Direct and Diffuse Radiation in the Shallow Cumulus-Vegetation System: Enhanced and Decreased Evapotranspiration Regimes

X. Pedruzo Bagazgoitia*, Huug Ouwersloot, M. Sikma, C.C. van Heerwaarden, C.M.J. Jacobs, J. Vilà-Guerau De Arellano

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

18 Citations (Scopus)

Abstract

Guided by a holistic approach, the combined effects of direct and diffuse radiation on the atmospheric boundary
layer dynamics over vegetated land are investigated on a daily scale. Three numerical experiments are designed that
are aimed at disentangling the role of diffuse and direct radiation below shallow cumulus at the surface and on
boundary layer dynamics. A large-eddy simulation (LES) model coupled to a land surface model is used,
including a mechanistically immediate response of plants to radiation, temperature, and water vapor deficit
changes. The partitioning in direct and diffuse radiation created by clouds and farther inside the canopy is explicitly
accounted for. LES results are conditionally averaged as a function of the cloud optical depth. The 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.Because of 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 because of the limited time and space where these perturbations occur. It is concluded that an
accurate radiation partitioning calculation is necessary to obtain reliable estimations on local surface processes.
Original languageEnglish
Pages (from-to)1731-1748
JournalJournal of Hydrometeorology
Volume18
DOIs
Publication statusPublished - 2017

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