Moist convection is a complex process which plays a large part in determining the thermodynamic state of the atmosphere, particular in the tropics. For this reason, thermodynamic descriptions of tropical moist convection can be useful in understanding the process itself, an example of which is the "heat engine" concept. In the present paper, we propose an analogue to the "thermodynamic state" to be used in describing the evolution of the tropical convective system.
For a system in thermodynamic equilibrium, its state can be uniquely defined by any two of the thermodynamic variables (functions of state) such as temperature, pressure, internal energy and entropy. Any evolution in the system can then be taken as a sequence of thermodynamic equilibrium states and hence is completely described by appropriate phase diagrams. We attempt to extend this type of description to the whole vertical column of the tropical moist convective system.
Two limiting types of convective available potential energy (CAPE), irreversible and reversible, have been selected to be our vertically integrated quantities used to describe the evolution of the state of an atmospheric column involving moist convection. Such a phase diagram can in no way define a complex state of the atmospheric column completely but it is anticipated to describe its bulk state. When GATE, TOGA COARE and MCTEX soundings are plotted on this phase diagram a simple compact characteristic emerges. The characteristic is easily divided into regimes which are associated with both the strength of the available buoyancy and the large-scale state of the atmosphere.