Mesoscale aerosol variability dominates stratocumulus-climate interactions

Stratocumulus clouds cover large parts of the subtropical oceans, and they dominate the net cooling effect of clouds in the Earth’s energy balance. Their non-linear response to anthropogenic aerosol forcings makes them a major source of uncertainty for climate projections. Part of this sensitivity arises from transitions between the two distinct states of stratocumulus (closed and open cells), which are associated with abrupt changes in the cloud’s radiative properties. These transitions can occur locally (pockets of open cells), or as a result of advection with the prevailing winds (stratocumulus-to-cumulus transition by drizzle). Here, we investigate the interaction of such transitions with aerosol perturbations and identify the perturbations that most strongly influence stratocumulus radiative properties.

The mesoscale evolution of stratocumulus decks is modeled using a data-driven, physics-informed stochastic dynamical system with time-dependent parameters. This description encapsulates the scales of cloud formation, mesoscale self-organization, and large-scale conditions through fluctuations, deterministic evolution, and slowly varying parameters, respectively. For relevant parameter conditions, the system features bistability, showcasing the coexistence of open and closed cells. This approach allows us to replicate previous LES results while efficiently extrapolating to a much wider range of parameters and initial conditions, enabling the study of regimes and transitions that LES cannot practically sample.

We find that aerosol-related processes, like rain-formation-washout feedback in open cells and slow aerosol accumulation in closed cells, lead to a lack of timescale separation. As a result, the system’s state is not equilibrated to the steady state prescribed by the large-scale parameters but instead strongly depends on its history. Combined with the system’s bistability, this results in mesoscale pollution plumes dominating the radiative response of stratocumulus, outweighing the effects of background aerosol forcing, cloud feedback, and small-scale fluctuations. It can also lead to delayed radiative responses to intermittent perturbations, such as ship-tracks. This strong mesoscale memory can complicate process attribution from satellite snapshot observations.

Our results highlight that mesoscale cloud organization needs to be considered in numerical modeling as well as in interpreting observations if we are to accurately constrain the response of stratocumulus to aerosol perturbations.