This study investigates the effects of resolved deep convection on tropical rainfall and its multi-scale variability. A series of aquaplanet simulations are analyzed using the Model for Prediction Across Scales-Atmosphere (MPAS-A) with horizontal cell spacings from 120 km to 3 km. The 3-km experiment uses a novel configuration with 3-km cell spacing between 20°S-20°N and 15-km cell spacing poleward of 30°N/S. A comparison of those experiments shows that resolved deep convection yields a narrower, stronger, and more equatorward intertropical convergence zone, which is supported by stronger nonlinear horizontal momentum advection in the boundary layer. There is also twice as much tropical rainfall variance in the experiment with resolved deep convection than in the experiments with parameterized convection. The extra variance is associated with westward propagating systems. Resolving deep convection yields at least two orders of magnitude more frequent heavy rainfall rates ( > 2 mm hr−1) than the experiments with parameterized convection. Tropical convection organizes into linear systems that are associated with stronger and deeper cold pools and upgradient convective momentum fluxes when convection is resolved. In contrast, parameterized convection results in more circular systems, weaker cold pools, and downgradient convective momentum fluxes. These results suggest that simulations with parameterized convection are missing an important feedback loop between the mean state, convective organization, and meridional gradients of moisture and momentum.
Many biases in conventional climate models are ascribed to deficiencies in parameterized convection. This study uses a model configured with different horizontal resolutions, from conventional climate model scales (~100km) down to convection-permitting scales (3km). The model is configured as an aquaplanet to focus on the physics of the system and cleanly isolate the impact of parameterized convection compared to the 3km simulation with explicit convection. The results suggest that parameterized physics damp tropical rainfall. Analysis shows that the difference arises because of changes in the physics of momentum transport that feedback onto the organization of convection.
Parameterizing convection appears to damp tropical rainfall and its variability. This benchmark study provides direct evidence for the impact of parameterized convection on tropical precipitation by using a model that explicitly resolves deep convection. Careful analysis of individual convective events shows how momentum transport changes when convection is resolved. This results in changes in the organization of convective events into more linear systems that have deeper, stronger cold pools than when convection is parameterized. The change in organization also manifests as a change in tropical precipitation variability, specifically an enhancement in westward propagating disturbances. This idealized study provides a new context for the emerging generation of convection-permitting models, and also shows that simulations with parameterized convection are missing an important feedback loop that could be crucial for accurately capturing tropical climate.