Using enthalpy as a prognostic variable in atmospheric modelling with variable composition

Specific enthalpy emerges from a general form of the internal energy equation as a convenient prognostic thermodynamic variable for atmospheric modelling with variable composition, including models of moist air. This choice presents a general and flexible alternative to the common formalism of virtual temperature employed in most numerical weather prediction and climate models to account for the presence of water vapour and other constituents. The new approach eliminates the need for additional terms in the energy equation, resulting from composition variations along the air‐parcel trajectories and routinely neglected in models. This note presents a derivation of relevant equations from first principles and outlines the changes to existing model codes necessary to accommodate the new formulation. Published in 2008 by John Wiley & Sons, Ltd.