Strong extratropical cyclogenesis is typically accompanied by significant precipitation. The impact of the corresponding condensational heating on the dynamics of the storm should be important. Moist baroclinic instability has been theoretically investigated only in the context of a supposedly equivalent dry model. In contrast, we will report an instability analysis that has an explicit treatment of the moisture field. The basic state under consideration has a linear baroclinic shear, a constant stratification and a monotonic decrease of specific humidity with height. The heating and moisture parameterization consist of two components. Its intensity is assumed to be proportional to the boundary layer specific humidity perturbation. The vertical distribution of the released heat and moisture depends upon the small scale cloud processes which has to be parameterized for different cloud types. Guided by the observed distribution of heat source/sink and moisture source/sink rates, we introduce three canonical profiles. Profile-A would arise from altocumulus clouds that has a lower-layer condensational heating and moisture sink. Profile-B would arise from stratocumulus cloud systems characterized by an upper-layer condensational heating and a lower layer evaporative cooling. There is a corresponding moisture sink in the upper layer and a moisture source in the lower layer in this case. Profile-C would arise from deep cumulus clouds that has a lower-layer moisture sink but an upper-layer condensational heating.
The objective of this study is to ascertain the extent to which we can deduce the dynamical impacts of self-induced condensational heating according to the formulation outlined above. Specifically, we would like to deduce how the instability properties would vary under three representative situations exemplified by the three canonical profiles of diabatic processes. One of the findings is that one branch of unstable modes is a modified version of the modes that would exist without heating. The interior moist potential vorticity in this case plays a secondary role compared to the temperature anomalies at the top and bottom boundaries in determining its structure. A new finding is that the self-induced condensational heating can also induce a new branch of unstable modes that has a considerably shorter short-wave cutoff. This is particularly true for the cases of Profile-A and Profile-B. These modes have a lower tropospheric baroclinic structure. They primarily stem from interaction between the temperature anomaly at the bottom surface and the interior moist potential vorticity. The three profiles give rise to substantially different impacts on the instability properties. The detailed instability properties of the different branches of mode and the associated energetics will be presented.
Close window or click on previous window to return to the Conference Program.
12th Conference on Atmospheric and Oceanic Fluid Dynamics