Stratocumulus radiative effect, multiple equilibria of the well‐mixed boundary layer and transition to shallow convection

Abstract

The present work investigates the equilibrium of the atmospheric boundary layer (ABL) over the ocean using a large‐eddy simulation model, with different lower‐boundary sea‐surface temperatures (SSTs) and different initial conditions. For low SSTs, one of two equilibria is reached depending on the initial conditions. One equilibrium is characterized by a deep, stratocumulus‐capped ABL, while the other is a thin, cloud‐free ABL. Cloud radiative cooling at the top of the stratocumulus is crucial to the existence of the stratocumulus equilibrium. If the SST is increased, the stratocumulus equilibrium becomes deeper and the cloud radiative effect reaches a maximum before decreasing due to buoyancy reversal and decoupling. This decrease of cloud radiative effect with SST eventually suppresses the stratocumulus equilibrium at higher SSTs–all initial conditions then lead to the cloud‐free equilibrium. If the SST is increased further, the cloud‐free equilibrium becomes a deeper, shallow convective equilibrium with cumuli in the upper part of the ABL. This shallow convective ABL is deepened by the cloud radiative effect. Our results suggest that the transition from stratocumulus decks in the eastern subtropical oceans to the trade‐wind shallow convective regions is essentially a transition from one basin of attraction, which disappears, to another.