Large Eddy Simulations of Stratocumulus Response to Changing CO2 Concentrations

The fate of marine stratocumulus cloud decks under greenhouse warming is a critical component of the climate system’s response to rising CO2 concentrations due to the efficiency of these clouds in reflecting incoming solar radiation. However, climate models must rely on parameterized representations of stratocumuli which can at best approximate their complex dynamics that depend on interactions between cloud-top longwave radiative cooling, turbulent mixing of the boundary layer, and entrainment of free tropospheric air. The uncertainty in stratocumulus parameterizations thus contributes substantially to the overall uncertainty in quantifying climate sensitivity.

Here we perform a series of high-resolution large eddy simulations capable of explicitly resolving stratocumulus clouds at scales of tens of meters to understand the details of their dynamical response to greenhouse warming. The LES are coupled to a slab ocean model to ensure a physically realizable surface energy budget and simulations are run until sea surface temperatures reach an approximate steady state. An innovative aspect of these simulations is the coupling between the state of the representative subtropical domain simulated by the LES and the large-scale forcings of heat and moisture applied to it, which are obtained through an idealized but physically relevant model of the tropical climate response. Using this framework, we identify a critical CO2 concentration above which stratocumulus cloud decks cannot be sustained. Subsequently, CO2 concentrations must be decreased significantly below this critical level to reform stratocumuli, demonstrating the hysteresis of the response to CO2.