ENTROPY OF A CONVECTING WATER-AIR SYSTEM AND THE INTERPRETATION OF CLOUD MORPHOGENESIS

Cloud-top entrainment in the stratocumulus-to-cumulus transition can b e viewed as an entropy-decreasing, self-organizing process. An express ion is derived for the entropy of a two-phase convecting water-air sys tem which locates the entropy deficit in the region of the convection itself, generalizing a similar formulation of Button's for the one-com ponent case. This expression is applied to a buoyancy-reversal model t hat simulates entrainment through convection driven by evaporative coo ling. The overall entropy budget suggests that the evaporation/precipi tation cycle is an essential ingredient of the stratocumulus-to-cumulu s transition, underlying the difference between the behaviour of this system and that of the buoyancy-reversal system. The cumulus cloud sys tem is thus rendered as a dissipative structure. The class of dynamica l mechanisms that can explain the stratocumulus-to-cumulus transition, together with the associated high entrainment rates, is restricted to those which depend on the water cycle. The relocalized entropy expres sion, in conjunction with the second law of thermodynamics, implies an upper bound on the possible extent of change in cloud form for given flux of water substance through the evaporation/precipitation cycle ov er the history of cloud morphogenesis.