Phase: RAMI IV
The Analytical Clumped Two-Stream (ACTS) model is a simple but physically based canopy RT scheme, which accounts for the effect of hierarchical foliage clumping on the light environment of vegetation canopies by combining a two-stream scheme with geometric optical (GO) theory.
The GO component yields a well-described actual vertical foliage profile and an analytically derived clumping factor allowing to account for the impact of the vertical and horizontal vegetation structure.
This scheme has the same computational cost as the schemes currently being used in GCMs, but provides better photosynthesis, radiative fluxes and surface albedo estimates for the ENT Dynamic Global Terrestrial Ecosystem Model [Kiang et al. , 2006].
The ACTS model generates vertical profiles of reflectance,transmittance and absorbance within the canopy.
The reflectance at the top level is selected as canopy reflectance, the transmittance at the bottom level is selected as canopy transmittance, and the absorbance profile is integrated under consideration of the sunlit/shaded components to generate the canopy absorption.
Since the embedded two-stream scheme cannot accept zero as input value for the background albedo, the black background cases were run with an albedo value of a = 10-8 instead.
A typical RAMI4PILPS run took less than 1 s on a i386 Linux workstation with 8 Gb of RAM and a 3.2 GHz processor.
The source code of ACTS is written in Fortran 95.