AERMOD Tech Guide
3.2.4 Mechanical Mixing Height(z) in the SBL
The mixing height in the SBL results exclusively from mechanical (or shear induced) turbulence. The value of zim is calculated from eq. (12), which is the same expression as is used in the CBL. The SBL height is smoothed in time using eq. (15) in the same manner as in the CBL.