Representations of the atmospheric boundary layer in global climate models
Representing the atmospheric boundary layer (ABL) within global climate models (GCMs) are difficult due to differences in surface type, scale mismatch between physical processes affecting the ABL and scales at which GCMs are run, and difficulties in measuring different physical processes within the ABL.
Various parameterization techniques described below attempt to address the difficulty in ABL representations within GCMs.
[1] Surface forcings must be accounted for in GCMs in order to have accurate simulations of the Earth's climate.
It consists (broadly) of land, water, and ice, and each surface interacts differently with the atmosphere.
Prognostic equations for each relevant physical process are run for each grid point.
Furthermore, when the environment is sufficiently unstable, convection may help wash away the temperature inversion that caps the ABL.
Also, "the convective motions associated with clouds produce important fluxes of mass, momentum, heat, and moisture".
"Large-scale supersaturation clouds occur when the relative humidity in a grid box at some model level exceeds a critical value".
Additionally, this scheme assumes quasi-equilibrium, with cloud dissipation at a "rate sufficient to keep the atmosphere near equilibrium in the face of large-scale destabilization.
Additionally, local closure likens turbulent transport to molecular diffusion, and is usually first or second order.
Turbulence does not just depend on local values and gradients due to the superposition of many individual eddies.
K-theory follows a similar concept as molecular viscosity, in that the turbulent flux of a quantity is proportional to its spatial gradient, with K as the eddy viscosity/diffusivity.
Depending on the vertical resolution of the model, K-theory may be used to account for the various fluxes within the ABL.
As an alternative to K-theory, ABL fluxes can be accounted for through the use of bulk aerodynamic formulae.
The image below shows the equations used for calculating the exchange coefficients and fluxes, with u* as the frictional velocity.
