Direct and diffuse radiation in the shallow cumulus-vegetation system: enhanced and decreased evapotranspiration regimes

Abstract

uided by a holistic approach we investigate the combined effects of direct and diffuse radiation on the atmospheric boundary-layer dynamics over vegetated land on a daily scale. We design three numerical experiments aimed at disentangling the role of diffuse and direct radiation below shallow cumulus at the surface and on boundary-layer dynamics. We use a Large Eddy Simulation (LES) model coupled to a land-surface model, including a mechanistically immediate response of plants to radiation, temperature and water vapor deficit changes. We explicitly account for the partitioning in direct and diffuse radiation created by clouds and further inside the canopy. LES results are conditionally averaged as function of the cloud optical depth. Our findings show larger photosynthesis under thin clouds than under clear sky, due to an increase in diffuse radiation and a slight decrease in direct radiation. The reduced canopy resistance is the main driver for the enhanced carbon uptake by vegetation, while the carbon gradient and aerodynamic effects at the surface are secondary. Due to the coupling of CO2 and water vapor exchange through plant stomata, evapotranspiration is also enhanced under thin clouds, albeit to a lesser extent. This effect of diffuse radiation increases the water-use efficiency and evaporative fraction under clouds. The dynamic perturbations of the surface fluxes by clouds do not affect general boundary-layer or cloud characteristics due to the limited time and space where these perturbations occur. We conclude that an accurate radiation partitioning calculation is necessary to obtain reliable estimations on local surface processes.