smoke spread out

[Alpa24]

Hi All,

I have made pyroclusterflow of volcano.
Every thing is ok but at the end smoke spread out.
which is not wanted.

Dissipation Min. Dens : 1.0
Dissipation straingth : 3.0
Diffussion : 0.0

I cant able to undertand how to set this parameter from reference menual because my
english is poor.

Please help.
Here i attech video.