Thermodynamically consistent versions of approximations used in modelling moist air

Abstract: Some existing approaches to modeling the thermodynamics of moist air make approximations that break $\textit{thermodynamic consistency}$, such that the resulting thermodynamics do not obey the 1st and 2nd laws or have other inconsistencies. Recently, an approach to avoid such inconsistency has been suggested: the use of $\textit{thermodynamic potentials}$ in terms of their $\textit{natural variables}$, from which all thermodynamic quantities and relationships are derived. In this paper, we develop this approach for $\textit{unapproximated}$ moist air thermodynamics and two widely used approximations: the constant $\kappa$ approximation and the dry heat capacities approximation. The consistent constant $\kappa$ approximation is particularly attractive because it leads to, when using virtual potential temperature $\theta_v$ as the thermodynamic variable, adiabatic dynamics that depend only on total mass, independent of the breakdown between water forms. Additionally, a wide variety of material from different sources in the literature on thermodynamics in atmospheric modelling is brought together. It is hoped that this paper provides a comprehensive reference for the use of thermodynamic potentials in atmospheric modelling, especially for the three systems considered here.