Simplifying the interaction of land surfaces with radiation for relating remote sensing products to climate models

Abstract

[1] Remote sensing products, such as the fraction of reflected solar radiation flux, as well as the amount of radiation absorbed in the photosynthetically active spectral region and the Leaf Area Index (LAI), are operationally available from Space Agencies. Climate models may benefit from these products provided their one dimensional (1-D) radiation transfer schemes effectively represent the three dimensional (3-D) effects implied by the internal spatial variability of vegetation canopies, e.g., the leaf area density, at all scales and resolutions involved (say from 1 to 100 kilometers). Failing to do so leads to inherent inconsistencies between the domain-averaged reflected and absorbed fluxes, and the implied Leaf Area Index. We propose a comprehensive approach which introduces a parameterization of the internal variability of the LAI in the 1-D representation of the radiation scheme, called a domain-averaged structure factor, and provides a description of the radiant fluxes fully consistent with the LAI specified by remote sensing. We take this opportunity to revisit and update the two-stream formulations implemented in climate models to accurately estimate the fractions of radiation absorbed separately by the vegetation canopy and the underlying surface. This is achieved by isolating the contributions of the vegetation canopy alone, the background as seen through the canopy gaps and the multiple scattering between the vegetation layer and the background. The performance of this formulation is evaluated against results from Monte Carlo simulations relative to explicit realistic 3-D canopies to show that the proposed scheme correctly simulates both the amplitude and the angular variations of all radiant fluxes with respect to the solar zenith angle.Citation: Pinty, B., T. Lavergne, R. E. Dickinson, J.-L. Widlowski, N. Gobron, and M. M. Verstraete (2006), Simplifying the interaction of land surfaces with radiation for relating remote sensing products to climate models,

A multi-level canopy radiative transfer scheme for ORCHIDEE (SVN r2566), based on a domain-averaged structure factor

“…Conceptually, this picture is directly comparable to the original model of Pinty et al (2006) for the case of a nonreflecting background.…”

“…The one-layer scheme is described in detail by Pinty et al (2006). We here use the same notation and terms as developed in that paper, for consistency.…”

Tropical Atlantic biases and their relation to surface wind stress and terrestrial precipitation

“…Nevertheless, there have been efforts to leverage existing satellite and in-situ observations to evaluate and improve model parameterizations (e.g. Wang et al 2005;Pinty et al 2006;Abramowitz et al 2008). Further efforts along these lines will be necessary to achieve substantial improvements of GCM land surface parameterizations in the tropics.…”

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

“…(8) and (9) with the definitions given in the lowest row of Table 4 of Pinty et al (2006). Pinty et al (2006) also provided more general expressions for leaves with a preferential alignment. The rates of lateral exchange of radiation between regions that appear in Eqs.…”

“…Ni-Meister et al, 2010), but this lacks a physical basis and fails to represent horizontal fluxes into and out of individual tree crowns. Pinty et al (2006) described one of the most sophisticated yet affordable schemes to date that attempts to overcome these limitations. Their scheme sums three terms: the reflection from the vegetation assuming a black underlying surface, the reflection from the surface assuming no interaction with the vegetation, and a term representing interactions between the surface and the vegetation.…”