Fate and Transport of Fine Volcanic Ash

Transcription

1 Fate and Transport of Fine Volcanic Ash William I Rose Michigan Tech University HOUGHTON, MI USA raman@mtu.edu 26 May 2010 ESRIN Frascati MODIS Aqua 17 April 2010

2 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?

3 Origin of particles in volcanic clouds Explosive vesiculation As pressure drops in ascending magma overpressured bubbles burst Hydrothermal explosions rock rock fractured by thermal shock from contact between magma and water Milling in the vent abrasion and grinding of particles can occur in pyroclastic flows s and Chemical and meteorological processes condensation, sublimation, surface chemical reactions forming acids, salts, hydrometeors and aggregates of mixed origin

4 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.

5 Lognormal size distributions are expected and we use a biased system to define them BOMBS, BLOCKS LAPILLI Φ phi Φ = log 2 d (mm) ASH

6 Wt.% May 1980 Mount St Helens Fall deposit Mass-weighted Isopach-weighted Carey and Sigurdsson [1982] 40% of mass is <30 microns in diam φ Total Grain-size distribution weighted by mass and by isopach volume, compared to Carey and Sigurdsson [1982].

7 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:

8 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

9 Volcanic Cloud Stages Volcanic Cloud stage Duration, hrs after eruption stops ~ Ash fallout, km from volcano < >400 Area of ash fallout, km 2 <300 ~5 x10 4?,discontinuous Ash fallout diameter range, mm > <0.01 <0.01 Ash fallout rate, kilotonnes/hr >10 4 < "very low" Fraction of fine ash(d= 1-25 m), % < >50 Cloud Area, km 2 < ,decreasing Cloud Area change, %/hr > to +10 Mean Optical Depth, 11 m > <0.3 Cloud ash burden, tonnes/km 2 > <3 Fraction of ash mass suspended, % 100-~30 ~30-3 <3 Rose et al, 2001, J Geology, 109:

10 Keflavik Radar

11

12 freezing during volcanic plume rise Influences maximum plume height attained Release of latent heat as all droplets freeze; burst of positive buoyancy freezing level -17<T< 17<T<-24 C Freezing occurs rapidly over narrow height range (not to scale!) Adam Durant Droplets form as T falls during rise

13 Ingredients Thunderstorm Formation warm, moist air (often mt) unstable (or conditionally unstable if lifting mech.) encouraged by diverging air aloft

14 ice-rich (>20 MT) stratospheric umbrella cloud ~20 km max. height ice-rich stratospheric umbrella cloud ~16 km max. height Rabaul, PNG, 1994 Ice affects plume buoyancy and maximum plume height, and particle fallout convective column Maximum plume height is an essential input parameter for volcanic cloud dispersion modelling used in real-time hazard mitigation Reventador, Ecuador, 2002 Adam Durant

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

16 RN =dv t / Fine and very fine ash Coarse ash D = 1µm D = 1mm.01 cm/s 10 m/s Laminar flow; RN = 10-2 RN = 20 RN = 40 RN = 10 4 Turbulent flow; RN = 10 6 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

17 Fall of spherical particles in earth s atmosphere Schneider et al., 1999, J Geophys Res

18 d < 1000 μm Φ > 0 d < 30 μm Φ > 5 Each stage 2 ashfall has a size distribution which does not reflect distance or age, and which includes ash down to submicron diameters.

19 2 Map from Sarna-Wojcicki et al. [1981]; isomass contours in g/cm 3.

20 Ave. Cumulative Mass Fraction 3

21 Aggregate Growth Collisions: differences in particle terminal fall velocities electrostatic attraction (if separation distance is low) 6 Binding forces: electrostatic surface-tension from liquid films ice formation mechanical interlocking From Gilbert and Lane [1994]

22 MSH80 Aggregate Fall 5 Map adapted from Sarna-Wojcicki et al. [1981]; isomass contours in g/cm 3

23 Durant et al., 2008, JGR 113 Few IN Bergeron Large Ice HM Precipitation Many IN Small ice HM Little Precip Sublimation Meteorological Cloud Volcanic Cloud

24 Mammatus simulation: thunderstorm cirrus outflow anvil snowflake aggregation induced Simulation time: 20 minutes! Cloud descent rate: ~6.5 ms 1 10 µm ice crystal descent rate: <10 2 ms 1 QuickTime and a GIF decompressor are needed to see this picture. 10 µm snow aggregate diameter contours dry sub cloud layer ~6000 m Kanak and Straka, Atmos. Sci. Let. 7: 2 8 (2006)

25 Conceptual Model: Distal Fallout 9

26 QuickTime and a decompressor are needed to see this picture. FLEXPART forecasts ash cloud motion, but how does it account for stage 2 fallout, and beyond?

27 CIMSS experimental product using SEVIRI and showing ash loading, cloud height and particle size. QuickTime and a GIF decompressor are needed to see this picture. 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.

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

29 Aged volcanic clouds Coarse ash falls out of cloud within ~30 minutes Most remaining ash has fallen out of cloud within hours Trackable up to 4 days with IR split window, mostly SO 2 and sulfate after that Aged = over 2 days old Do they still pose a risk to aircraft?

30 GOES - visible QuickTime and a GIF decompressor are needed to see this picture. Courtesy of Scott Bachmeier, U. Wisconsin, Madison