Moist Baroclinic Instability and Macroturbulence of the Midlatitude Atmosphere

Abstract

Water and its change of phase greatly enrich the dynamics of the midlatitude atmosphere and challenge us to extend our theories of baroclinic instability and macroturbulence beyond dry adiabatic dynamics. Two specific phenomena in which latent heating plays a key role and that are poorly understood form the central focus of this thesis.

Past research has identified a special class of storm, dubbed the Diabatic Rossby Vortex (DRVs), which derives its energy from latent heating rather than baroclinic effects and as such goes beyond the traditional understanding of midlatitude storm formation. DRVs have been implicated in extreme and poorly predicted forms of cyclogenesis along the east coast of the US and west coast of Europe and have recently emerged as the dominant mode of instability in an idealized GCM with climate warming. While we have a good theoretical understanding of dry cyclogenesis, our understanding of DRV formation, and propagation as well as their growth rate and length scale is poor. In chapters 2 and 4 of my thesis, a fluid dynamical theory is developed for DRVs both in terms of simple conceptual models of moist instability and potential vorticity dynamics of finite-amplitude storms. In particular, the dispersion relation for the growth rate and length scale of DRVs is derived analytically, and it is shown that DRVs become faster than both dry or moist baroclinic waves in the limit of a convectively-neutral stratification.

Latent heating also makes upward motion stronger than downward motion, and this asymmetry has important implications for the distribution of precipitation and its extremes. Current theories based around small-amplitude modes greatly overestimate the change in asymmetry with warming. In chapter 3, we develop a toy-model that takes into account adjustment of the atmosphere to a state of moist macroturbulence and show that it better reproduces the slow increase in the asymmetry from winter to summer over the seasonal cycle in reanalysis and with climate warming in idealized simulations.