Vertical Mixing in Clear Air and Clouds

Abstract

The realization that clouds are diluted by the entrainment of dry air from outside the cloud boundaries began more than 40 years ago. Cloud-top entrainment was first clearly recognized by Squires in 1958. Warner and Squires showed that the dilution increased towards cloud top and was uniform across the cloud perpendicular to the shear. Not until the mid 1970s, however, did more evidence begin to show that cumulus clouds have a structure from up hear to downshear where rapid dilution by the mixing of dry air from above cloud top eventually totally evaporates the cloud parcels. This slow development was partially caused by the lack of appropriate thermodynamic diagrams for cloud microphysics analysis. In this paper, an easy approach to thermodynamic analysis is introduced for studying cloud mixing and entrainment structure. The advantages of using the temperature and wet-bulb temperature of air parcels, after compression to 1000 mb, as the coordinates of a general thermodynamic diagram are discussed and illustrated with examples. These variables are easily calculated on a small computer and then directly plotted. No further graphical calculations are needed for parcel mixing or for comparing densities. Total water mixing ratios and condensation pressures are directly readable for each composition. An important point is that these displays can be automatically produced without including any significant approximations. Thus, the details of how to process the data, or plot points and draw lines, is no longer a consideration in the use of such analysis, and no analog calculations are needed to produce the graphs. Observational examples are discussed, and the use of the computer program raises new questions about radiative cooling in and above cloud top and about the penetration by turbules carrying heat and moisture of marine stratus clouds and subcloud air.