Vertical diffusion above the boundary layer depth.

[Andrew_M]

I seek clarification of HYSPLIT's treatment of particles/puffs that penetrate the boundary layer and enter the free troposphere (free atmosphere) such as in modelling volcanic eruptions or when the (night-time) boundary layer depth (mixing height) falls below an emitting stack height (chimney stack) or greenhouse-gas monitoring tower. I have determined from the available documentation the following:

NOAA Technical Memorandum ERL ARL-224 (www.arl.noaa.gov/documents/reports/arl-224.pdf)

1. HYSPLIT (section 4.3)

Mixing through the inversion layer at z = Zi during convective conditions (W*>0) is computed based on the surface flux parameters and the strength of the inversion, where
Kh = - Cs u* T* (∂θ/∂z)-1, where Cs = 0.4.
During stable conditions, the mixing is calculated as follows - above the BL, pollutant mixing in the remainder of the atmosphere is defined by the vertical diffusivity for heat using mixing length theory
Kh = l2 ∣ ∂V/∂z ∣ φh(ℓ/Lo)-1,
where ℓ is a Blackadar-type mixing length in meters, Lo is the local Obukhov length and the stability function φh.

Are these values computed at each time step and each grid point in the meteorological field (so they vary in time and space) or are they kept constant? Is it fair to say that the vertical dispersion parameters used above the boundary layer depth are kept as a fraction of the surface/boundary layer variables?

2. HYSPLIT - STILT scheme (section 4.5) - does this scheme only determine when particles are transmitted or reflected and the top of the boundary layer depth? Is the term for sigma_w computed as in section 4.3?

3. HYSPLIT - Turbulent Velocity Variances (section 4.6) - are these schemes only applied below the boundary layer depth or through the full atmosphere (so affecting particles in the free troposphere)?

For comparison - the Met Office NAME-III dispersion model uses fixed ( default or user-defined) values for the vertical diffusion [ Kz = vertical velocity variance x vertical lagrangian timescale] for particles in the free troposphere.

I note here that I have no experience in using HYSPLIT.

Any clarification would be gratefully received.

Thank you.

Andrew.