Volcanic Ash Clouds -- Why do they flatten at certain altitudes when rising?

[RJ Emery]

Why do plumes of smoke or ash rise to a certain elevation and then appear flattened?

 

[Drakkith]

My guess is that the ash cloud rises until it hits the tropopause, where the air ceases to cool with increasing altitude. This means that the ascending warm air, which has been gradually cooling as it ascends, suddenly finds itself cooler than the above air, halting its rise. The lower levels of the cloud are still rising, however, which forces the top level outwards to create the flat top. This is why thunderstorms typically have an 'anvil' shape when they mature.

That's my guess at least.

 

[jim mcnamara]

The Planetary Boundary Layer can have similar effect on a lesser scale.

https://en.wikipedia.org/wiki/Planetary_boundary_layer - check out the second picture in the right margin.

 

[256bits]

The neutral density height where the volcanic ash spreads out is called the umbrella region.
Below that is the convective region where there is an upthrust and where particle fallout is determined by their terminal velocity, with larger heavier particles falling out sooner than lighter. Wind will move these particles away from the volcano following the wind direction.
http://www.geo.umass.edu/courses/volcanology/Pyroclastics 1.pdf
goes into some more detail about the umbrella region and downdrift and fallout of tephra.

 

[Drakkith]

The neutral density height where the volcanic ash spreads out is called the umbrella region.
Below that is the convective region where there is an upthrust and where particle fallout is determined by their terminal velocity, with larger heavier particles falling out sooner than lighter. Wind will move these particles away from the volcano following the wind direction.
http://www.geo.umass.edu/courses/volcanology/Pyroclastics 1.pdf
goes into some more detail about the umbrella region and downdrift and fallout of tephra.

Glad to know that this appears to support my guess to some degree:

The base of the umbrella region (HB) often occurs at the tropopause (boundary between atmosphere and stratosphere) because of temperature inversions.

I'd love to read more on this topic.

 

[256bits]

Glad to know that this appears to support my guess to some degree:
I'd love to read more on this topic.

Pretty spot on I think.

For an eruption that reaches the neutral density and still has upward velocity, momentum will carry the column higher and subsequently laterally as it spreads out.
Some lessor eruptions might not reach the troposphere tropopause
, but those that do can rise 20+ miles, depending upon the ejection flow and other conditions. These eruptions are by the way labelled as Plinian eruptions - after Pliny the Younger ( I suppose ) who described the look of a particular tree as being the shape of the column of Mount Vesuvius 79 AD.

The umbrella name was given before the nuclear age ( mushroom cloud ).

The troposphere tropopause altitude is latitude dependent - lower in the higher latitudes - so plumes will rise to a lessor height than nearer the equator.

this site gives some info.
http://ffden-2.phys.uaf.edu/645fall2003_web.dir/Ben_Andrews/Eruption_volc.html
as well as
https://www.ucl.ac.uk/volcanoscope/files/Types of Eruption/Cary_Bursik_Volcanic Plumes_Encyc of Volcanoes_2000.pdf

Edit: Corrected , The tropopause is the boundary between the troposhere and the stratosphere.

 

[anorlunda]

Might the jet stream play a role here?

 

[RJ Emery]

They might or could. They are typically near the top of the troposphere and flow westerly (west to east), but they may not be above an erupting volcano.

 

[anorlunda]

What made me think of it was the shape of the classical anvil cloud. In the northern hemisphere, the tip of the anvil usually points East.

 

[Baluncore]

The jet streams are much more complex and dynamic than is usually pictured. There are seasonal changes of strength. There is merging of the polar and subtropical streams. Only the equatorial zone is not swept by jets, and so provides some isolation between the the N & S hemispheres.

The jet streams are well predicted and reported here.
https://earth.nullschool.net/#curre...winkel3=-205.41,-0.97,218/loc=147.281,-42.645

Give it time to render. Then drag map, mouse wheel to zoom. Height = 250hPa is 30k feet.

 

[RJ Emery]

What made me think of it was the shape of the classical anvil cloud. In the northern hemisphere, the tip of the anvil usually points East.

In the continental United States, all weather generally moves west to east or northeast, so your observation about the tip of the anvil does not surprise me. Nevertheless, thanks for sharing.

 

[rsk]

My guess is that the ash cloud rises until it hits the tropopause, where the air ceases to cool with increasing altitude. This means that the ascending warm air, which has been gradually cooling as it ascends, suddenly finds itself cooler than the above air, halting its rise. The lower levels of the cloud are still rising, however, which forces the top level outwards to create the flat top. This is why thunderstorms typically have an 'anvil' shape when they mature.

That's my guess at least.

This - although it can also be lower than the tropopause, if there's what Meteorologists refer to as an inversion which basically means that, as Drakkith describes, the temperature above is warmer rather than colder. It can happen at lower levels and especially in winter when the temperature at the surface can be very cold with warmer air not far above. It explains why on cold winter days smog and pollution can be worse as they remain trapped below the inversion.

 

[davenn]

What made me think of it was the shape of the classical anvil cloud. In the northern hemisphere, the tip of the anvil usually points East.

As they do in the southern hemisphere as well

 

[Drakkith]

As they do in the southern hemisphere as well

I blame the great blacksmith in the sky. George.