Geoscience Reference
In-Depth Information
2.2 Ash plume monitoring
Long-range trajectory tracking of ash clouds is achieved primarily by means of satellite
imagery. Although Delene et al. (1996) showed the utility of a satellite-based microwave
imager passively measuring radiations (19-85 GHz) of millimetric volcanic particles from an
ash cloud of Mount Spurr in 1992, satellite visible-infrared radiometric observations from
geostationary platforms are usually exploited (e.g., Rose et al., 2000). The evolution of the
ash cloud spatial distribution, in particular, can be imaged at intervals of 15-30 min.
Important parameters can be further retrieved like the approximate plume height assuming
thermal equilibrium with the atmosphere (non unicity of solutions for altitudes above the
tropopause), and the concentration and size of distal particles (< 20 microns) transported in
the atmosphere, assuming particle sphericity and vertically homogeneous concentration.
Using these assumptions, the mass of SO
2
and ash can be integrated on successive images
(e.g. Wen & Rose, 1994). Scollo et al. (2010) also showed the potential of Multiangle Imaging
SpectroRadiometer (MISR) working in four wavelengths in visible and near-infrared bands,
for the 3-D reconstruction of ash plume shape, and for the retrieval of column height, optical
depth, type and shape of the finest particles, among the most sensitive inputs for ash
dispersal modeling.
Yet, the exploitation of satellite images for monitoring purposes is limited by (1) the
presence of clouds at higher levels, (2) an insufficient acquisition rate for event onset
detection, (3) a relatively poor spatial resolution, (4) errors of the “split-window” method
(brightness temperature difference) when the volcanic plume lies over a very cold surface or
when the plume lies above a clear land surface at night where strong surface temperature
and moisture inversions exist (Prata et al., 2001). In addition, low ash content and/or small
ash plumes might not be clearly observed and near-source emissions are obscured by the
emitted tephra. For these reasons, ground-based radar systems represent an optimal
complementary solution for real-time monitoring of these phenomena, by providing higher
spatial resolution and data acquisition rates, as well as the ability to make observations at
night and under any weather conditions. Real-time monitoring of ash plumes is crucial, in
particular for the initialization of dispersion models. In this respect, essential input
parameters such as plume height, mass flux, and particle concentration can be assessed
quantitatively from radar data and directly contribute to improve ash dispersion forecasts.
2.2.1 Radar monitoring of ash plumes
2.2.1.1 Weather radar observations
2.2.1.1.1 Characteristics and advantages
Although ash plume hazards to aviation safety raised concerns early on about the detection
capacity of ash clouds by airborne radar (Musolf, 1994; Stone, 1994), most observations of
large volcanic ash clouds have been opportunely carried out by fixed meteorological radars
of national weather services. Weather radars operate at microwave frequencies from S band
(7.5-15 cm wavelength, generally about 10 cm) up to C band (3.75-7 cm wavelength,
commonly around 6 cm), X band (2.5-3.75 cm wavelength, commonly around 3 cm) and Ka
band (0.75-1.11 cm wavelength, often around 1 cm). With peak powers up to 250 kW or even
higher, they have sufficient sensitivity to detect volcanic ash clouds with small particle sizes.
Pulsed systems ensure a relatively high spatial range resolution of a few hundreds of
