The size distribution of volcanic ash is rarely measured in real time and Volcanic Ash Advisory Centres (VAACs) often rely on a default particle size distribution (PSD) to initialise their dispersion models when forecasting the movement of ash clouds. We conducted a sensitivity study to investigate the impact of PSD on model output and consider how best to apply default PSDs in operational dispersion modelling. Compiled grain size data confirm that, when considering particles likely to be in the distal ash cloud (< 125 µm diameter), magma composition and eruption size are the dominant controls on grain size distribution. Constraining the PSD is challenging but we find that the grain size of deposits from large hydromagmatic eruptions remains relatively constant with distance, suggesting that total (whole-deposit) grain size distributions (TGSDs) for these eruptions could be estimated from a few samples. We investigated the sensitivity of modelled ash mass loadings (in the air and on the ground) to input PSDs based on coarse to fine TGSDs from our dataset. We found clear differences between modelled mass loadings and the extent of the plume. Comparing TGSDs based on ground-only and ground-plus-satellite data for the Eyjafjallajökull 2010 eruption, we found that basing input PSDs on TGSDs from deposits alone (likely missing the finest particles) led to lower modelled peak ash concentrations and a smaller plume.

中文翻译：

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基于水成岩和岩浆沉积物的火山灰弥散度模型对输入粒度分布的敏感性

火山灰的粒度分布很少实时测量，而火山灰咨询中心（VAAC）在预测灰云运动时通常依靠默认的粒度分布（PSD）初始化其扩散模型。我们进行了一项敏感性研究，以调查PSD对模型输出的影响，并考虑如何在运营离散模型中最好地应用默认PSD。汇编的粒度数据证实，当考虑可能存在于远处烟灰云中的颗粒（直径<125 µm）时，岩浆成分和喷发大小是粒度分布的主要控制因素。约束PSD具有挑战性，但是我们发现，大型水磁喷发形成的沉积物的晶粒尺寸随距离保持相对恒定，这表明可以从一些样本中估算出这些喷发的总（整个沉积）粒度分布（TGSD）。我们研究了建模的灰分质量载荷（在空中和地面上）对基于来自数据集的粗略到精细TGSD的输入PSD的敏感性。我们发现建模的质量载荷与羽流范围之间存在明显差异。根据Eyjafjallajökull2010喷发的仅地面数据和卫星卫星数据比较TGSD，我们发现仅将PSD输入基于沉积物的TGSD（可能会丢失最细的颗粒）会导致较低的模拟峰值灰分浓度和较小的羽流。我们发现建模的质量载荷与羽流范围之间存在明显差异。根据Eyjafjallajökull2010爆发的仅地面数据和卫星地面数据比较TGSD，我们发现，将仅基于沉积物（可能缺少最细颗粒）的TGSD输入PSD导致较低的模拟峰值灰分浓度和较小的羽流。我们发现建模的质量载荷与羽流范围之间存在明显差异。根据Eyjafjallajökull2010喷发的仅地面数据和卫星卫星数据比较TGSD，我们发现仅将PSD输入基于沉积物的TGSD（可能会丢失最细的颗粒）会导致较低的模拟峰值灰分浓度和较小的羽流。