Interactions between the Amazonian rainforest and cumuli clouds:A large-eddy simulation, high-resolution ECMWF and observational intercomparison study

The explicit coupling at metre and second scales of the vegetation responses to the atmospheric-boundary layer dynamics drives a dynamic heterogeneity that influences surface fluxes and cloud formation. Focusing on a representative day during the Amazonian dry season characterized by a transition from a clear boundary layer to shallow cumuli, we investigated the diurnal cycle of the energy, moisture and carbon dioxide at the surface, and the coupling during this transition. Three different methodologies are applied: a large-eddy simulation technique, a high-resolution global weather model and a complete observational data set collected during the GoAmazon campaign.

The overall model-observation process comparisons of radiation and surface fluxes, turbulence and cloud dynamics are very satisfactory with all the modelled variables within the standard deviation of the monthly aggregated observations. Our analysis indicates that the switch of the diel carbon exchange in the rainforest, from a negative sink to a positive source, remains uncertain in time and is related to the stomata closure due to the afternoon increase vapour pressure deficit and larger radiation perturbations by clouds. Related to the moisture transport from the sub-cloud layer into the cloud layer, we found the need to represent adequately this process to ensure realistic values of specific humidity and thermal instability. More complex studies on radiation, microphysics and chemistry can follow up this discussed benchmark study to improve our understanding the sub-daily variability in the Amazonian rainforest.