RESEARCH FOCUS RESEARCH FOCUS: Toward a realistic formulation of fine-ash lifetime in volcanic clouds

Adam J. Durant1,2 1Meteorology and Oceanography Section, Department of Geosciences, University of Oslo, Blindern, 0371 Oslo, Norway 2Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, USA

Ash cloud detection and forecasting became a research priority and cloud forecast is dependent on the sink description; errors increase as the societal concern (Bonadonna et al., 2014) after the 2010 Eyjafjallajökull forecast is extended into the future. (Iceland) eruptions caused widespread flight cancellations and an esti- Cloud instabilities reduce fine ash atmospheric lifetime and have mated U.S.$5.0 billion impact on global gross domestic product (GDP) been observed on many recent volcanic clouds: ‘Ash veils’ on the base (5 day period up to 24 May 2010; [The Economic Impacts of Air Travel of a 3 h old ash cloud from the 1990 eruption of Mount Redoubt, Alaska, Restrictions Due to Volcanic Ash, 2010 [www.oxfordeconomics.com/ USA (Hobbs et al., 1991); ‘finger-like protrusions’ on a cloud from the my-oxford/projects/129051]). In 2011, Grímsvötn volcano (Iceland) 1997 Montserrat (Caribbean) eruptions (Bonadonna et al., 2002); and ice- again caused widespread cancellations and in 2014 an aircraft sustained rich, turbulent mammatus lobes (Schultz et al., 2006) on the 1980 Mount ~AU$20.0 million engine damage after flying through a volcanic cloud St. Helens eruption cloud (Durant et al., 2009). The properties of an ash from Gunung Kelud (Java) (The West Australian, 22 February 2014 [au. deposit, i.e., poor sorting, polymodal size distributions and localized lat- news.yahoo.com/thewest/a/21618743/20m-ash-cloud-damage-bill/]). eral thickness variations, may provide evidence of particle settling influ- Since 1953, over 129 incidents involving aircraft and volcanic ash clouds enced by cloud instabilities (e.g., Carazzo and Jellinek, 2013). Fine-ash have occurred and more than 79 sustained serious damage (Guffanti et settling through the ocean water column may also be enhanced by trans- al., 2010). Fundamental to managing risk and economic impacts is fore- port in gravitational convective instabilities (Carey, 1997; Manville and casting of volcanic ash clouds (e.g., Casadevall, 1994), which depends Wilson, 2004). on accurate models of ash sedimentation. Manzella et al. (2015, p. 211 in this issue of Geology) used high- Volcanic cloud modeling consists of a description of the source (char- resolution imagery to link observations of gravitational instabilities on the acteristics of the eruption column such as geometry, height, mass erup- Eyjafjallajökull eruption cloud to rapid fine-ash sedimentation. Layers at tion rate, vertical separation of ash and gases) (e.g., Mastin et al., 2009; the cloud base peeled away forming ‘fingers’ that carried fine-ash par- Moxnes et al., 2014) and the sink (cloud microphysical processes and sedi- ticles downward at ~1 m/s, orders of magnitude faster than the predicted mentation) (e.g., Costa et al., 2010). Source and sink terms are coupled to terminal fall velocities of the smallest particles. The resulting deposit a model describing atmospheric dynamics and microphysics to simulate within 10 km of the vent consisted of a coarse, unimodal size population particle transport (e.g., Stohl et al., 2011). Quantification of the source has of coarse ash; beyond 10 km, fallout was poorly sorted with bimodal par- been improved, while the sink has received less attention. ticle size related to aggregate fallout (Bonadonna et al., 2011). The authors Volcanic tephra comprises of particles ranging from meters to sub- develop a gravitationally driven convective settling law for fine ash in vol- micrometer size (Durant et al., 2010). ‘Classical’ models of tephra sedi- canic clouds based on laboratory experiments using glass microspheres mentation focus on gravitational settling of single particles (e.g., Sparks in a density-stratified aqueous isothermal solution (building on work by et al., 1997). Large particles fall out quickly, less-affected by inertial Carazzo and Jellinek, 2013; Hoyal et al., 1999). Instability propagation and viscous drag; good agreement is achieved between predicted and reduced fine-ash particle lifetime by 1–3 orders of magnitude relative to observed coarse deposit characteristics within 10–100 km of the volcano single particle settling. This research offers the basis for simple, effective (e.g., Pyle, 1989). In distal (100–1000 km) sections, the particle size is parameterization in operational modeling . much finer and observed and predicted characteristics agree poorly (e.g., The role of hydrometeors in aggregation and formation of instabili- Fierstein and Nathenson, 1992). Calculating particle fall based on Reyn- ties should also be considered. Volcanic clouds commonly contain more olds number in different flow regimes (Bonadonna et al., 1998; Rose et water than the background atmosphere (e.g., Williams and McNutt, 2004) al., 1993) and correcting for non-spherical particle shape provides some and ash particles act as nucleation sites for water phases (e.g., Durant et improvement (e.g., Bursik, 1998; Ganser, 1993; Riley et al., 2003; Wil- al., 2008). Understanding of mammatus clouds provides a meteorologi- son and Huang, 1979). cal analog to gain insight into the mechanisms driving ash cloud insta- The 18 May 1980 eruption of Mount St. Helens (USA) provided the bilities (e.g., Schultz et al., 2006). Such features typically occur on the opportunity to observe distal ash sedimentation (e.g., Sarna-Wojcicki et underside of thunderstorm anvils, and there have been numerous sight- al., 1981). Particle aggregate fallout was observed over a vast area (e.g., ings on recent volcanic clouds. Within volcanic mammatus lobes, ash- Sorem, 1982) and enhanced fine-ash (<63m m; Mastin et al., 2009) sedi- hydrometeors­ accumulate at the base of the cloud layer and evaporate or mentation was required to reconcile observations and predictions (Carey sublimate (resulting in cooling of the immediate atmosphere due to the and Sigurdsson, 1982). Aggregation (or coagulation) of ash particles (e.g., associated latent heat exchange). Gravitational loading, combined with Brown et al., 2012) into larger composite particles with a higher terminal the associated increase in air density, triggers the onset and propagation fall velocity leads to a reduction in the atmospheric lifetime of fine ash. As of instabilities leading to bulk sedimentation (Durant et al., 2009). A more most operational forecasting models do not account for aggregation, prox- contentious issue concerns the factors leading to the onset of cloud base imal ash deposition may be underestimated and distal airborne ash frac- instabilities. Specifically, does the process of aggregation drive the forma- tion overestimated, which is particularly relevant for forecasting aviation tion of convective instabilities, or does the development of instability lead hazards (e.g., the pan-European airspace closures in 2010 were largely to an increase in particle aggregation (through turbulent particle interac- guided by model-based predictions). Even sophisticated source inversion tions)? A complete model for fine-ash settling and removal will, therefore, modeling constrained by satellite observations (e.g., Stohl et al., 2011) need to consider hydrometeor formation and water phase changes within may have similar limitations: while the source term is optimized, the ash the frame of particle aggregation and bulk cloud instabilities.

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