由于turbulence分层，火山灰云层在数小时至数天内发展
The development of volcanic ash cloud layers over hours to days due to turbulence layering
摘要：
火山灰云通常会随着距离喷口的距离而变得多层和稀薄。我们探索这种分层结构的发展机制之一。我们回顾数据的特点，在自由大气的湍流分层，以及观测的层状云近喷口和远端的例子。然后，我们探讨色散模型，明确使用所观察到的分层结构的大气湍流。结果表明，湍流和静止大气层的交替提供了一种机制，多层灰云的发展，通过调制垂直粒子运动。最大的颗粒，通常为$> 100 μ$ m，几乎不受湍流的影响。对于沉降和湍流扩散对垂直运动都很重要的颗粒，大多数在10-100 $μ$ m的范围内，在某些层中更大的湍流强度和更快的湍流扩散导致这些颗粒在更湍流的层中花费更长的时间，导致浓度分层。对于较小的颗粒，主要是在亚微米范围内，在湍流层中更快的扩散导致这些颗粒迅速“洗掉”。
英文摘要：
Volcanic ash clouds often become multilayered and thin with distance from the vent. We explore one mechanism for development of this layered structure. We review data on the characteristics of turbulence layering in the free atmosphere, as well as examples of observations of layered clouds both near-vent and distally. We then explore dispersion models that explicitly use the observed layered structure of atmospheric turbulence. The results suggest that the alternation of turbulent and quiescent atmospheric layers provides one mechanism for development of multilayered ash clouds by modulating vertical particle motion. The largest particles, generally $> 100 μ$m, are little affected by turbulence. For particles in which both settling and turbulent diffusion are important to vertical motion, mostly in the range of 10-100 $μ$m, the greater turbulence intensity and more rapid turbulent diffusion in some layers causes these particles to spend greater time in the more turbulent layers, leading to a layering of concentration. For smaller particles, mostly in the submicron range, the more rapid diffusion in the turbulent layers causes these particles to ``wash out'' quickly.
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