Geoscience Reference
In-Depth Information
5.2.5 Gas flux estimates
Although maximum velocities measured by the radar result from particles, initial gas
velocities can be estimated at first order using the corrections seen in section 5.2.2. and
adequate model laws for the coupling of gas and particle velocities and gas velocity
decrease with height. The volume and mass gas fluxes
Q
g
can also be estimated from the
initial gas velocity (V
0
g
) and vent section, the radius (
r
) of which can be observed in the field
or inferred from modeling of acoustic signals:
g
2
g
vol
Qr
V
(11)
0
g
mass
g
g
Q
Q
(12)
vol
where
g
is the gas density at the considered atmospheric pressure (elevation) and
temperature; water vapor being the ultra-dominant species, its density can be used for
g
.
Note that gas fluxes evolve similarly to the maximum radial velocities and therefore have a
large peak at the explosion onset followed by a rapid decrease in a matter of seconds.
Averaging over the duration of the emission for a large number of Strombolian explosions,
Gouhier & Donnadieu (2011) found volume and mass gas fluxes of 3-11×10
3
m
3
/s and 0.5-2
ton/s during the paroxysmal stage of a Strombolian eruptive episode of the SE crater at
Etna. Radar-derived gas flux estimates at the source can then be compared with fluxes
inferred from other ground-based techniques, like combined OP-FTIR gas spectroscopy and
SO
2
flux measurements by DOAS, or ground-based thermal imagery.
6. Investigations of ash plumes with VOLDORAD
There is a continuum in the types of activity between Strombolian explosions seen
previously and ash plumes. Ash plumes display a variety of behaviours depending on
whether they are short-lived or sustained (steady state or not), whether they are jet plumes
or buoyant plumes according to their momentum (mass loading, particle size distribution,
fluxes), and also depending on environmental conditions including crosswind, elevation of
the emission point, humidity and atmospheric temperature profiles, among the main ones.
As an example, figure 13 shows three ash plumes with varying ash concentration and
momentum, and also differently affected by the wind advection.
Fig. 13. Examples of various ash plumes at Arenal and Popocatépetl volcanoes. (a): low
concentration buoyant plume bent over by wind advection at Arenal.
(b): vertical jet plume,
a few hundreds of meters in height, on same day (May 23, 2005). (c): dense ash plume of
Popocatépetl buoyantly rising to about 2 km in height and drifting to the North on July 28
2007. Photos courtesy: Hotel Kioro Arenal and CENAPRED.
