The Effect of Thermal Stratification and Evaporation on Geostrophic Drag Coefficient in the Atmospheric Boundary layer

The turbulent energy equation, together with a modified mixing length, is adopted to close the governing equations in the turbulent planetary boundary layer under stratification conditions. The system of governing equations is decoupled by introducing a stratification parameter, Q. Numerical integration of the system, in general, is outlined and should be easily implemented. Results for the momentum fluxes, eddy viscosity, geostrophic drag coefficient, and the angle between the geostrophic wind and the surface shear stress, which are obtained from the decoupled system with Q as a parameter, are presented for Q ranging from -100 to 100. They are compared, when possible, with the measurements. The effects of both the thermal and humidity stratification are indexed by Q. The model shows that, in the presence of evaporation or humidity stratification, the geostrophic drag coefficient may differ greatly from the case of thermal Stratification only. The planetary boundary thickness and other main features of the turbulence depend on Q.