WRF Modelling of Volcanic Ash Dispersion Ralph Burton1, Stephen Mobbs2, Alan Gadian1 1 NCAS Weather 2 NCAS Director email: ralph@env.leeds.ac.uk The model can be initialised with a single column (for large-scale runs) or with a point source plus large surface thermal perturbation (for high-resolution runs.) The plume height and emission rates are a function of time Three ash bins are included: 0.1μm, 1 μm and 10 μm (with an assumed density of 2500 kg/m3) Additionally, the tracer can be removed at the surface or when precipitation occurs – although these representations of deposition are not switched on for the results shown. The WRF model has been used to simulate the dispersion of volcanic ash using several passive tracers. The tracers has been given settling velocities to mimic the effect of sedimentation. The settling velocities are a function of height AMSL (see e.g. Sparks et al. 1997). WRF is run with its usual full range of microphysical schemes, etc. This allows a dispersion model to be embedded within a full NWP model. Eyjafjallajökull, May 2010 Grimsvötn, May 2011 The plots below show isosurfaces of ash concentration at several times after eruption. The isosurface is coloured by hydrostatic pressure to show the height of the ash cloud. The top-left image is from MODIS taken on the morning of the 6th May 2011 (from NASA Earth Observatory, 2010). The ash cloud is clearly seen. The lower left image shows modelled isosurface of total ash concentration (brown colour) for 6th May 12Z. Also shown is an isosurface of cloud water mixing ratio. Note the model has the cloud in the correct location. N.B. both images use the same domain. T+18 T+36 Both the Grimsvötn and Eyjafjallajökull runs are initialised with operational ECMWF analyses. The WRF model resolution is 15km with 121 vertical levels. T+54 T+72 High-resolution runs Here, the model is initialised with a sounding from Keflavikurflugvollur (24th April 2010 , see left) and is run over a limited domain at 100m resolution (with boundary layer parametrisation switched off.) Note the fine structure visible in the ash cloud, and the layering due to the stable layers in the profile. Side view: Aspect ratio 1:1 Ash from above 25 km Further work: Progress is currently underway at Leeds to implement a multiphase version of WRF, whereby ash particles will have mass and there is a two-way coupling between the fluid and particle phases. Acknowledgements Thanks to Wei Wang, NCAR, for details of the tracer code; thanks to Sue Laughlin for plume heights from Grimsvotn and to Steve Sparks for communications regarding settling velocities. References Sparks, R. S. J. et al. Volcanic Plumes, Wiley, 1997 MODIS image retrieved from NASA Earth Observatory, 27th June 2011: http://earthobservatory.nasa.gov/IOTD/view.php?id=43894