From needles to plates: The origin of extreme volcanic ash shapes and implications for dispersion modelling and retrieval algorithms

火山灰在大气中的下落速度以及由此产生的距离取决于颗粒的大小、形状和密度。为了预测灰的运输，需要灰扩散模型包括适当的颗粒特性。然而，英国气象局用于预测火山灰扩散的程序假设是球形颗粒。然而，也有火山反复产生具有极端形状的火山灰，包括针状火山灰，典型的卡特拉火山喷发，以及Vulcano（意大利）的一些火山喷发，尖形颗粒，如2011年格里姆火山（冰岛）喷发的颗粒，这些颗粒沉积在英国，以及非常大的火山喷发特征的大扁平碎片。与相同密度的球形颗粒相比，具有极端形状的颗粒将具有明显较低的下落速度，在大气中停留的时间更长，距离火山更远。为了评估特定极端灰形状的起源，学生将使用布里斯托大学（UoB）收集的样本和学生从卡特拉和武尔卡诺收集的针状灰的新样本量化各种喷发的灰形状，并将其置于已发表的灰形态描述的背景下。通过光学显微镜和扫描电子显微镜（SEM）在2D（投影形状和灰切片）中定量颗粒形状。将使用MeX软件从不同角度拍摄的多个SEM图像构建3D形状，并通过X射线断层扫描对选定的较大颗粒进行成像。将在UoB的实验室中通过高速视频观察单个灰颗粒下落的速度和方向。将通过随时间积累的质量和分析作为存款中高度函数的灰的大小和形状来研究散装样品的沉降特性。这将通过将理想化的颗粒与3D打印机制成的颗粒一起沉降在水中来补充，以便可以系统地修改形状。在与英国气象局的合作下，实验结果将被开发成一个计划，然后将被纳入NAME，以解释非球形粒子的下落速度。学生将对火山灰形状对NAME预测的影响进行敏感性测试，并对卡特拉火山爆发的可能情况进行模拟。球形灰假设也是在实验室和喷发期间空气中测量灰粒度分布的标准。学生将测量灰样的表观粒度分布，已知（有限）

The fall velocity, and consequently the distance volcanic ash travels through the atmosphere, depends on the size, shape and density of the particles. To forecast the transport of ash requires that ash dispersion models include appropriate particle characteristics. The programme used by the Met Office to forecast ash dispersal, however, assumes spherical particles. Yet there are volcanoes that repeatedly produce ash with extreme shapes including needles, typical of silicic eruptions at Katla also some eruptions of Vulcano (Italy), cuspate-shaped particles such as those from the 2011 eruption of Grimsvotn (Iceland) that were deposited in the UK, and large flat shards characteristic of very large silicic eruptions. Particles with extreme shapes will have a significantly lower fall velocity, remain in the atmosphere for longer and travel further from the volcano than spherical particles of the same density. To evaluate the origins of specific extreme ash shapes, the student will quantify shapes of ash from various eruptions using samples in the University of Bristol (UoB) collection and new samples of needle-shaped ash from Katla and Vulcano collected by the student, and place these within the context of published descriptions of ash morphology. The shapes of particles will be quantified in 2D (both projected shapes and slices through ash) by optical microscopy and scanning electron microscopy (SEM). 3D shapes will be constructed with MeX software from multiple SEM images taken at different angles, and selected larger particles will be imaged by X-ray tomography. The velocities and orientations of falling individual ash particles will be observed in a laboratory at UoB with high-speed video. Settling properties of bulk samples will be studied by the mass accumulated with time and analysis of size and shape of the ash as a function of height in the deposit. This will be complemented by settling idealized particles in water with particles made with a 3D printer so that shape can be systematically modified. In collaboration with the Met Office, the results of the experiments will be developed into a scheme that will then be incorporated into NAME to account for the fall velocity of non-spherical particles. The student will run sensitivity tests of the effects of ash shape on NAME forecasts and run simulations of likely scenarios for eruptions of Katla volcano. The spherical ash assumption is also standard in measurements of ash size distributions both in the lab and in the air during eruptions. The student will measure the apparent size distributions of ash samples with known (limited)