|Internship at the Canadian Meteorological Centre: Adam Durant Winter 2006|
|Adam Durant and Marika Dalton were fortunate enough to be able to visit and work at the Canadian Meteorological Centre (CMC) during their Ehaz exchange: this is the home of the Montreal VAAC.|
|The Canadian Meteorological Centre, Montreal, Quebec|
|Using software developed at CMC, Adam carried out an analysis of the trajectories of volcanic clouds produced by the 1992 eruptions of Mount Spurr, Alaska: a Lagrangian particle model was used in conjunction with wind fields from reanalysis data to investigate cloud dispersal and particle fallout. Cloud heights and thicknesses were analysed over time for the August and September eruptions by comparing Lagrangian particle heights to satellite-observed cloud position. This information is important for hazard assessment and mitigation. Particle fallout from the September eruption cloud was investigated by applying Stokes� settling to individual particles.|
Explosive volcanic eruptions (VEI=4) produce plumes that inject fine pyroclasts called ash (<2mm in diameter) into the stratosphere approximately every 2 years [Textor et al., 2003], with larger events every 5.5 years [Grainger and Highwood, 2003]. Volcanic clouds resulting from these eruptions contain significant proportions of fine ash particles 25 μm diameter [Rose et al., 1980], with atmospheric residence times of hours to days. Fine volcanic ash presents hazards to: (1) aviation [e.g., Casadevall, 1994]; (2) human and animal health [e.g., Baxter et al., 1999]; and (3) infrastructure [e.g., Spence et al., 2005]. Volcanic ash transport and dispersion models (VATDMs) are used to model cloud trajectory and are employed operationally by Volcanic Ash Advisory Centres (VAACs) around the globe to mitigate hazards from fine volcanic ash in the atmosphere following an explosive eruption. The Montreal VAAC is based at the CMC, Montreal, and integrates volcanological, meteorological and aeronautical information to provide warnings to aircraft in the event that a volcanic cloud enters Canadian airspace. Eruptions that may impact this region generally orig- inate from volcanoes in the Pacific �Ring of Fire� and the Mid-Atlantic Ridge.
Why is Cloud Trajectory Analysis Useful?
Satellite remote sensing is a powerful method for making rapid observations over a wide area of the Earth. For this reason, it is especially suited to observing the dispersal of volcanic clouds over long distances. Using the �split window techique� (e.g. Prata, 1989) under certain conditions, volcanic clouds can be discriminated through an associated negative brightness temperature difference (BTD). Determination of cloud top temperature can be used to approximate cloud height. However, information on cloud thickness (important for modeling dispersion) cannot be determined using satellite-based remote sensing. Lagrangian VATDMs can be employed to estimate cloud parameters such as thickness and height as a function of time. A simulation is carried out under specified boundary conditions that reflects observations of a given eruption (e.g. maximum cloud height, emission duration, grain-size, mass erupted). Cloud height and thickness can be constrained through comparison of model predicted particle position with satellite-observed cloud position after dispersion from a specified point source (volcano). Included in the boundary conditions is a description of the emission source with initial vertical particle number/mass distribution, which subsequently evolves according to the atmospheric dynamics.
1992 Eruptions of Mount Spurr
Crater Peak and Mount Spurr are located approx. 125 km west of Anchorage, Alaska. Crater Peak is a basaltic-andesite stratocone which has been active for at least the last 5,000 years. It is the surface expression of a satellite vent that formed 10,000-20,000 B.P. and truncates an ancestral stratovolcano. Mount Spurr is an ice-covered, andesite lava dome complex adjacent to Crater Peak, and has not erupted in historic times. Crater Peak erupted on 27 June, 18 August, and 16-17 September 1992. Many sources of observational data exist for the volcanic cloud and associated fallout deposit from this eruption including radar [Rose et al., 1995], satellite remote sensing [Bluth et al., 1995; Rose et al., 2001; Schneider et al., 1995] and field-focused studies [McGimsey et al., 2002; Neal et al., 1995]. For this reason, the 1992 Spurr eruptions provide an excellent source of data to test atmospheric dispersion and fallout models, essential for the mitigation of volcanic clouds.
|The figure below shows a cross-sectional view of the simulated emission from Mount Spurr from the September 1992 eruption (without particle settling applied; vertical exaggeration �26.7); upper image shows Lagrangian particle dispersion at 0900 UTC on 17 September 1992; lower image shows dispersion at 1400 UTC on 17 September 1992. A dispersion of Lagrangian particles were released into the model atmosphere; these were then dispersed by the wind field input into the model. For analysing dispersion, no particle settling was applied; to look at fallout patterns, particle settling was applied.|
|The figure below shows Lagrangian particle trajectory analysis of the 17 September 1992 eruption of Mount Spurr, Alaska with satellite imagery from Schneider et al.  (12:40 UTC) and MLDP0 simulation results at 1300 UTC (without particle settling applied); upper image shows the range of particle heights (ZH) from 0 to 13 km AMG; lower image shows the range of particle heights from 8 to 13 km AMG. By comparing the distribution of Lagrangian particles in the model with the observed position of the cloud, it is possible to estimate cloud height and thickness.|
|The figure below shows simulated trajectory of the cloud from the August 1992 eruption of Mount Spurr, Alaska combined with GAC satellite composite image and MLDP0 simulation results from 0 to16 km AGL (without particle settling applied); image shows particle height (ZH) at 0600 UTC on 22 August 1992. Using VATDMs, it is possible to investigate the long-range dispersion of volcanic particulates, even in the absence of robust satellite observations.|
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Many thanks to Rene Servranckx and others at CMC for making this possible. Please contact Adam Durant for more information: ajdurant @ mtu.edu