Hierarchies of dynamical and microphysical control on tropical convective instability

Convection in the tropics is driven by large-scale instability and the buoyancy of convective plumes, but has a variety of representations in Earth system models (ESM). In model convective schemes, subgrid parameterizations approximate the processes that regulate plume buoyancy, leading to substantial spread in simulated tropical thermodynamic structure. While entrainment is known to strongly influence plume buoyancy, recent work highlights an important role for cloud microphysical processes—particularly mixed-phase water physics and precipitation efficiency—in shaping buoyancy during ascent (Emmenegger et al. 2024). The influence of these microphysical processes on tropical buoyancy, and how they should be constrained in convective schemes, is explored here.

Using a perturbed-parameter ensemble (PPE) of the Community Atmosphere Model version 6 (Eidhammer et al. 2024), together with numerical single-column simulations, we construct process-oriented diagnostics to identify controls on convective sensitivity over the tropical western Pacific. Across the ensemble, variability in the convection sensitivity metric is dominated by deep convective parameters, while shallow convective and microphysical parameters exert weaker but systematic influence, revealing a clear hierarchy of physical controls. This hierarchy provides a useful way to organize the role of cloud processes, but does not explain how these parameter sensitivities arise or interact across scales.

To interpret the ensemble behavior, we use a simple bulk plume framework that isolates the effects of entrainment and key microphysical processes on plume buoyancy. Observations from the US Department of Energy’s Atmospheric Radiation Measurement field campaigns together with theoretical considerations of plume buoyancy are used to derive constraints for model representation of these processes. Together, the PPE diagnostics, observational constraints, and bulk plume framework clarify how microphysical and dynamical processes interact to shape simulated tropical convective instability. This approach provides a physics-informed basis for interpreting parameter sensitivity and improving the representation of convection in coarse resolution ESMs.