The role of moisture in the dynamics and energetics of turbulent baroclinic eddies

The effects of moisture on nonlinear baroclinic eddies are examined in the context of a horizontally homogeneous two-layer quasigeostrophic model. Using an explicit equation for moisture and a simple parameterization of latent heat release, the present study focuses on how moisture affects the statistically steady state of a baroclinically unstable flow. It is shown that, when latent heating is weak, the flow is dominated by jets and baroclinic waves, just as in the corresponding dry model. In this regime, the concept of an effective static stability can be used, which allows one to interpret some aspects of the moist simulations in terms of an equivalent dry model. It is found that a useful way of diagnosing the effective static stability is by relating it to the eddy fluxes of moisture and moist potential vorticity; no a priori theory for its value is presented here. As the strength of latent heating is increased, the flow rather suddenly becomes vortex dominated, with an asymmetry between strong low-level cyclones and weak anticyclones that has no analog in the dry model. It is argued that this asymmetry develops because of a correlation between low-level vorticity and moisture that results from the correlated horizontal transports of moisture and vorticity. The energetics of the simulations and the efficiency of energy production by latent heat release are discussed.