Clouds, Noncloudy Latent Heat Convection, Entrainment, and Horizontal Averaging

Abstract

Crucial observations describing the mechanisms active in the convective planetary boundary layer are reviewed. Attention is drawn to the observed transport of heat and moisture upward against vertical stratifications stable to cloud-free convection. Occasional saturated parcels carry air of a different composition through regions well below cloud base. These parcels do not form sustained or easily visible cloud and are observed to penetrate even where no sustained cloud forms higher up. This mechanism is likely to be of major importance. The possibility of erosion into the warm overlying air at the top of the convective layer due to a negative heat flux is discussed. One must conclude from the observations discussed here that it does not actually occur. Despite claims to the contrary, the air above appears to enter the convecting layer only when the convection has effectively reached its temperature. When stratus cloud is present, mixtures with the overlying unsaturated air often have the same density as the cloudy air below. Both these phenomena illustrate the inadequacy of conceptual thinking based on averaged horizontal uniformity in the fluid, without recognizing that convective transport is a process where isolated thermal elements in the fluid carry practically all of the quantities such as heat and moisture from one level to another. Gradient turbulent diffusion usually has no role to play and is not a satisfactory concept in describing the planetary boundary layer.