On the Local Available Energetics in a Moist Compressible Atmosphere

A new derivation of local available energetics for a fully compressible, nonhydrostatic, moist atmosphere is presented. The available energetics is defined relative to an arbitrary dry reference state in hydrostatic balance with stable stratification. By introducing the modified potential temperature, a positive-definite expression of the moist available potential energy (APE) is derived. The change of the moist APE must include the role of convection to function both as a source of latent heat and as an atmosphere dehumidifier. The sum of this moist APE and the available elastic energy (AEE) is the moist available energy. In the local energy cycle, the moist available energy is partly used to generate kinetic energy (KE) and partly used to lift the water vapor to the higher level where it precipitates, resulting in the increase of gravitational energy of moist species. The moist APE is converted into vertical KE through the buoyancy term; the vertical KE is converted into the AEE through the vertical perturbation pressure gradient term; and the AEE is converted into horizontal KE through the horizontal divergence/convergence term. In addition, there exist two adiabatic nonconservative processes, which act on the AEE and APE, respectively. A suitable choice of the reference state should make these two processes much less significant than the conversions between the available energy and KE. An alternative method is presented to construct such a reference state. Application to the idealized baroclinic atmosphere shows that this reference state is much more relevant to the local available energy analysis than the isothermal one.