Sack Lunch Seminar (SLS) – Matthieu Kohl (MIT, pre-defense)

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. In this talk, I will discuss two phenomena that illustrate the role of latent heating in midlatitude dynamics.

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. A fluid dynamical account is developed for DRVs both in terms of simple conceptual models of moist instability and potential vorticity dynamics of finite-amplitude storms.

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. Here 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.

About this series: The Atmosphere, Ocean and Climate Sack Lunch Seminar Series is an informal seminar series within PAOC that focuses on more specialized topics than the PAOC Colloquium. Seminar topics include all research concerning the science of atmosphere, ocean and climate. The seminars usually take place on Wednesdays from 12-1pm. The presentations are either given by an invited speaker or by a member of PAOC and can focus on new research or discussion of a paper of particular interest. Contact: sacklunch-committee@mit.edu