Geoscience Reference
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the overall reflected signal, whereas values of between 0 and 30 dBz characterized the cloud
advected downwind. The plume head had a mean ascent rate of 30 to 50 m/s up to 12 km in
altitude (upper limit of the radar). Using the same radar, Marzano et al. (2006a, 2010a) found
maximum reflectivities of 34 dBZ for the 2004 ash cloud of Grímsvötn volcano (260 km
away), at a height of 6 km (minimum detection altitude). From an inversion technique based
on a classification scheme of particles, they estimated ash concentrations of up to 6 g/m
3
,
and ashfall rates of up to 31 kg/h. Likewise, for the 2010 Eyjafjöll eruption, Marzano et al.
(2011) determined an ash mass of up to 15×10
8
kg on April 16, and 8×10
7
kg on May 5.
Recently, Marzano et al. (2010b) used volume scan data acquired in the S-band by a
NEXRAD WSR-88D ground-based weather radar at Augustine vocano in Alaska in 2006
(Wood et al., 2007). From their model-based technique, ash aggregate concentrations of up
to 0.2g/m
3
were found to correspond to measured reflectivities of up to 55 dBZ at an ash
column height of about 4 km. Maki et al. (2001) first reported observations of ash plumes
from Mount Oyama in Japan by a 3-cm wavelength polarimetric mobile radar about 40 km
away. They discussed the possibility of detecting volcanic ash particles and estimating their
size distribution from polarimetric radar parameters such as the differential reflectivity and
specific differential phase shift.
2.2.1.1.3 Limitations
As seen previously, ground-based weather radar systems are powerful tools for volcanic ash
cloud detection and quantification. Their Doppler capacity has not, so far, been much
exploited in the study of ash clouds and could aid understanding of the interplay between
their dynamics and their environmental conditions (wind, atmospheric properties such as
humidity and temperature profiles, etc). Their main limitations are, in general: (i) their
limited sensitivity tending to render invisible to the radar the cloud parts where particle
concentration is too low (ultimately all of the ash cloud). This leads to an underestimation of
the ash cloud lateral extension, and also of its height because the top of the ash column may
be coarse-depleted. Another source of error on column heights, and hence an
underestimation of height-derived eruption rates, may come from the incomplete filling of
the highest volume scanned by the plume top. The sensitivity of the ground-based radar
measurements will decrease as the ash cloud moves farther away. (ii) By using single-
polarization weather radar, however, it is fairly difficult to discriminate between ash,
hydrometeors, and mixed particles. Ice nucleation and subsequent loss in reflectivity also
make ash detection more difficult (Marzano et al., 2006b). These authors suggest that
polarimetric radars may improve discrimination of the impact of cloud ice and liquid water
on ash aggregates. According to Hannesen and Weipert (2011), however, significant overlap
exists between meteorological targets and volcanic ash, so that, even if all polarimetric
observables of dual-polarized radars are used, automatic detection might be tricky. With
polarimetric data, however, the retrieval of volcanic parameters could be improved by
taking into account the mixed particle composition and their shape (Marzano et al., 2012).
(iii) Path attenuation effects are not always negligible. According to Marzano and Ferrauto
(2003), in the case of hydrometeors, any radar technique above S band should take into
account, and possibly remove, path attenuation effects in order to correctly convert
measured reflectivity into rain rate. For ash clouds, Marzano et al. (2006b) concluded that C-
band may offer some advantages in terms of radar reflectivity response and negligibility of
path attenuation. While still tolerable at X-band, the path attenuation cannot be handled at
