Geoscience Reference
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at Stromboli, Scharff et al. (2008) found a correlation between the radiative energy of
Strombolian lava jets and the backscattered energy, suggesting that both methods record the
relative variations of mass. They also found pulsations in the power time series of 40% of the
eruptions, likely reflecting variations in mass eruption rate and originating in multiple
consecutively exploding bubbles.
Scharff et al. (2012) also report the pulsed release (2-5 s) of ash clouds from the dome of
Santiaguito with particle radial velocities between 10 and 25 m/s, and preceded by a vertical
dome uplift of about 50 cm, as recorded with a FM-CW radar. Using VOLDORAD,
Donnadieu et al. (2008) had already reported staccato pressure release in the ash emissions
of Arenal volcano, along with a variety of ash plume dynamics from short-lived explosive
events with radial velocities of up to 90 m/s, to sustained pulsed ash jetting and to passive
dilute ash emissions. Donnadieu et al. (2011) successfully reconstructed the 3D vector of the
ash plume transport speed from the echo onsets in contiguous range gates.
3. Specificity of radar signals of volcanic origin
3.1 Examples of meteorological signals
While abundant literature describes the effects of meteorological targets on weather radar
signals, few studies characterize volcanic targets from a radar perspective. Not only the
dynamics of volcanic eruptions strongly differs from that of common meteorological
phenomena but also the target properties. This section points out some differences relevant
to the study of radar signals of volcanic origin, for measurements near the emission source
and in the distal part of ash clouds.
Fig. 2. Examples of meteorological Doppler spectra from a 24 GHz Micro Rain Radar.
Reflectivity is shown versus radial velocity (i.e. fall speed with vertical beam) at a 1 hour
interval (6:05 U.T. in red and 07:05 U.T. in green) on 21/12/2011: (a) snow crystals at 1430-
1730 m a.s.l., (b) mixture of melting snow and water droplets in the radar bright-band (930-
1230 m a.s.l.), (c) melt water droplets at 430-730 m a.s.l.. Data of MRR4 at Aulnat Airport
(France): courtesy of Yves Pointin (OPGC).
Typical Doppler spectra of meteorological targets showing reflectivity versus fall speeds are
presented in figure 2. Because the sounding is vertical, radial velocities (toward the radar)
directly indicate fall speeds, unlike in the oblique radar soundings of volcanic emissions. At
altitude, low reflectivity snow crystals fall at low speed (Gaussian shape spectrum). At
intermediary altitude, a mixture of melting snow and water droplets (radar bright-band)
