Fate and Transport of Fine Volcanic Ash

William I RoseMichigan Tech UniversityHOUGHTON, MI 49931 USA

raman@mtu.eduwww.geo.mtu.edu/~raman

26 May 2010 ESRIN Frascati

MODIS Aqua 17 April 2010

• fine ash proportions erupted

• radar and---ascending ash

• ice or water in volcanic cloud?

• remote sensing dynamics----early fallout stage 2

• ash size and distance

• t-storm analogy---forecast possible?

• is the aged cloud dangerous?

• http://www.geo.mtu.edu/~raman/Ashfall/

Origin of particles in volcanic Origin of particles in volcanic cloudscloudsExplosive vesiculationExplosive vesiculation----As pressure drops in ascending magma--overpressured bubbles burstAs pressure drops in ascending magma--overpressured bubbles burst

Hydrothermal explosions--Hydrothermal explosions--rock fractured by thermal shock rock fractured by thermal shock from contact between magma and waterfrom contact between magma and water

Milling--Milling-- abrasion and grinding of particles can occur in pyroclastic flows and abrasion and grinding of particles can occur in pyroclastic flows and in the ventin the vent

Chemical and meteorological processes-- Chemical and meteorological processes-- condensation, sublimation, surface chemical reactions forming acids, salts, condensation, sublimation, surface chemical reactions forming acids, salts, hydrometeors and aggregates of mixed originhydrometeors and aggregates of mixed origin

Tephra is classified on the basis of pyroclast size: 

ASH -- Very fine-grained fragments (< 2 mm), generally dominated by broken glass shards, but with variable amounts of broken crystal and lithic (rock) fragments. Courtesy of USGS. LAPILLI -- Pea- to walnut-size pyroclasts (2 to 64 mm). They often look like cinders. In water-rich eruptions, the accretion of wet ash may form rounded spheres known as accretionary lapilli (left). Courtesy of USGS. BLOCKS AND BOMBS -- Fragments >64 mm. Bombs are ejected as incandescent lava fragments which were semi-molten when airborne, thus inheriting streamlined, aerodynamic shapes. Blocks (not shown) are ejected as solid fragments with angular shapes. Courtesy of J.P. Lockwood, USGS.

www.geology.sdsu.edu/how_volcanoes_work

Φ phiΦ = - log2 d (mm)

Lognormal size distributions are “expected” and we use a “biased” system to define them

ASHASH

LAPILLILAPILLI

BOMBS, BLOCKSBOMBS, BLOCKS

Total Grain-size distribution weighted by mass and by isopach volume, compared to Carey and Sigurdsson [1982].

18 May 1980

Mount St Helens

Fall deposit

40% of mass is <30 microns in diam

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Fine and very fine ash• Linked to fall and air resistance• volcanic ash , <2 mm diameter• fine ash , <1 mm : intermediate flow regime• very fine ash, < 30 microns : laminar flow regime• ash > 1 mm falls in ~30 min• as we progress after 30 min, exponential

thinning decreases and may reverse, and atmospheric sorting decreases to zero

Rose & Durant, 2009, JVGR 186: 31-39

Three stages of volcanic clouds• Stage 1--near volcano, first 1-2 hrs of ash

residence, exponentially thinning fallout of pyroclasts in turbulent flow, radar and webcam

• Stage 2-- several hours to 2 days, accelerated aggregate fallout of 90% of fine and very fine ash, infrared remote sensing (mie scattering)

• Stage 3--days to weeks? drifting volcanic clouds, very fine ash present but danger uncertain... trajectory models, particle/SO2 remote sensing

Volcanic Cloud StagesVolcanic Cloud stage 1 2 3

Duration, hrs after eruption stops ~1-2 18-24 24-96+

Ash fallout, km from volcano <25 25-400 >400

Area of ash fallout, km 2 <300 ~5 x104 ?,discontinuous

Ash fallout diameter range, mm >0.5 0.5-<0.01 <0.01

Ash fallout rate, kilotonnes/hr >10 4 <10 4-10 2 "very low"

Fraction of fine ash(d= 1-25 μ ), %m <1 10-50 >50 , Cloud Area km 2 <10 4 10 4-10 6 10 6 ,decreasing

, %/Cloud Area change hr >100 30-50 -10 +10to

, 11 Mean Optical Depth μm >2 0.5-2 <0.3 , / Cloud ash burden tonnes km 2 >25 3-10 <3

, %Fraction of ash mass suspended 100-~30 ~30-3 <3

Rose et al, 2001, J Geology, 109: 677-694

Keflavik Radar

Freezing occurs Freezing occurs rapidly over narrow rapidly over narrow height rangeheight range

Release of latent heat as Release of latent heat as all droplets freeze; burst all droplets freeze; burst of positive buoyancyof positive buoyancy

Influences maximum Influences maximum plume height plume height attainedattained

Droplets form as T Droplets form as T falls during risefalls during rise

-17<T<--17<T<-24˚C24˚C

(not to scale!)

freezing during volcanic plume risefreezing during volcanic plume rise

freezing level

Adam Durant

Thunderstorm Formation• Ingredients

–warm, moist air (often mT)–unstable (or conditionally unstable if lifting mech.)–encouraged by diverging air aloft

Rabaul, PNG, 1994

Reventador, Reventador, Ecuador, 2002Ecuador, 2002

Ice affects plume buoyancy and Ice affects plume buoyancy and maximum plume height, and particle maximum plume height, and particle falloutfallout

Maximum plume height is an essential Maximum plume height is an essential input parameter for volcanic cloud input parameter for volcanic cloud dispersion modelling used in real-time dispersion modelling used in real-time hazard mitigationhazard mitigation

convectivconvective columne column

ice-rich ice-rich stratospheric stratospheric ‘umbrella’ cloud‘umbrella’ cloud

~16 km max. ~16 km max. heightheightice-rich (>20 MT) ice-rich (>20 MT)

stratospheric stratospheric ‘umbrella’ cloud‘umbrella’ cloud ~20 km max. ~20 km max.

heightheight

Adam Durant

During much of the Eyjafjallajökull eruption, the ash column did not reach the level of ice formation. This affects the aggregation and fallout.

Laminar flow; RN = 10-2

Turbulent flow; RN = 106

RN = 20 RN = 40 RN = 104

Coarse ashCoarse ash

Fluid dynamics applies dimensionless analysis of fall of spheres in the atmosphere, which shows that experience with large pyroclasts might not apply to smaller ones which fall much more slowly…

RN =dvRN =dvttρρ//ηη

Fine and very fine Fine and very fine ashash

10 m/s

D = 1mmD =

1µm

.01 cm/s

Fall of spherical particles in earth’s atmosphere

Schneider et al., 1999, J Geophys Res 104 4037-4050

d < 1000 μmΦ > 0

d < 30 μmΦ > 5

Each stage 2 ashfall has a size distribution which does not reflect distance or age, and which includes ash down to submicron diameters.

2Map from Sarna-Wojcicki et al. [1981]; isomass contours in g/cm3.

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Ave. Cumulative Mass Fraction

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Aggregate Growth

From Gilbert and Lane [1994]

Binding forces:

electrostatic

surface-tension from liquid films

ice formation

mechanical interlocking

Collisions:

differences in particle terminal fall velocities

electrostatic attraction (if separation distance is low)

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MSH80 Aggregate Fall

Map adapted from Sarna-Wojcicki et al. [1981]; isomass contours in g/cm3

Meteorological Cloud Volcanic CloudMeteorological Cloud Volcanic Cloud

Many IN

Small ice HM

Little Precip

Sublimation

Few IN

Bergeron

Large Ice HM

Precipitation

Durant et al., 2008, JGR 113

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Mammatus simulation:thunderstorm cirrus outflow anvil

Kanak and Straka, Atmos. Sci. Let. 7: 2–8 (2006)

~6000 m

Simulation time: 20 minutes!

10 µm snow aggregate diametercontours

dry sub-cloud layer

snowflake aggregation induced

Cloud descent rate:

~6.5 ms-1

10 µm ice crystal descent rate:

<10-2 ms-1

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Conceptual Model: Distal Fallout

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FLEXPART forecasts ash cloud motion, but how does it account for stage 2 fallout, and beyond?

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CIMSS experimental product using SEVIRI and showing ash loading, cloud height and particle size.

This data is a potential step forward, and may allow for quite sophisticated interpretation, and fits well after ground based radar and before/with trajectory models.

M Pavolonis, NOAA CIMSS.

The sizes of ash particles sensed optimally by these methods are about 1-25 µm (~5-9 φ).

“Aged” volcanic clouds“Aged” volcanic cloudsCoarse ash falls out of cloud within

~30 minutes

Most remaining ash has fallen out of cloud within 24-36 hours

Trackable up to 4 days with IR split window, mostly SO2 and sulfate after that

Aged = over 2 days old

Do they still pose a risk to aircraft?

GOES - visibleGOES - visible

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Courtesy of Scott Bachmeier, U. Wisconsin, MadisonCourtesy of Scott Bachmeier, U. Wisconsin, Madison