Forecasting Volcanic Ash Deposition Using HYSPLIT Tony Hurst1* and Cory Davis2
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Hurst and Davis Journal of Applied Volcanology (2017) 6:5 DOI 10.1186/s13617-017-0056-7 RESEARCH Open Access Forecasting volcanic ash deposition using HYSPLIT Tony Hurst1* and Cory Davis2 Abstract A major source of error in forecasting where airborne volcanic ash will travel and land is the wind pattern above and around the volcano. GNS Science, in conjunction with MetService, is seeking to move its routine ash forecasts from using the ASHFALL program, which cannot allow for horizontal variations in the wind pattern, to HYSPLIT, which uses a full 4-D atmospheric model. This has required some extensions to the standard version of the HYSPLIT program, both to get appropriate source terms and to handle the fall velocities of ash particles larger than 100 microns. Application of the modified HYSPLIT to ash from the Te Maari eruption of 6 August 2012 from Tongariro volcano gives results similar to the observed ash distribution. However, it was also apparent that the high precision of these results could be misleading in actual forecasting situations, and there needs to be ways in which the likely errors in atmospheric model winds can be incorporated into ash models, to show all the areas in which there is a significant likelihood of deposited ash with each particular volcanic eruption model. Keywords: Volcanichazard,Volcanicash,Ashfall,Atmosphericmodels,MtTongariro,TeMaari Introduction processing requirement, based on the processing power One aspect of GeoNet, the geological hazard monitoring of mid-1990s Personal Computers. To obtain reduced system for New Zealand run by GNS Science, is fore- processing times, ASHFALL ignored vertical diffusion of casting the expected thickness distribution of volcanic the ash, although its effect was partly accounted for by ash if any volcano was to erupt. As well as the four vol- using a large value for the horizontal diffusion coeffi- canoes that have erupted in the last 50 years (Ruapehu, cient. This horizontal diffusion coefficient is effectively a Ngauruhoe, Tongariro and White Island), possible ash “fudge factor”, which spreads out the expected ash distri- distributions are calculated daily for six other volcanoes bution to incorporate uncertainties in the wind pattern seen as significant risks, as shown in Table 1. There are and other parameters. ASHFALL uses MetService (New six cases calculated for each volcano, with two column Zealand Meteorological Service) forecasts of the wind at heights and three eruption volumes. If any volcano is various levels above each volcano. However, it does not erupting or has erupted recently, the forecasts for that allow for any horizontal variations in the wind pattern, volcano are distributed to relevant organisations and the i.e. ASHFALL can only use 1-D spatial wind fields, public, whereas during normal times the information is although it does allow for wind changes with time. kept within GNS Science to avoid causing confusion. Hurst and Turner (1999) found that the forecast wind These forecasts are currently produced by the program field was the major source of error in the volcanic ash ASHFALL, originally described in Hurst (1994), which is distribution from Ruapehu eruptions. During investiga- now somewhat out-dated. ASHFALL follows the ash tions as to how the wind information used by ASHFALL particles from an initial vertical eruption column as they might be improved, we looked at the atmospheric are moved horizontally by the wind, and at the same dispersion model HYSPLIT (Stein et al., 2015), already time fall vertically with a velocity which depends on in use by MetService for the movement of fine particles their size and density. It was designed to have a minimal travelling in the atmosphere, such as volcanic ash, and found it has the capability for estimating the distribution * Correspondence: [email protected] of ash particles on the ground. As well as capturing the 1GNS Science, Lower Hutt, New Zealand influence of realistic atmospheric motion in ash Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Hurst and Davis Journal of Applied Volcanology (2017) 6:5 Page 2 of 8 Table 1 The standard eruption model input parameters used in New Zealand by GNS Science, with three volumes and two column heights for each volcano Volcano column height column height tephra volume tephra volume tephra volume ash size (high) km ASL (low) km ASL large km3 medium km3 small km3 distribution Auckland 5 3 0.1 0.01 0.001 Basaltic Mayor Island 10 3 0.5 0.05 0.005 Rhyolitic White Island 5 3 0.01 0.001 0.0001 Andesitic Haroharo 16 4 1 0.1 0.01 Rhyolitic Tarawera 16 4 1 0.1 0.01 Rhyolitic Taupo 15 3 1 0.1 0.01 Rhyolitic Tongariro 11.5 4.5 0.1 0.01 0.001 Andesitic Ngauruhoe 12 6 0.1 0.01 0.001 Andesitic Ruapehu 12.5 6.5 0.1 0.01 0.001 Andesitic Taranaki 12 6 0.5 0.05 0.005 Andesitic transport, HYSPLIT can also model in-cloud (rainout) not a consideration in the aviation applications of HYS- and below-cloud (washout) wet deposition processes. PLIT, as these particles are the ones that rapidly fall out HYSPLIT is a hybrid Lagrangian dispersion model, de- of ash clouds. veloped by NOAA/ARL which is used by MetService, in its role as a Volcanic Ash Advisory Centre (VAAC), to Ash sizes and fall velocities model airborne volcanic ash, with meteorological data HYSPLIT was originally designed to use atmospheric provided by external and in-house NWP (Numerical models to calculate the tracks of pollutants in the at- Weather Prediction) models, which operate with 3 mosphere (Stein et al., 2015). In regard to volcanic ash, spatial dimensions and time. The spatial resolution of it is used for tracking the small ash particles with long these models is 4 km, and the temporal resolution one residence times that are hazardous to aircraft generally hour. HYSPLIT is operationally used by several of the 9 at considerable distances from the volcano. Accordingly Volcanic Ash Advisory Centres for aviation forecasting. by default it only considers ash particles less than 100 A by-product of the HYSPLIT volcanic ash dispersion microns in diameter (e.g. in VAFTAD, Heffter and Stun- simulations is the ash deposition at the ground surface. der (1993)). The terminal velocity for these small parti- The ground surface elevation is modelled at the same cles can be calculated by Stokes Law, which applies in resolution as the atmospheric model. Aside from the dis- the laminar flow region, with Reynolds Number less persion of volcanic ash, HYSPLIT is used in several than 0.4. other atmospheric transport applications, including the The ash size distributions used for calculating ash de- dispersion of hazardous materials (e.g. nuclear material posit thicknesses in the work presented here were based after the Fukushima reactor accident), air quality model- on historic and well-preserved ash deposits, after making ling (e.g. ozone, visibility, haze, and dioxin), dust storms, allowance for the fraction of very fine particles that do smoke, and the transport of biological material (e.g. not fall out within the area of identifiable ash deposits. pollen and mould spores) (Stein et al., 2015). Rather In New Zealand, we use three size distributions, based than using a horizontal dispersion coefficient, the disper- on the rock type of the volcano (Fig. 1 and Table 1). For sion was calculated from the friction velocity, height and rhyolitic volcanoes the distribution is based on the boundary layer height following Kantha and Clayson Hatepe Ash (Walker, 1981), for andesitic volcanoes it is (2000), where there are different equations for stable/ based on the AD 79 eruption of Vesuvius (Macedonio neutral and unstable layers for both the surface and et al., 1988), while for the basaltic volcanoes it is based boundary layer. Above the boundary layer a mixing coef- on the 1973 eruption of Heimaey (Self et al., 1974). ficient obtained from mixing length theory is used to These ash size distributions are similar to those used for calculate the velocity variances (Draxler and Hess, 1997). hazard analysis in Hurst and Smith (2010). Size- We initially compared the ash deposition results from dependent densities are used to convert size to mass for HYSPLIT with ASHFALL for similar eruptions and wind each of the three ash types. The distribution shown as patterns. This showed that alterations to the standard VAAC in Fig. 1 is derived from Heffter and Stunder fall velocity model of HYSPLIT were required to deal (1993). The normal practice at MetService, who operate with ash particles larger than about 100 microns, which the Wellington VAAC, when using HYSPLIT for Avi- make up the bulk of ash deposits near a volcano. This is ation Hazard purposes, is to assume that 5% of any total Hurst and Davis Journal of Applied Volcanology (2017) 6:5 Page 3 of 8 Fig. 1 The three ash size distributions used by ASHFALL, for Andesitic, basaltic and Rhyolitic volcanoes, compared to the ash size distribution used for VAAC modelling of ash as an aviation hazard. On the same particle diameter scale are also shown particle fall speeds for the three equations of Bonadonna et al. (1998) and the Ganser equation eruption volume is comprised of fine ash, with this Comparisons of HYSPLIT and ASHFALL showed VAAC size distribution, and to ignore the 95% of coarser broadly similar results on days when there were uniform ash as it is falling out quickly.