Dear all,

I am using HYSPLIT to model insect dispersal including dry deposition. Insects are not a perfect sphere. I am not sure how to set ‘Diameter (µm)’, and ‘Shape’.

I guess that ‘Diameter (µm)’ refers to the diameter of the cross section (a circle) of a particle. If so, would I be able to use the area of an insect’s body to calculate ‘Diameter (µm)’?

For example, I could use Area = π*(Diameter/2) to calculate Diameter inversely, since the cross section of a particle in HYSPLIT is a circle.

How about ‘Shape’? Should I set it 2? According to the manual, 1 represent a perfect sphere and a tiny insect is not a perfect sphere.

It would be great if you could explain ‘Diameter (µm)’, ‘Density (g/cc)’, and ‘Shape’ in detail. Your help would be much appreciated. Thanks.

Kind Regards,

Ming

## Diameter (µm) Density (g/cc) and Shape

### Re: Diameter (µm) Density (g/cc) and Shape

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.

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. .**### Re: Diameter (µm) Density (g/cc) and Shape

Dear alicec,

Thank you very much for your help. I will look at these papers and get back to you later.

I have another two questions now. I use NCEP/NCAR Global Reanalysis Data to run HYSPLIT.

Question 1: Do I have to set ‘starting time (year, month, day, hour)’ as the UTC time in HYSPLT, regardless of meteorological data? I guess that I should not set as the local time?

Question 2: For each grid cell on the ground in the output file, the coordinate (longitude and latitude) represents the centre point of a grid cell? If so, does it work the same for the grids in the meteorological data (NCEP/NCAR Global Reanalysis Data)?

Your help would be much appreciated. Thanks.

Kind Regards,

Ming

Thank you very much for your help. I will look at these papers and get back to you later.

I have another two questions now. I use NCEP/NCAR Global Reanalysis Data to run HYSPLIT.

Question 1: Do I have to set ‘starting time (year, month, day, hour)’ as the UTC time in HYSPLT, regardless of meteorological data? I guess that I should not set as the local time?

Question 2: For each grid cell on the ground in the output file, the coordinate (longitude and latitude) represents the centre point of a grid cell? If so, does it work the same for the grids in the meteorological data (NCEP/NCAR Global Reanalysis Data)?

Your help would be much appreciated. Thanks.

Kind Regards,

Ming

### Re: Diameter (µm) Density (g/cc) and Shape

HYSPLIT uses the UTC time. More info about the CONTROL file can be found in the user guide (starting p24): https://www.arl.noaa.gov/documents/repo ... _guide.pdf

For the conc grid (output file from HYSPLIT runs), it is in lat/long coordinate and is independent of the input met data. You can set up the conc grid (horizontal/vertical resolution and size of the domain) in the CONTROL file. More info is in the user guide p96.

For the conc grid (output file from HYSPLIT runs), it is in lat/long coordinate and is independent of the input met data. You can set up the conc grid (horizontal/vertical resolution and size of the domain) in the CONTROL file. More info is in the user guide p96.