In terms of light responses of canopy NEE this implies consideration of different shapes for the response curve according to the sky conditions. Not all sites do measure diffuse and total available radiation. So other methods to distinguish clear, cloudy and overcast time periods have to be considered. The method here calculates for each half-hour the potential radiation at the top of the atmosphere depending on latitude, longitude, Julian day, and daytime. The threshold lines are site-specific, somehow arbitrary and more an engineering approach.
More sophisticated suggestions are welcome.
The data in this figure is also pre-sorted in temperature classes. For the AmeriFlux sites Howland, Shidler, and Little Washita Fig. 11 summarizes the shift of maximum NEE towards lower light intensities at a given temperature range when sky conditions are cloudy or overcast. The implications of a relative higher net photosynthesis during periods with large diffuse proportion on estimates of future carbon uptake are worth considering, as global climate change models predict increasing cloudiness. Also indicated the threshold lines for distinguishing clear, cloudy and overcast sky conditions.