Turbulent Exchange of CO 2 ${\text{CO}}_{2}$ in the Lower Tropical Troposphere Across Clear-to-Cloudy Conditions

Abstract

This study investigates the roles of clear air entrainment and shallow cloud ventilation, alongside rainforest ${\text{CO}}_2$-assimilation, in the turbulent exchange of ${\text{CO}}_2$ within the lower tropical troposphere under clear-to-cloudy conditions. Constrained by comprehensive observations from the CloudRoots-Amazon22 campaign, spanning leaf stomatal to upper atmosphere, we design and evaluate a representative shallow convective numerical experiment with the turbulence-resolving Dutch Atmospheric Large Eddy Simulation model, incorporating a bulk rainforest representation. We assess contributions from the rainforest, clouds, and environment through the vertically integrated, domain-averaged ${\text{CO}}_2$ budget by comparing simulations with and without the dynamic effects of clouds. Our findings reveal three distinct diurnal regimes named: entrainment-diluting, cloud-ventilation-and-entrainment, and ${\text{CO}}_2$-assimilation. Shallow convective clouds ($\sim$23%), clear air entrainment ($\sim$21%), and rainforest ${\text{CO}}_2$-assimilation ($\sim$56%) collectively influence the diurnal evolution and vertical exchange of ${\text{CO}}_2$ within the clear-to-cloudy boundary layer, with their relative importance varying per diurnal regime. In the absence of clouds, ventilation ceases and ${\text{CO}}_2$ exchange is driven solely by entrainment and ${\text{CO}}_2$-assimilation, resulting in a 20%–25% reduction in mixing effectiveness. In the vertical, shallow clouds ventilate ${\text{CO}}_2$ to heights reaching twice the boundary layer depth, significantly affecting the vertical distribution until late afternoon. Analysis of the correlation between ${\text{CO}}_2$ and $H_2$O shows that shallow convective clouds organize the turbulent exchange at shallow cloud-scales, shaping a vertical pattern of negative to positive ${\text{CO}}_2$-$H_2$O correlation from the roughness sublayer into the cloud layer. These findings highlight key processes crucial for accurately representing the lower tropical tropospheric ${\text{CO}}_2$ budget across clear-to-cloudy conditions.