storm rotation in vulcanic clouds

[Menno van der Haven]

In my opinion perhaps only a vulcanic ash cloud can beat the esthetics of a supercell storm. Although I have never witnessed a vulcanic eruption, I wonder about a few things:

-Updraft speed in vulcanoes is for the largest part determined by the enormous pressure below the earth. But what CAPE values are possible when for example a Mt.St.Helens kind of eruption takes place? Perhaps 10,000 J/kg or only a few thousand J/kg?

-If windprofiles are favourable for supercell development when a vulcano eruption takes place, the whole pyroclastic cloud must start to rotate. Is this observed? Or is this impossible because when a "vulcanic storm" moves off the vulcano, it's simply been cut off of his heat source?

I've seen in television programmes that there is indeed rotation in vulcanic clouds, but I think that this is not the effect of dynamic perturbation pressures but due to something else, like you can see in forest fires or even in smoking plumes of a factory.


A nice example of a supercell like vulcanic eruption can be found here:

http://www.calstatela.edu/faculty/acolvil/volcanos/pinatubo_cloud.jpg

 

[John Diel]

I think something to rremember with Volcanic vs. Convective "Supercells" there art two entirely different forces at play.

Volcanic Clouds are composed primarily of ash ejecta from the volcano and forced upwards into the higher atmosphere by heat and pressure from the eruption. Yes, they do generate some electricity (Static Discharges or lightning) and yes, they do eventually cause some rainfall, but they are not atmosphere driven.

Atmospheric Supercells are driven by High and Low pressure areas, differences in the atmosphere itself given water, heat, density, etc.

I don't know that you could measure the actual j/kg on a volcanic ash cloud. You certainly couldn't do it as you would a Midwestern Supercell. The cloud composition is all different.

A Pyroclastic Cloud is something else altogether and I don't believe is influenced by local weather conditions. These are ash clouds that move superheated gases and ash from the eruption along the ground terrain and thus are affected by that terrain and how long the ash/gases are given force from the initial eruption.

An upward expanding ash cloud will be affected by weather patterns as the ash cloud moves higher in the atmosphere and becomes less dense with the fallout of the hevier ash componenets, Depending on the eruption this is either carried by the prevailing winds or, if ejected high enough, into the jetstream.

I think any "rotation" you may be seeing is simply heat/pressure forces from the eruption itself rather than by any convective weather forces.

Interesting thoughts though. Some of the higher learned Meteorologists should be able to give better insight than I can though.

John Diel

 

[Bob Schafer]

This is a fabulously interesting post!!

I can't answer your questions very well, but I have a couple thoughts:

CAPE would likely be extreme, yet there would probably not be polar air at mid and upper levels, and you might or might not have a moist adiabatic lapse rate to work with... so I dunno.

Having an eruption coincident with supercell hodograph parameters would also be quite a coincidence, and not likely.

Yes, as soon as a storm "moved off the mountain", it would be deprived of it's heat source, and any ability the storm would have to maintain itself would be on it's own merit, which, again, would be quite a coincidence. In that case, the volcano would have been little more than a lifting mechanism.

I've never seen that pic before. Simply awesome.

 

[Aaron Kennedy]

Both phenomena involve fluid flow so they are all governed by the same laws. With storms and supercells we are concerned with bouyancy owing to warm, moist air (less dense). With volcanoes, bouyancy will be driven by superheated air from lava etc. Either updraft can tilt environmentally present vorticity into the vertical. My concerns with a volcanically forced updraft is its small scale (100s of meters?) and extreme vertical velocity. Turbulence and entrainment probably destroy vertical vorticity pretty quickly (bouyancy term >> shear term). On the other hand, I'm sure you get some nice vorticity rings surrounding the updraft (think microburst). The cool laminar features seen with some volcanic and nuclear blasts are probably courtesy to mid level inversions. I'd like to see some shots of these clouds overlaid with a sounding taken nearby.

but due to something else, like you can see in forest fires or even in smoking plumes of a factory.

Yep. Flows driven by thermal plumes.

 

[Terry Tyler]

i dont know about organized rotation, but i have seen a night-time video with small lightning occuring inside of a volcanic ash cloud...

since windshear influences clouds of water vapor...i dont see why it wouldent influence a major thing like that...interesting...and perplexing at the same time...i can imagine a big tornadic water vapor supercell, but not a big tornadic ash supercell...

as far as the instability generated?

i guess you would just have to measure the temperature of the ash cloud and the surounding air...or, no thats the LI...oh well, something along those lines...

 

[David Draun]

i dont know about organized rotation, but i have seen a night-time video with small lightning occuring inside of a volcanic ash cloud...

since windshear influences clouds of water vapor...i dont see why it wouldent influence a major thing like that...interesting...and perplexing at the same time...i can imagine a big tornadic water vapor supercell, but not a big tornadic ash supercell...

as far as the instability generated?

i guess you would just have to measure the temperature of the ash cloud and the surounding air...or, no thats the LI...oh well, something along those lines...

LOL, subzero degrees F surrounding air at 500mb - ash cloud of over several hundered degrees F at 500mb = one HELL of a negative LI!

 

[cdcollura]

Oh, by the way...

That plume from the Pinatubo eruption in the Phillipines reached an altitude of over 40 km, which is 25 miles (132,000 feet), about two and a half times as high as a Great-Plains supercell!

In this case, it's like punching a "hole in the sky" - literally.

Updrafts in such a plume can be well in excess of 500 MPH. The core temperatures can be 600 - 1,000 degrees C (about 1,500 degrees F) or more (so don't even think of core-punching these things unless you wanna be carbonized).

Updrafts can be from 100 to 600 M/Sec (or in MPH, 225 to 1,350 MPH - Yup that's up to a super-sonic Mach 1.8 straight up). At such speeds the upper atmosphere is reached, and ash / debris is pumped high into the upper stratosphere.

The plume does spread out at the tropopause, just as thunderstorm anvils do, due to the cooler edges of the hot plume. This forms what is called the "umbrella region", and depends on the topopause (start of stratosphere) height, which can be 35,000 to 45,000 feet in temperate regions like Alaska during the winter, but above 65,000 feet in tropics in places like Monserrat or the Phillipines.

The plumes should have an over-shooting top as well, much - MUCH higher than any thunderstorms because of the high temperatures in the core of the plume. These often do not stop at the stratosphere during big eruptions, they continue above into the mesosphere, that's why the fine ash can circle the globe and cause global cooling.

The element of rotation is very tricky in a volcanic plume. Obviuously, environmental winds and atmospsheric conditions prevailing DURING the eruption will play a big role in this. If an eruption occurred with strong SE surface winds and a screaming 150 Knot jet aloft, sure, maybe you'll have some rotation.

An eruption in the tropics, where many such volcanoes are located, often will be in a light-wind field environment, as the jet stream is far away from places like the Caribbean or the Phillipines. The Pinatobo eruption did, however, occur in the strong low-level wind environment of a tropical system (typhoon).

Maybe this is what caused the rotation in the updraft in that picture?

Or, maybe it's not rotation, but a stacked plates of pileus (cap) clouds that encircle the updraft due to its sheer upward speed, like what is noted along gust front (example - the Bobby Eddins shots in May 27, 2001)?

The updtaft plume should also displace a large volume of air, creating inflow and strong winds. Valleys and mountains should interact with this flow.

In other cases, especially Mt St Helens, the blast was not directed straight up, it was a lateral blast, like a gun-shot, which traveled from the north side and obliverated many square miles of forest, yet trees were still standing on the south side of the mountain. Would such a plume create "shear" vortices on it's edges near the ground?

The Surtsey Volcano in Iceland DID create water spouts and tornadoes HAVE been observed with Mount Etna eruptions.

This is an area of great interest since Volcanic ash plumes can have some nasty tricks of their sleeves besides heat and ash!

 

[rdale]

I don't think Coriolis operates on a scale that would impact circulation around a smokestack plume or building fire...

 

[rdale]

Already had one person say that Coriolis affects his toilet - before that gets too far read http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html

 

[rdale]

So that my reply doesn't get confused - I was tackling Richard's comment that a building fire or smokestack is impacted by Coriolis. That's what I am denying. I would imagine it's possible there could be something on a widescale event like a volcanic plume, and a quick google of "coriolis smoke" came up with a paper that indicates under the right conditions the impact of Coriolis can be seen...

http://www.springerlink.com/content/w124246844029jrt/

But NOT from a building fire or smokestack. Rossby numbers would be way too high.

 

[Glenn Rivers]

Volcanic Plumes

I have done a lot of reading of reading about the dynamics of volcanic jets, plumes, and pyroclastic density currents. Some interesting factoids about them.....

-In large eruptions, the thermal buoyancy forces acting on the hot ash cloud are numerically much more important than the initial upward jet, even at "muzzle" velocities around 300 m/s.

-The bulk density of most pyroclastic jets at the vent are about 10 times more dense than the surrounding atmosphere, therefore the initial jet is very negatively buoyant. as the upwards momentum of the heavy jet is gradually lost, the entrainment and expansion of ambient air helps make the jet lighter. If this mixing can make the jet positively buoyant before it runs out of its initial upwards inertia, the jet will transform into a buoyant thermal plume, and often accelerate upwards reaching a second vertical velocity maximum high in the troposphere or even in the stratosphere. When the mass flux of ash is as large as one million tons per second or about a billion kilograms per second, the upward velocity of the thermal plume can become even greater then the initial muzzle velocity of the jet at the vent, and the level of neutral buoyancy can be located as high as 35 km, or 110,000 feet, high within the stratosphere, with an overshooting top as high as 50 km or about 160,000 feet!

-If the initial jet does not become buoyant before it runs out of vertical momentum, it will collapse, forming a plunging fountain thousands of feet high, which then spreads across the terrain around the eruption as a negatively buoyant, ground hugging density current somewhat like an outflow current fed by a collapsing downburst. These "pyroclastic density currents" are enormously destructive and can cover hundreds of square miles of terrain with speeds of up to ~200 m/s or 400 knots, and temperatures of hundreds of degrees. These pyroclastic density currents can be more lethal than a hydrogen bomb and devastate an even larger area.

-A giant pyroclastic density current over-ran 6,000 to 10,000 square miles of North Island New Zealand in just minutes around 186 AD when an eruption caused the collapse of the roof of a shallow magma chamber, opening up a giant vent that hemorrhaged about 100 billion tones of ash and pumice in perhaps 10-20 minuts.....an eruption rate of perhaps 100 million tones per second. This rate is 10,000 times the rate of an eruption with a minimum flux necessary to create a thermal plume that can penetrate the troposphere.....about 10,000 tons per second.

-To produce tall thermal plumes, it is helpful for an eruption to produce a total grain size distribution that contains abundant fine ash. Fine particles transfer their heat to the surrounding atmosphere rapidly and efficiently.

-Eruptions of high viscosity, high silica magma that produce pumice and ash produce a lot of vary fine particles......perhaps 80% of the erupted mass is composed of particles finer than 1 mm (course sand) and about 50% of the erupted mass is made of particles finner than 64 microns (silt sized). A key process for producing abundant fine fragments is that magma, especially high viscosity magma that is capable of flowing as a viscous fluid at low strain rates becomes brittle and literally shatters at higher strain rates....a rather counter-intuitive behavior. The size of the particles also seams to be related to the size of the gas bubbles in the magma at the moment that it is subject to brittle fracture and is shattered largely to dust. Many bubbles in high silica magma are very small...tens of microns, rather than the mm to cm sized bubbles often found in low silica, low viscosity basaltic magma. Much of the finer ash in silicic eruptions is composed of platty, curved fragments derived from the walls of the tiny bubbles found in silicic magma.