We investigate a set of Energy Exascale Earth System Model Multi-scale Modeling Framework (E3SM-MMF) simulations that vary the dimensionality and momentum transport configurations of the embedded cloud-resolving models (CRMs), including unusually ambitious 3D configurations. Issues endemic to all MMF simulations include too much ITCZ rainfall and too little over the Amazon. Systematic MMF improvements include more on-equatorial rainfall across the Warm Pool. Interesting sensitivities to CRM domain are found in the regional time-mean precipitation pattern over the tropics. The 2D E3SM-MMF produces an unrealistically rainy region over the northwestern tropical Pacific; this is reduced in computationally ambitious 3D configurations that use 1024 embedded CRM grid columns per host cell. Trajectory analysis indicates that these regional improvements are associated with desirably fewer tropical cyclones and less extreme precipitation rates. To understand why and how the representation of precipitation improved in 3D, we propose a framework that dilution is stronger in 3D. This viewpoint is supported by multiple indirect lines of evidence, including a delayed moisture-precipitation pickup, smaller precipitation efficiency, and amplified convective mass flux profiles and more high clouds. We also demonstrate that the effects of varying embedded CRM dimensionality and momentum transport on precipitation can be identified during the first few simulated days, providing an opportunity for rapid model tuning without high computational cost. Meanwhile the results imply that other less computationally intensive ways to enhance dilution within MMF CRMs may also be strategic tuning targets.

Vertical resolution is an often overlooked parameter in simulations of convection. We explore the sensitivity of simulated deep convection to vertical resolution in the System for Atmospheric Modeling (SAM) convection resolving model. We analyze simulations run in tropical radiative convective equilibrium with 32, 64, 128, and 256 vertical levels in a small (100 km) and large domain (1500 km). At high vertical resolution, the relative humidity and anvil cloud fraction are reduced, which is linked to a reduction in both fractional and volumetric detrainment. This increases total atmospheric radiative cooling at high resolution, which leads to enhanced surface fluxes and precipitation, despite reduced column water vapor. In large domains, convective aggregation begins by simulation day 25 for simulations with 64 and 128 levels, while onset is delayed until simulation day 75 for the simulation with 32 vertical levels. Budget analyses reveal that mechanisms involved in the generation and maintenance of convective aggregation for the 32-level simulation differ from those for the 64- and 128-level simulations. Weaker cold pools in the 32-level simulation allow the boundary layer in dry regions to become extremely dry, which leads to an aggregated state with very strong spatial gradients in column-integrated moist static energy. Understanding both the triggering and maintenance of convective aggregation and its simulated sensitivity to model formulation is a necessary component of atmospheric modeling. We show that vertical resolution has a strong impact on the mean state and convective behavior in both small and large domains.