Understanding how the connection between rainfall and tropical forests will respond to increasing CO2 concentrations is a key piece for understanding how the tropical water cycle will respond to increasing CO2. The physiological and radiative impacts of CO2 on rainfall patterns over tropical forest regions are examined in the Energy Exascale Earth System Model version 1 (E3SMv1) biogeochemistry experiments. Composite analysis reveals a dampening of the diurnal cycle of rainfall over the Amazon, Congo, and Maritime Continent in response to rising CO2 levels. The effect occurs both for experiments where only the radiative response to CO2 is enabled and for those where only the plant physiological response to CO2 is enabled. Moisture budget analysis reveals that the dampening of the diurnal cycle occurs for different physical reasons for the two components of the CO2 response examined here. For the physiological response, it is the reduction in surface latent heat flux that dampens the diurnal rainfall signal; while for the radiative response, it is changing moisture storage within the atmosphere. More analyses are underway to better understand the physical pathways responsible for the physiological and radiative responses.