Geoscience Reference
In-Depth Information
is complex and rapidly varying in time and space. The moments and shapes of Doppler
spectra also fluctuate rapidly, with spatial variations among the range gates related to the
dimension of the phenomenon with respect to the sounded volumes. The power spectral
density results in particular from the complex combination of the particle velocity field and
particle load distribution, i.e. the amount of particles, their velocities and trajectories. Being
either short-lived or sustained, volcanic emissions can be roughly viewed as two-phase
flows generally oriented vertically upward with a continuum of dynamic behaviors from
inertial large blocks mostly following ballistic trajectories soon after their ejection down to
the finest low-inertia particles nearly following the gas behavior involving a stronger
deceleration soon after their emission. Crosswinds also affect the particles' motion
differentially according to their diameter and residual momentum. Gravity further implies
that ejecta are propelled upward while others fall out simultaneously and both imprint their
signature in near-source radar measurements.
Volcanic emissions are generally highly turbulent close to the source (gas thrust region and
convective part) and less turbulent in the distal cloud. The effect of turbulence in the cloud
is difficult to assess because the particle behavior is highly size-dependent, from inertial
large blocks to gas-entrained fine particles. Although assumed not to be dominant in radar
measurements near the source where large particles are present, the turbulence effects
affecting overall the small particles should nevertheless tend to increase the spectral width,
as observed in radar meteorology.
3.3 Target properties
At second order, the intrinsic properties of the targets, their movements and chemico-
physical evolution, also play a role in the measured reflectivity. Because volcanic tephra
generally originate from the violent fragmentation of magma by the expanding gas, their
shape is also complex and their surface highly irregular at various scales. The effects of
shape and roughness of volcanic particles on reflectivity have been little investigated at
radar wavelengths. Yet they might be non negligible, at least at short wavelength, as
suggested for meteorological targets. In examining the effects of ice crystal shapes on
reflectivity at 3 mm wavelength, Okamato (2002) found, for instance, 8 and 5 dB effects of
non-sphericity and orientation respectively, for particle sizes approaching the wavelength.
In the volcanic case, the analysis is further complicated by in-flight modifications of the
ejecta shape and orientation, especially close to the source. Large lava fragments, in
particular, deform in-flight due to their plastic nature, as attested by the specific shapes of
volcanic bombs (e.g. fusiform), or break up upon impact with other ejecta and because of
high strain rates imposed by acceleration, rotation, and drag force. It must be expected that
most fragments have a rapidly changing orientation in flight, especially close to the source
where turbulence occurs.
Water vapor being the dominant gas species exsolved from magma (commonly >85%),
major condensation by the cold atmosphere occurs during eruptions. There is 2.4 factor
difference between the dielectric factors of ash (0.39: Adams et al., 1996; Oguchi et al., 2009;
Rogers et al., 2011) and liquid water (0.93). According to studies by Oguchi et al. (2009) from
3 to 13 GHz, a water film coating 10-20% of the radius of a sub-millimetric volcanic particle
is sufficient to raise the radar cross section to that of a whole liquid water particle (0.93
dielectric factor). Water vapor further promotes the nucleation of ice (0.197 dielectric factor)
