has led to the development of several active remote
sensing compact instruments in the last decade or so. The first attempt to bring
transportable sounders close to volcanic craters to measure the near-source dynamics was
achieved by Weill
et al.
(1992) who successfully determined vertical velocities in the range
20-80 m/s for over 100 mild Strombolian explosions at Stromboli using a Doppler sodar.
This cumbersome acoustic sounder could operate only at a few hundred meters from the
vent and, hence, was not well suited to the sounding of larger magnitude, hazardous
eruptions. Besides, velocity determinations using sodar require the knowledge of sound
velocity at the jet temperature and gas composition, which was not available. Two main
types of dedicated portable radars have since been used with the primary goal of studying
