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 〖(ν〗^2/〖g^')〗^(1/3 ), where ν 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. 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. 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.

中文翻译：

---

粒子浓度对火山云基底沉降驱动引力不稳定性形成的影响

在火山灰云层底部观测到的沉降驱动重力不稳定性可能在火山灰沉降中起重要作用。它们起源于一个狭窄的，在重力上不稳定的区域，称为粒子边界层（PBL），该区域在较低的云层-大气界面处形成，并产生向下移动的灰烬指，从而提高了灰烬的沉降速率。我们结合颗粒成像和平面激光诱导荧光（PLIF）技术使用规模化实验室实验来研究颗粒浓度对PBL和手指形成的影响。结果表明，随着颗粒沉积在初始密度界面上并掺入稠密的底层流体中，PBL会在界面下方生长，成为较小的过剩密度的狭窄区域。这会在达到临界厚度时脱离，并以〖（ν〗^ 2 /〖g ^'）〗^（1/3）缩放，其中ν是运动粘度，而g ^'是PBL的减小的重力，导致形成手指。在此过程中，界面上方和下方的流体混合不良，只有少量上部流体相通过手指注入。此外，我们的测量结果证实了先前在更广泛的初始条件下的发现，这些发现表明手指的数量及其速度都随颗粒浓度的增加而增加。我们还量化了颗粒悬浮液下方的垂直颗粒质量通量如何随时间和颗粒浓度变化。我们的结果表明，火山灰中高灰分含量的手指不仅更容易发育，但是它们也有望比与贫灰云有关的手指更快地形成并以更高的速度传播。最后，我们确定了无量纲数，该无量纲数取决于可测量的云质量负荷和厚度，可用于评估沉降驱动的重力不稳定性形成的可能性。