Abstract
Settling-driven gravitational instabilities observed at the base of volcanic ash clouds have
the potential to play a substantial role in volcanic ash sedimentation. They originate from a
narrow, gravitationally unstable region called a Particle Boundary Layer (PBL) that forms at
the lower cloud-atmosphere interface and generates downward-moving ash fingers that
enhance the ash sedimentation rate. We use scaled laboratory experiments in combination
with particle imaging and Planar Laser Induced Fluorescence (PLIF) techniques to
investigate the effect of particle concentration on PBL and finger formation. Results
show that, as particles settle across an initial density interface and are incorporated
within the dense underlying fluid, the PBL grows below the interface as a narrow region of
small excess density. This detaches upon reaching a critical thickness, that scales with
(v^2/g′)^1/3, where v is the kinematic viscosity and g′ is the reduced gravity of the PBL,
leading to the formation of fingers. During this process, the fluid above and below the
interface remains poorly mixed, with only small quantities of the upper fluid phase being
injected through fingers. In addition, our measurements confirm previous findings over a
wider set of initial conditions that show that both the number of fingers and their velocity
increase with particle concentration. We also quantify how the vertical particle mass flux
below the particle suspension evolves with time and with the particle concentration. Finally,
we identify a dimensionless number that depends on the measurable cloud mass-loading
and thickness, which can be used to assess the potential for settling-driven gravitational
instabilities to form. Our results suggest that fingers from volcanic clouds characterised by
high ash concentrations not only are more likely to develop, but they are also expected to
form more quickly and propagate at higher velocities than fingers associated with ash-poor
clouds.
the potential to play a substantial role in volcanic ash sedimentation. They originate from a
narrow, gravitationally unstable region called a Particle Boundary Layer (PBL) that forms at
the lower cloud-atmosphere interface and generates downward-moving ash fingers that
enhance the ash sedimentation rate. We use scaled laboratory experiments in combination
with particle imaging and Planar Laser Induced Fluorescence (PLIF) techniques to
investigate the effect of particle concentration on PBL and finger formation. Results
show that, as particles settle across an initial density interface and are incorporated
within the dense underlying fluid, the PBL grows below the interface as a narrow region of
small excess density. This detaches upon reaching a critical thickness, that scales with
(v^2/g′)^1/3, where v is the kinematic viscosity and g′ is the reduced gravity of the PBL,
leading to the formation of fingers. During this process, the fluid above and below the
interface remains poorly mixed, with only small quantities of the upper fluid phase being
injected through fingers. In addition, our measurements confirm previous findings over a
wider set of initial conditions that show that both the number of fingers and their velocity
increase with particle concentration. We also quantify how the vertical particle mass flux
below the particle suspension evolves with time and with the particle concentration. Finally,
we identify a dimensionless number that depends on the measurable cloud mass-loading
and thickness, which can be used to assess the potential for settling-driven gravitational
instabilities to form. Our results suggest that fingers from volcanic clouds characterised by
high ash concentrations not only are more likely to develop, but they are also expected to
form more quickly and propagate at higher velocities than fingers associated with ash-poor
clouds.
| Original language | English |
|---|---|
| Article number | 640090 |
| Journal | Frontiers in Earth Science |
| Volume | 9 |
| DOIs | |
| Publication status | Published - 3 Jun 2021 |
Bibliographical note
Funding Information:Fr?d?ric Arlaud and Jean-Jacques Lasserre are thanked for their technical expertise in the laboratory. We thank Mark Jellinek, TM, LM and Valerio Acocella for their comments that substantially improved the manuscript.
Publisher Copyright:
© Copyright © 2021 Fries, Lemus, Jarvis, Clarke, Phillips, Manzella and Bonadonna.