RAMI4PILPS: An intercomparison of formulations for the partitioning of solar radiation in land surface models

Abstract

[1] Remotely sensed, multiannual data sets of shortwave radiative surface fluxes are now available for assimilation into land surface schemes (LSSs) of climate and/or numerical weather prediction models. The RAMI4PILPS suite of virtual experiments assesses the accuracy and consistency of the radiative transfer formulations that provide the magnitudes of absorbed, reflected, and transmitted shortwave radiative fluxes in LSSs. RAMI4PILPS evaluates models under perfectly controlled experimental conditions in order to eliminate uncertainties arising from an incomplete or erroneous knowledge of the structural, spectral and illumination related canopy characteristics typical for model comparison with in situ observations. More specifically, the shortwave radiation is separated into a visible and near-infrared spectral region, and the quality of the simulated radiative fluxes is evaluated by direct comparison with a 3-D Monte Carlo reference model identified during the third phase of the Radiation transfer Model Intercomparison (RAMI) exercise. The RAMI4PILPS setup thus allows to focus in particular on the numerical accuracy of shortwave radiative transfer formulations and to pinpoint to areas where future model improvements should concentrate. The impact of increasing degrees of structural and spectral subgrid variability on the simulated fluxes is documented and the relevance of any thus emerging biases with respect to gross primary production estimates and shortwave radiative forcings due to snow and fire events are investigated.

Fast matrix treatment of 3-D radiative transfer in vegetation canopies: SPARTACUS-Vegetation 1.1

“…To test the application of the SPARTACUS methodology to the vegetation problem, we use two 3-D scenarios from the RAMI4PILPS 1 intercomparison exercise (Widlowski et al, 2011). The first scenario is an idealized representation of an open forest canopy, and consists of spheres of leafy vegetation of diameter 10 m, while the second represents shrubland and consists of spheres of diameter 1 m. Details are provided in Table 1, including the three different area coverages of vegetation that are used.…”

“…Two spectral intervals are simulated, representing the photosynthetically active visible region and Table 2. Variables describing the optical properties of the leaves and the surface in the visible and near-infrared in the RAMI4PILPS cases (see Widlowski et al, 2011 the near-infrared, and both snow-free and snow-covered surfaces are considered. Table 2 lists the optical properties of the leaves and the surfaces in the two spectral intervals.…”

“…The dashed and dot-dashed lines depict the two-region and one-region SPARTACUS calculations, respectively, where the latter involves complete horizontal homogenization of the vegetation properties through the domain. Also shown are the corresponding Monte Carlo calculations of Widlowski et al (2011) at solar zenith angles of 27, 60 and 83 • . v i+1/2 .…”

“…The assumption that tree crowns do not touch was used by Widlowski et al (2011) in generating the idealized scenes that we use in Sect. 3 to evaluate SPARTACUS.…”

Consistent retrieval of land surface radiation products from EO, including traceable uncertainty estimates

“…C(d mod ) in Eq. (1), a further exercise in the RAMI frame (termed RAMI4PILPS) addressed the accuracy and consistency of the absorbed, reflected, and transmitted radiative fluxes retrieved by inverse models of the soil-vegetation-atmosphere transfer (Widlowski et al, 2011). This exercise thus offers a possibility to assess the performance of the JRC-TIP with regard to its ability to partition the incoming solar radiation.…”