Scaling approaches to quasi-geostrophic theory for moist, precipitating air

Abstract

Quasi-geostrophic (QG) theory is of fundamental importance in the study of large-scale atmospheric flows. In recent years, there has been growing interest in extending the classical QG plus Ekman friction layer model (QG-Ekman) to systematically include additional physical processes known to significantly contribute to real-life weather phenomena. This paper lays the foundation for combining two of these developments, namely Smith and Stechmann's family of \emph{Precipitating Quasi-Geostrophic} (PQG) models (J.\ Atmos.\ Sci, {\bfseries 74}, 3285--3303, 2017) on the one hand, and the extension of QG-Ekman for dry air by a strongly \emph{Diabatic Layer} (DL) of intermediate height (QG-DL-Ekman) in (J.\ Atmos.\ Sci, {\bfseries 79}, 887--905, 2022) on the other hand. To this end, Smith and Stechmann's PQG equations for sound-proof motions are first corroborated within a general asymptotic modeling framework starting from a full compressible flow model. The derivations show that the PQG model family is naturally embedded in the asymptotic hierarchy of scale-dependent atmospheric flow models introduced by one of the present authors in (Ann.\ Rev.\ Fluid Mech., {\bfseries 42}, 249--274). Particular emphasis is then placed on an asymptotic scaling regime for PQG that accounts for a generic Kessler-type bulk microphysics closure and is compatible with QG-DL-Ekman theory. The detailed derivation of a moist QG-DL-Ekman model is deferred to a future publication.