**An attempt to describe the MRF PBL scheme
(Troen and Mahrt, 1986)**

**1. PBL height : a bulk Richardson number is computed from
layer 1 to K successively until it exceeds a critical number (.5). A
first estimate of the PBL height is obtained for Ri equals the
critical number. For unstable surface condition, a thermal
enhancement to the first layer virtual potential temperature is
done based on the surface sensible and latent heat flux. A second
estimate of the PBL height is obtained based on the enhanced
surface temperature. **

**2. Coefficient of diffusivity in the PBL: A cubic function is
constructed based on the PBL height. The magnitude of the K
within PBL is scaled to match the surface layer by the use of the
convective velocity scale. Prandtl number is used to scale the
thermal diffusivity from the momentum diffusivity coefficients. A
base value of 1 m**2/sec is added for numerical stability. **

**3. Coefficient of diffusivity above the PBL: Local Richardson
number is used to scale the coefficient with a limiting length scale
specified (currently lambda=150 m).**

**4. Vertical diffusion: Fully implicit time integration scheme is used
to calculate the update of prognostic variables due to vertical
diffusion.**

** **