Aspects of bulk atmospheric boundary layer similarity under free‐convective conditions

For many hydrologic and atmospheric dynamic purposes the turbulent surface fluxes of sensible and latent (or evaporative) energy and of momentum must be formulated over areas covering a wide range of spatial scales; these are characteristically the scales of river basins and of the grid sizes for integration in current atmospheric circulation models. The bulk atmospheric boundary layer (ABL) similarity (BAS) approach continues to be one of the very few available formulations to describe turbulent surface fluxes over the range of scales from roughly 1 to 10 km, in terms of average characteristics aloft in the upper reaches of the ABL. The approach is reexamined as it relates to atmospheric stability and surface characteristics under conditions of free convection. Since the largest gradients occur near the surface, first a formulation is proposed for the Monin-Obukhov profile functions for convective conditions that is consistent with the theoretical advances by Kader and Yaglom [1990] and with recent experimental data. This is then combined with a slab representation of the mixed layer to derive the most plausible form of the BAS functions, on the basis of presently available information. Finally, it is illustrated with available experimental data that any alternative formulation of bulk ABL transport, regardless of its appearance, is of necessity equivalent with BAS and that the same original variables are at work.