Hello,

I am curious as to where the particle shape factor input from 1-2 comes from for the Hysplit model? The particles I use for a volcanic plume have a shape factor of 0.43. How would I convert the 0.43 shape to fit within Hysplit's 1-2 shape factor input?

Best,

Kyle

## Particle Shape Factor

### Re: Particle Shape Factor

You may find this technical report by Richard Dare useful.

It explains the calculation of the terminal fall velocity of the particles in detail.

https://cawcr.gov.au/technical-reports/CTR_079.pdf

Here is a relevant section of the HYSPLIT user manual. You may also wish to consult the reference here.

https://ready.arl.noaa.gov/hysplitusersguide/S314.htm

If gravitational settling is on and the Shape is set to a negative value then the Ganser (1993) calculation is used to replace Stokes equation for estimating particle fallspeeds. The absolute value of the Shape factor is used for the calculation. The Stokes equation overestimates particle fallspeeds for particles larger than about 20 micron diameter. As this diameter often lies within size distributions of volcanic ash particles, it is desirable to use the Ganser formulation so that particle fallspeeds can be computed accurately. Ganser, G.H., 1993: A rational approach to drag prediction of spherical and nonspherical particles. Powder Technology, 77, 143-152. .

It explains the calculation of the terminal fall velocity of the particles in detail.

https://cawcr.gov.au/technical-reports/CTR_079.pdf

Here is a relevant section of the HYSPLIT user manual. You may also wish to consult the reference here.

https://ready.arl.noaa.gov/hysplitusersguide/S314.htm

If gravitational settling is on and the Shape is set to a negative value then the Ganser (1993) calculation is used to replace Stokes equation for estimating particle fallspeeds. The absolute value of the Shape factor is used for the calculation. The Stokes equation overestimates particle fallspeeds for particles larger than about 20 micron diameter. As this diameter often lies within size distributions of volcanic ash particles, it is desirable to use the Ganser formulation so that particle fallspeeds can be computed accurately. Ganser, G.H., 1993: A rational approach to drag prediction of spherical and nonspherical particles. Powder Technology, 77, 143-152. .