Dispersion Simulations Using HYSPLIT
HYSPLIT is set to use the same volcanic ash source term and produce look-alike output graphics as VAFTAD (Heffter and Stunder, 1993). The main differences between running HYSPLIT for volcanic ash and running HYSPLIT at the ARL HYSPLIT web page (unregistered or registered) is in the source term (the initial conditions of the substance being modeled) and the output graphics. For volcanic ash, a set of particles of a given size distribution are uniformly distributed in a vertical layer above the volcano. Additional options, such as mass emission rate and wet deposition, are not available for running HYSPLIT on ARL’s volcanic ash webpage. All HYSPLIT options are available in the Windows-based version that may be downloaded from the HYSPLIT main page.
Brief description of volcanic ash dispersion modeling:
Required inputs are:
- Volcano latitude and longitude
- Volcano summit height
- Eruption date and time
- Eruption duration
- Ash column height
- Ash reduction level (none-small-medium-large)
Preset input is a unit source (1 g) since the actual amount is not known in real-time. The ash particle distribution is described below.
Meteorological data input
HYSPLIT inputs gridded meteorological data in a format described in the meteorological archives web page. Forecast meteorological fields in this format are routinely generated from NWS National Centers for Environmental Prediction (NCEP) runs of the Global Forecast System (GFS) and the North American Mesoscale (NAM) meteorological models and are available on an NCEP ftp server. Information on these HYSPLIT-compatible forecast datasets, as well as others produced by ARL, are available in the meteorological forecast data web page. In addition to these forecast files, GFS and NAM “archive” data (analysis and short-term forecasts) for the previous two days are routinely created at NCEP and are available on the NCEP ftp server. This permits running volcanic ash forecasts for large eruptions that occurred up to two days ago.
ARL also archives meteorological data in the HYSPLIT-compatible format. Information on these files is available in the meteorological archives web page.
Ash particle distribution
HYSPLIT calculates transport and dispersion of volcanic ash from an ash column extending from the volcano summit to the column top. The model uses spherical particles of density 2.5*10^6 g m-3 with diameters ranging from 0.3 to 30 microns. A particle distribution was derived from aircraft sampling of Mount St. Helens and Redoubt Volcano ash clouds (Hobbs, P. V., L. F. Radke, J. H. Lyons, R. J. Ferek, D. J. Coffman, and T. J. Casadevall, 1991: Airborne measurements of particle and gas emissions from the 1990 volcanic eruptions of Mount Redoubt, J. Geophys. Res., 96, 18 735-18 752.).
Calculated concentrations relative to the unit emission have been correlated with satellite imagery (see verification discussion below) for defining the ash cloud. The model computation of the ash cloud includes the magnitude of the volcanic eruption as determined by an algorithm based on the ash column top height and column depth.
For some volcanic eruptions, HYSPLIT forecasts a larger ash cloud than what is observed, partly because of water vapor in the eruption column. HYSPLIT is typically run in a “reduced ash” mode when satellite imagery shows a smaller ash cloud. Differences between the default (no reduction) run and a reduced ash run, if any, reflect some of the uncertainty in defining the eruption column.
An example 8-panel “VAFTAD-format” chart of the forecast ash cloud is shown in Figure 1. The four panels in any column are for a single valid time after eruption. Individual panels are for layers applicable to aviation operations and are identified at the side of a panel with upper and lower flight levels (FL) in hundreds of ft. The bottom panel is a composite layer, from the SURFACE to FL550, and is useful for satellite imagery comparisons. For each column, the forecast valid time separates the upper three panels from the composite panel. Volcano eruption information is at the lower left. A description of the input meteorology is at the lower right. The ash cloud symbol is at the lower center. If a reduced ash level was used, it is indicated below the ash cloud symbol. HYSPLIT volcanic ash output issued by NOAA in response to a volcanic eruption includes a message to “SEE VOLCANIC ASH SIGMETS AND ADVISORIES” at the lower right. The example output shown is valid for ERUPTION+18 hours (left column) and ERUPTION+24 hours (right column).
Volcanic Ash Dispersion Model Verification
Verification has included three eruptions of Spurr in Alaska (June, August, and September, 1992), multiple eruptions of Rinjani in Indonesia (June & July 1994), a continuous 24-h eruption of Klyuchevskoi in Kamchatka – eastern Russia (starting September 1994), Soufriere Hills, Monserrat and Popocatepetl, Mexico (1996-1997).
List of Publications
- Mastin, L.G., M. Guffanti, R. Servranckx, P. Webley, S. Barsotti, K. Dean, A. Durant, J.W. Ewert, A. Neri, W.I. Rose, D. Schneider, L. Siebert, B. Stunder, G. Swanson, A. Tupper, A. Volentik, C.F. Waythomas, 2009: A multidisciplinary effort to assign realistic source parameters to models of volcanic ash-cloud transport and dispersion during eruptions Journal of Volcanology and Geothermal Research, 186:10-21.
- Webley, P.W., B.J.B. Stunder, and K.G. Dean. 2009: Preliminary sensitivity study of eruption source parameters for operational volcanic ash cloud transport and dispersion models — A case study of the August 1992 eruption of the Crater Peak vent, Mount Spurr, Alaska. Journal of Volcanology and Geothermal Research, 186:108-119.
- Stunder, B.J.B., J.L. Heffter, R.R. Draxler (2007), Airborne Volcanic Ash Forecast Area Reliability, Weather and Forecasting, 22:1132-1139, DOI: 10.1175/WAF1042.1
- Tupper, A., J. Davey, P. Stewart, B. Stunder, R. Servranckx, and F. Prata, 2006: Aircraft encounters with volcanic clouds over Micronesia, Oceania, 2002/03. Australian Meteorological Magazine, 55, 289-299.
- Heffter,J.L., 1996: Volcanic ash model verification using a Klyuchevskoi eruption. Geophy. Res. Letters, 23-12, 1489-1492.
- Heffter, J.L. and B.J.B. Stunder, 1993: Volcanic Ash Forecast Transport And Dispersion (VAFTAD) Model. Wea Forecasting, 8, 534-541.
- Heffter, J.L., B.J.B. Stunder, and G.D. Rolph, 1990: Long-range forecast trajectories of volcanic ash from Redoubt volcano eruptions. Bull. Amer. Meteor. Soc. 71(12):1731-1738.