Moist convection and the injection of volcanic ash into the atmosphere
Moist convection and the injection of volcanic ash into the atmosphere
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潮湿对流和火山灰注入大气
DOI：
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发表时间：
1993
期刊：
影响因子：
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通讯作者：
A. Woods
中科院分区：
文献类型：
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作者：
A. Woods
如果不饱和水蒸气被火山喷发柱向上携带，由于柱子通过减压膨胀并将热量传递给夹带的空气，温度降低，因此它最终可能会变得饱和。随后水蒸气冷凝释放的热量导致塔内的空气膨胀。我们表明，这增加了浮力，从而增加了柱子上升的总高度。在相对较小的亚普林尼式和斯特龙博式喷发中，高度的增加很显着，其中大规模喷发速率在 103 至 106 千克/秒之间。在此类喷发中，夹带的水蒸气凝结时释放的潜热可能提供驱动火山灰和碎屑向上的主要热源。然后，上升高度对在通风口喷出的质量通量变得相对不敏感，并且主要取决于大气的蒸汽负载。在潮湿的大气中，火山灰的上升可能比在干燥环境中同等强度的喷发高出几公里。潮湿的对流会导致火山灰更广泛地扩散，特别是在非常小的喷发中。随后，雨伞云中的灰烬可能会因夹带的蒸汽凝结而形成的水而产生。灰烬为一些夹带的蒸气提供了自然凝结核，其质量可能比灰烬大得多。在小柱（107 kg/s）中，与热碎屑提供的热能相比，蒸汽冷凝释放的潜热相对较小，因此水分对喷发柱动力学没有显着影响；此外，源自喷发挥发物的水蒸气质量通常与周围空气中夹带的水蒸气质量相当或更大。我们的模型还表明，如果喷发混合物变得有浮力，那么与风岩浆喷发相关的喷发柱的上升几乎与具有相同质量喷发率的普林尼式喷发柱一样高。这是因为在源头被热灰分蒸发的水在塔中的较高处凝结，从而将其潜热恢复到上升的灰分中。
If unsaturated water vapor is carried upward by a volcanic eruption column, it may eventually become saturated owing to the decrease in temperature of the column as it expands through decompression and transfers heat to entrained air. Heat released as a result of the subsequent condensation of water vapor causes the air within the column to expand. We show that this increases the buoyancy and therefore the total height of rise of the column. The increase in height is significant in relatively small sub-Plinian and Strombolian eruptions in which mass eruption rates lie in the range 103 to 106 kg/s. In such eruptions, the latent heat released as the entrained water vapor condenses may provide the main source of heat which drives the ash and clasts upward. The height of rise then becomes relatively insensitive to the mass flux erupted at the vent and depends primarily upon the vapor loading of the atmosphere. In a moist atmosphere, ash may rise several kilomtres higher than in an eruption of comparable strength in a dry environment. Moist convection leads to much wider ash dispersal, particularly from very small eruptions. Subsequently, rain flushing of ash from umbrella clouds may result from the water which forms through the condensation of entrained vapor; the ash provides natural condensation nuclei for some of this entrained vapor, whose mass may be much greater than that of the ash. In small columns ( 107 kg/s), the latent heat released by condensation of vapor is relatively small in comparison with the thermal energy provided by the hot clasts and therefore moisture has no significant effect upon the eruption column dynamics; furthermore, the mass of water vapor originating from the erupted volatiles is usually comparable to, or greater than, that entrained from the ambient air. Our model also shows that, if the erupting mixture becomes buoyant, then the eruption columns associated with phreatomagmatic eruptions ascend nearly as high as Plinian columns with the same mass eruption rate. This is because the water which is vaporized by the hot ash at the source, condenses higher in the column and thereby restores its latent heat to the ascending ash.