Clouds reflect incoming solar radiation back into space, but also increase the amount of diffuse radiation that reaches the land surface. Our new study (lead author Xabier Pedruzo-Bagazgoitia) shows that thin clouds are beneficial for CO2 assimilation and evapotranspiration, due to the influence of diffuse radiation. This analysis is based on a typical Dutch early autumn day with cumulus convection.
The figure above shows the CO2 assimilation An, the evapotranspiration rate LE and the sensible heat flux SH as a function of the cloud optical depth $\tau$ relative to their magnitude under clear sky conditions. Under conditions of thin clouds ($\tau$ below 5) there is more CO2 assimilation and more evapotranspiration than under conditions without clouds. Under clouds of moderate thicknesses ($\tau$ between 5 and 8.5) CO2 assimilation is still enhanced compared to clear-sky conditions, but evapotranspiration is less.
This study highlights the importance of understanding cloud formation over land in order to understand the Earth's carbon and water budget.
Pedruzo-Bagazgoitia, X., H.G. Ouwersloot, M. Sikma, C.C. van Heerwaarden , C.M. Jacobs, and J. Vilà-Guerau de Arellano, 2017: Direct and Diffuse Radiation in the Shallow Cumulus–Vegetation System: Enhanced and Decreased Evapotranspiration Regimes. J. Hydrometeor., 18, 1731–1748, https://doi.org/10.1175/JHM-D-16-0279.1.