What causes volcanic mesocyclones to rotate in the direction they do?

March 29, 2009 10:55 AM

Recent research has concluded that strong volcanic plumes exhibit cyclonic, storm-like rotation, which helps explain why volcanic thunderstorms occur. The article indicates that the volcanic plume from Mt. Pinatubo in 1991 exhibited anticlockwise rotation, which I infer to be consistent with the rotation of a low-pressure system in the northern hemisphere. My question: Will the rotational direction of these volcanic mesocyclones be governed by the Coriolis Effect, local atmospheric conditions, compositional characteristics of the plume, or some combination of these factors?
posted by baphomet to Science & Nature (6 answers total)

Best answer: Not a meteorologist or a vulcanologist (hey, how cool is the disclaimer, IANYV?), but I am an active pilot and well-grounded (har!) in basic weather science. My guess would be that the Coriolis Effect would be the overwhelmingly dominant factor. It's hard to imagine a better lifting mechanism than a volcanic plume, and something's got to rush in to replace that rising airmass. The effect would seem to be similar to a well formed low pressure area. I suspect that significant local weather could have some effect, but I would still expect rotation consistent with Coriolis.
posted by dinger at 3:51 PM on March 29, 2009

Best answer: This paper might help you.

Baines, P. G., and Sparks, R. S. J. Dynamics of giant volcanic ash clouds from supervolcanic eruptions.
Geophysical Research Letters (2005): 32: L24808.

The largest explosive volcanic eruptions that have occurred on Earth generated giant ash clouds from rising plumes that spread in the stratosphere around a height of neutral buoyancy, with estimated supply rates that are in the range 10¹¹ to 10¹³ m³/s. These giant ash clouds are controlled by a balance between gravity and Coriolis forces, forming spinning bodies of nearly fixed proportions after a few hours and are initially insensitive to stratospheric winds. In contrast, volcanic plumes from eruptions with small to intermediate magnitude spread as inertial intrusions under the influence of stratospheric winds, with the Earth's rotation being unimportant. In the largest eruptions the giant spinning ash clouds typically develop diameters greater than 600 km and up to 6000 km in the most powerful super-eruptions, thus explaining why areas of continental size can be covered with volcanic ash. The radial expansion and spinning velocities are calculated at tens of metres per second and increase with eruption intensity. Higher spreading velocities carry larger ash grain sizes to a given radius, so that grain size at a given distance from the source increases with eruption intensity, consistent with geological observations.

The point of the paper, more or less, is that based on a simple physical model and some (reasonable) assumptions about the eruption flow volume and a few other parameters, you can calculate an eruption size threshold at which Coriolis forces begin to have significant effects on the development of a plume.

I don't know what your level of science literacy is, but I'll excerpt some sections:

Very large magnitude explosive eruptions have the potential for global catastrophe. Such eruptions can cover continental scale areas with volcanic ash and may cause severe environmental effects in the years and possibly decades that follow. Here we consider explosive eruptions of magnitude M = 6.5 and above, where M = Log₁₀ m – 7 and m is the mass of magma erupted in kg. Such eruptions range from the largest magnitude eruptions of the last century, such as Mount Pinatubo in 1991 (M ~ 6.5), to the largest explosive eruptions known, such as the eruption of Toba, Sumatra at 71 kya (M ~ 9). Explosive eruptions with M > 8 are commonly described as “super-eruptions”. Here we establish that the spreading of such clouds is controlled by the Earth's rotation. The giant ash clouds can have very large horizontal dimensions, with spreading and spinning velocities exceeding typical stratospheric wind speeds. The model can explain why continents can be covered with volcanic ash, and why the grain size of the ash tends to be much coarser than in smaller magnitude explosive eruptions . . .

. . . The calculations confirm that only explosive eruptions of magnitude 6.5 and above develop giant clouds a few thousand kilometres in diameter where rotational effects dominate. Mount Pinatubo in 1991 falls at the lower end of this magnitude range. The cloud for Pinatubo (magma flux ~ 7 × 10⁴ m³/s and stratospheric flow rate ~ 5 × 10¹⁰ m³/s) behaved like a gravity-driven intrusion during its initial expansion , reaching a diameter of 1200 km after 4 × 10⁴ seconds. Rotational waves were observed at a diameter of 800 km in qualitative agreement with our model . . . Above a magnitude of 6.5 the clouds have dimensions from several hundred kilometres to a few thousand kilometres in the largest magnitude cases. Beyond the threshold diameter the radial expansion speeds are commonly larger than typical stratospheric wind speeds but reduce to low values as the spin velocities increase with size; the cloud dimensions become constrained by the Earth's rotation . . .

. . . Hence a dynamic model of ash clouds from very large magnitude eruptions shows that rotational effects become dominant for eruptions that have magnitude much greater than about M ~ 6.5. The clouds expand to diameters of hundreds to thousands of kilometres in time scales of order of a day, and rotate in an anti-cyclonic manner much like a rigid body. The size of a giant cloud is limited by its volume and cyclostrophic balance, which helps to maintain its integrity against a mean stratospheric wind. This explains how these giant ash clouds can cover continent-sized areas and deposit ash over them, as has been observed from geological evidence of previous eruptions . . .

If you would like a copy of this paper PDF, let me know. A Google Scholar search for "volcanic" and "Coriolis" will turn up other papers too.
posted by jeeves at 5:42 PM on March 29, 2009

Response by poster: Thanks very much, folks. That's about the behavior I expected, but I wasn't entirely certain. Being a new student of geography, atmospheric science, and vulcanism, the question piqued my curiosity, and I'm glad to know that my reasoning was valid! It's especially interesting that 6.5 is the critical magnitude for rotational behavior to be observed, that's pretty high up on the VEI.
posted by baphomet at 7:54 AM on March 30, 2009

Perhaps I'm just thinking about this wrong, but I'm confused by the question and link you provide. You say that the Mt. Pinatubo plume exhibited cyclonic rotation, whereas jeeve's quote above says it should have exhibited anti-cyclonic rotation.

I would expect the Coriolis effect to cause anti-cyclonic rotation of the plume up to the point in time where it is in cyclostrophic balance, because the ash is clearly moving outward from the center.
posted by dsword at 8:38 AM on March 30, 2009

Response by poster: dsword: That is indeed interesting...the research clearly confirmed with satellite imagery that the rotation of Pinatubo's plume was indeed cyclonic, since (as dinger says) the updraft from the volcanic emission would necessarily create a local low-pressure system, which would exhibit cyclonic rotation in the northern hemisphere. That, I suppose, was the crux of my question: would an eruption similar to Pinatubo's in the southern hemisphere show anti-cyclonic rotation due to Coriolis?

I'm not sure I have a good answer for why Pinatubo exhibited characteristics in contradiction to this section of jeeves' quoted text (to which I assume you reference):

...the clouds expand to diameters of hundreds to thousands of kilometres in time scales of order of a day, and rotate in an anti-cyclonic manner much like a rigid body.

I would point out that the research I linked to is more recent than that paper, but there could be something we're missing.
posted by baphomet at 9:21 AM on March 30, 2009

Hey all — I think my excerpting is likely the cause of your confusion. I went back and skimmed the paper. "Anti-cyclonic" in the paper refers not to Pinatubo specifically but to the behavior of their simple model for large ash cloud formation.

Hopefully this'll work; I've uploaded the PDF here so that you can view it as needed.
posted by jeeves at 5:34 PM on March 30, 2009