The bottom fifth of the convective atmospheric boundary layer where virtual potential temperature, wind speed, and humidity vertical gradients are nonzero. A convective boundary layer comprises three layers: the radix layer touching the ground, a uniform layer in the interior of the convective boundary layer, and an entrainment zone at the top. The classical surface layer, where Monin–Obukhov similarity theory applies to small shear- driven turbulent eddies, is a bottom subdomain of the radix layer.