Geoscience Reference
In-Depth Information
Ka-band. The advantage of higher frequencies (X-, Ka-band) is the potential diminution of
the overall size of the system and a higher sensitivity to fine particles, hence a better
detection at low ash concentration. For near-source soundings, path attenuation effects are
presumably very important up to X-band and possibly non negligible up to L-band because
of the high particle concentrations and sizes (commonly pluri-decimetric), especially in the
gas thrust region. Further investigations are needed, even at L-band. (iv) Weather radars
cannot track ash clouds over the long-term, due to the low atmospheric residence time of
reflective coarse particles. (v) Their maximum detection range is generally within 200-300
kilometers of their fixed location. Portable radar systems overcome the limitation of
observing ash clouds from a far distance and always the same volcano. In many respects,
the synergetic role of satellite imagery in tracking volcanic ash, particularly after the initial
stages of an eruptive event is obvious. (vi) Weather radars are unable to image the lowest
few kilometers of the ash column when the volcano is too far away (and the top if above the
beam), preventing early detection and retrieval of the near-source ash plume characteristics.
To avoid some of these shortcomings, institutes in charge of volcano monitoring have
started to integrate nearby dedicated radars into their instrumental networks.
2.2.1.2 Radars dedicated to volcano monitoring
Given the benefits of continuous quantitative retrieval of parameters such as height and
mass loading which are crucial to initiate dispersion models, permanent volcano monitoring
using weather radars has become more widely used. Ground-based weather radar networks
are currently operational at several volcanoes, in Alaska, Iceland, Italy and Guadeloupe. The
U.S. Geological Survey first experimented in 1997 with a ground-based Doppler radar at the
National Center for the Prevention of Disasters (CENAPRED) in Mexico to track the
dispersal of ash plumes of Popocatépetl volcano and at least two eruptions were
successfully captured. In addition to the near contiguous network of weather-monitoring
Doppler radar NEXRAD operated by the U.S. National Weather Service, the U.S. Geological
Survey also deployed a new truck-transportable C-band Doppler radar (MiniMax-250C)
during the 2009 eruptions of Redoubt Volcano, Alaska (Hoblitt and Schneider, 2009). Results
for 17 ash plumes detected by the radar compared favorably well with those of a nearby
WSR-88D NEXRAD operated by the Federal Aviation Administration. The sector-scanning
strategy (45°) of the new mobile radar advantageously allowed event onset detection within
less than a minute. Heights (9-19 km) and vertical rise rates of the ash columns (25-60 m/s)
have been determined. The high radar reflectivity values of the central core of the eruption
column (50-60 dBZ) were interpreted as being the result of rapid formation of volcanic ash-
ice aggregates (Schneider, 2012).
The X-band is generally preferable providing higher sensitivity with respect to lower
frequency bands typically used for weather observations. The Japanese government recently
set up an X-band polarimetric radar near Sakurajima volcano, able to monitor its recurrent
vulcanian ash plumes (M. Maki, pers. comm.). Since November 2010, the Icelandic Met
Office has had on loan from the Italian Civil Protection a mobile X-band dual-polarization
radar for volcano monitoring. This radar (75 km from the volcano), along with the fixed
weather C-band radar in Keflavík (257 km from the volcano), monitored the ash plumes of
Eyjafjoll in 2010 and Grimsvötn in 2011 (Arason et al., 2011, 2012). These authors used in
particular the radar time-series of the plume heights to calculate the mean eruptive flow
