Impact of skewness and nonlocal effects on scalar and buoyancy fluxes in convective boundary layers

Large-eddy simulation (LES) results of three prototype convective boundary layers (CBL) are used to study flux budgets and to test simple expressions for scalar and buoyancy fluxes. The simulated CBLs differ in their characteristics of skewness of the vertical velocity field and the scalar flux gradients. This is accomplished by applying a surface heat flux or radiative cooling at the boundary layer top. In this way boundary layers with positive, zero, and negative skewness are simulated. Useful approximations for the transport and buoyancy terms in the flux budgets are achieved by using a generalized convective scaling. This directly provides an expression for scalar and buoyancy fluxes that combines downgradient and nonlocal effects on the fluxes. The nonlocal effects are due to vertical velocity variance and the integrated flux over the boundary layer. The result is compared with an alternative expression in which the nonlocal flux is related to the skewness of the vertical velocity field and the scalar flux gradient. Overall it appears that the nonlocal flux is mostly related to the vertical velocity variance and the integrated flux and not so much to the skewness. The final result is a simple expression for scalar and buoyancy fluxes, that can be used in mesoscale and global models.