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Dr. Kerry Emanuel joins us from the department of Earth, Atmospheric and Planetary Sciences group at the Massachusetts Institute of Technology for his seminar titled Radiative-Convective Instability: Implications for Tropical Weather and Climate. Refreshments will be served.

Abstract

The concept of radiative鈥恈onvective equilibrium (RCE) is the simplest and arguably the most elegant model of a climate system, regarding it as a statistically one鈥恉imensional balance between radiative and convective heat transfer. In spite of this, RCE is seldom studied and poorly understood today. Recent advances in cloud鈥恠ystem鈥恟esolving numerical models have made it possible to explicitly simulate such states, simulating the convective plumes themselves rather than representing them parametrically. The simulations reveal a startling phenomenon: Above a critical surface temperature, moist convection spontaneously aggregates into a single cluster, in a non鈥恟otating system, or into multiple tropical cyclones on a rotating planet. I will show that this results from a linear instability of the RCE state, and this this instability migrates the RCE state toward one of the two stable equilibria. This instability represents a subcritical bifurcation of the ordinary RCE state, leading to either a dry state with largescale descent, or to a moist state with mean ascent; these states may be accessed by finite amplitude聽 perturbations to ordinary RCE in the subcritical state, or spontaneously in the supercritical state.