Differences in Tropical Rainfall in Aquaplanet Simulations With Resolved or Parameterized Deep Convection

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 with horizontal cell spacings from 120 to 3 km. The 3-km experiment uses a novel configuration with 3-km cell spacing between 20°S and 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. All experiments show comparable precipitation variance associated with Kelvin waves; however, the experiment with resolved deep convection shows higher precipitation variance associated with westward propagating systems. Resolved deep convection also yields at least two orders of magnitude more frequent heavy rainfall rates (>2 mm hr⁻¹) than the experiments with parameterized convection. A comparison of organized precipitation systems demonstrates that 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.