Properties of Volcanic Clouds
There are three types of ejection activity of volcanic clouds with respect
to the time dependence as follows:
I. Explosive eruption, forming an eruption column,
II. Continuous ejection with relatively constant strength, forming a stationary plume.
III. Sequential small eruptions at short time intervals, forming a series of puffs or columns.
An eruption column rises upward almost straightly, with its height essentially
determined by the heat energy ejected.
On the other hand, the vertical shape of a continuously ejected plume depends on the wind velocities around and above the summit of the mountain. Namely, it may be an almost upright rise for mild wind, slant or no rise for medium or strong winds.
When the wind is strong enough, the plume is blowed down first along the slope of the mountain and then bounces up, forming a mountain lee wave. Related phenomena are cap and hunging clouds caused by strong winds over an isolated mountain, to be presented in another forthcoming section of this home page.
In any case, a plume at several km away from the crater turns out to be in the stage of horizontal drift at an equilibrium height with the ambient atmosphere.
In drifting to the downstream, a plume does not exhibit much vertical
diffusion except for the ashfall according to the grain size. On the other
hand, horizontal dispersions exhibit different patterns which are essentially
determined by the wind shear at the altitudes around 1000-2000 m within
the vertical thickness 200-800 m of the plume cloud, which depends on the
ejection strength. Typical ones are the followings:
(i) Linear advection with small spread under strong and/or collimated wind.
(ii) Fan type spread under weak wind with large vertical shear.
(iii) Belt type spread due to shear wind near an edge height of the plume.
These patterns are clearly seen in the satellite images ( SiNG/Public )
| References |
K. Kinoshita, Video Monitoring of Ejection and Dispersion of Volcanic Ash
Clouds from Mt.Sakurajima,
Proc. 3rd. Asian Symp. on Visualization, Chiba, Japan, 1994, pp.416-421.
K.Kinoshita, Observation of Flow and Dispersionof Volcanic Clouds from Mt.
Sakurajima, Atmospheric Environment, 30, 1996, 2831-2837.
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