Moist component potential vorticity

The role that atmospheric water, in both its liquid and vapor phases, plays in cyclogenesis is difficult to determine because of the complex interactions between dynamic and thermodynamic forcings. From a potential vorticity (PV) perspective, it is possible to decompose the atmospheric state into a set of superposed PV anomalies. The modification of these anomalies allows for sensitivity testing using numerical models. Although this approach allows for the determination of cyclogenetic contributions from individual PV features, its application has not accounted for the dynamically consistent modification of the moisture field. This paper develops a PV-based variable that describes the effects of water vapor, cloud, and rainwater on balanced dynamics. A special-case analytic form of this "moist component'' PV is developed and interpreted using an idealized model of the atmosphere. The application of the moist component methodology developed here provides the basis for future work, which includes sensitivity tests designed to separate the impacts of dynamics and thermodynamics on cyclogenesis.