In the present study, we analyzed the particle size distribution (PSD) of falling volcanic ash particles measured using optical disdrometers during six explosive eruptions of the Sakurajima volcano in Kagoshima Prefecture, Japan. Assuming the gamma PSD model, which is commonly used in radar meteorology, we examined the relationships between each of the gamma PSD parameters (the intercept parameter, the slope parameter, and the shape parameter) calculated by the complete moment method. It was shown that there were good correlations between each of the gamma PSD parameters, which might be one of the characteristics of falling volcanic ash particles. We found from the normalized gamma PSD analysis that the normalized intercept parameter and mass-weighted mean diameter are suitable for estimating the ash fall rate. We also derived empirical power law relationships between pairs of integrated PSD parameters: the ash fall rate, the volcanic ash mass concentration, the reflectivity factor, and the total number of ash particles per unit volume. The results of the present study provide essential information for studying microphysical processes in volcanic ash clouds, developing a method for quantitative ash fall estimation using weather radar, and improving ash transport and sedimentation models.

中文翻译：

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日本樱岛火山的光学测风仪测量的火山灰飘落物粒度分布特征

在本研究中，我们分析了在日本鹿儿岛县樱岛火山的六次爆炸性喷发过程中，使用光学测速仪测量的下落的火山灰颗粒的粒径分布（PSD）。假设雷达气象学中常用的伽玛PSD模型，我们检查了通过完整矩法计算出的每个伽玛PSD参数（截距参数，斜率参数和形状参数）之间的关系。结果表明，每个γPSD参数之间都具有良好的相关性，这可能是火山灰下落颗粒的特征之一。我们从归一化的γPSD分析发现，归一化的截距参数和质量加权平均直径适用于估计灰分沉降率。我们还推导了成对的综合PSD参数对之间的经验幂律关系：灰分沉降率，火山灰质量浓度，反射率因子和每单位体积的灰分粒子总数。本研究的结果为研究火山灰云中的微物理过程，开发使用气象雷达估算灰烬落定量的方法以及改进灰分运输和沉降模型提供了重要信息。