Moist baroclinic adjustment in an idealized 2-layer model

The impact of condensation of water vapor and latent heat release can be important for many atmospheric dynamical processes at the Tropics but also at midlatitudes. A new idealized atmospheric 2-layer Shallow Water model has been recently developed to take into account this dynamical effect of moisture (Lambaerts et al., 2011). The model includes an equation for the total water content in the atmospheric column and accounts for latent heat release as a mass exchange between the isentropic layers. This formulation offers the advantage over other similar models that it has a non-linear energy conservation and explicitly represents ageostrophic motions.

This model has been extensively tested in a systematic study of a moist baroclinic life-cycle (Lambaerts et al., 2012). It has been shown that the growth rate of baroclinic instability in its initial phase increases by latent heat release. Furthermore, both the lower layer cyclonic and the ageostrophic motions are strongly reinforced by condensation which modifies the standard non-linear development of the instability and leads to a different equilibrium state in this idealized framework. These results confirm the ones of previous similar studies (e.g. Emanuel et al., 1987) and somehow benchmark this model.

Here, we modify the original model to include evaporation, radiation and surface friction in order to study the equilibration of a midlatitude jet in a moist atmosphere. In particular, we are interested in studying how the meridional heat transport is related to the strength of the jet, how it is partitioned between sensible and latent heat transport, and how the midlatitude eddies affect the overall humidity distribution. In this model, the moisture content of the atmospheric columns crossing the jet increases by lower layer convergence such that the resulting precipitation induces a significant poleward latent heat transport on the southern side of the storm-track even without any meridional gradient in the initial total moisture field. Various closures have been tested to estimate the relative importance of these heat transports.