The Vertical Structure of Radiative-Convective Instability

Moist radiative-convective equilibrium is the simplest state of the Tropical atmosphere, in which convective heating balances radiative cooling on weekly timescales. Understanding its stability to small perturbations is key to exploring the physics of the tropical atmosphere (e.g. convective self-aggregation), and connecting its variability to what we learn from theory and modeling. Here, we aim at studying the observed variability of tropical water vapor as the result of internal water vapor instabilities, motivating the questions:

Is radiative-convective equilibrium unstable to small water vapor perturbations? What are the perturbation growth rates associated with the interaction between water vapor, radiation and convection?

By using the weak temperature gradient approximation, we diagnose the tendency of water vapor perturbations from moist radiative and convective perturbations, reducing the problem to linear stability analysis for water vapor perturbations. We combine analytical theory with one and two-dimensional runs of the MIT single column model, using interactive radiation, convection, water vapor and clouds. We couple the column model and a three-dimensional cloud-permitting model to a slab ocean, in order to capture the effect of air-sea interaction. We run a first set of experiments without interactive surface temperatures nor cloudy radiation; we find that the atmosphere is potentially unstable above fixed surface temperatures of ~300K and leads to typical growth rates of a week. Interactive cloudy radiation significantly increases these growth rates, while air-sea interactions trigger clear-sky instabilities for lower surface temperatures. Finally, we use both interactive surface temperatures and cloudy radiation, and observe that the atmosphere alternates between warm & aggregated states and cold & non-aggregated states.

In the short term, our work shows that the linear stability analysis of moist perturbations is an instructive way to quantify the response of any model that includes moist radiation and convection, which has led us to significantly improve the MIT Single Column model's performances. In the long term, this project aims at exploring how the interaction between water, radiation and convection impacts the genesis of tropical mesoscale phenomena, including the monsoonal oscillations mechanism over the Bay of Bengal, cyclogenesis and the onset of the Madden-Julian oscillation.