Modeling radiative transfer in tropical rainforest canopies: sensitivity of simulated albedo to canopy architectural and optical parameters

Sílvia N M Yanagi,Marcos H Costa

doi:10.1590/s0001-37652011000400010

Abstract

This study evaluates the sensitivity of the surface albedo simulated by the Integrated Biosphere Simulator (IBIS) to a set of Amazonian tropical rainforest canopy architectural and optical parameters. The parameters tested in this study are the orientation and reflectance of the leaves of upper and lower canopies in the visible (VIS) and near-infrared (NIR) spectral bands. The results are evaluated against albedo measurements taken above the K34 site at the INPA (Instituto Nacional de Pesquisas da Amazônia) Cuieiras Biological Reserve. The sensitivity analysis indicates a strong response to the upper canopy leaves orientation (χup) and to the reflectivity in the near-infrared spectral band (ρNIR,up), a smaller sensitivity to the reflectivity in the visible spectral band (ρVIS,up) and no sensitivity at all to the lower canopy parameters, which is consistent with the canopy structure. The combination of parameters that minimized the Root Mean Square Error and mean relative error are χup = 0.86, ρVIS,up = 0.062 and ρNIR,up = 0.275. The parameterizations performed resulted in successful simulations of tropical rainforest albedo by IBIS, indicating its potential to simulate the canopy radiative transfer for narrow spectral bands and permitting close comparison with remote sensing products.

Highlights

Global climate models (GCMs) simulate the evolution of the climate based on physical principles as well as on initial and boundary conditions
We model the radiative transfer using the radiative transfer module of the Integrated Biosphere Simulator (IBIS) and validate the simulations against albedo measurements taken at the tropical rainforest site at the Cuieiras Biological Reserve (K34)
Root Mean Square Error (RMSE) and mean error analyses indicate that the model is not sensitive to the parameters previously cited, and it is consistent with an ecosystem of tropical rainforest because the parameter χlo that controls the leaf orientation, as well as ρVIS,lo and ρNIR,lo that control the lower canopy reflectance in the visible and infrared wavebands, receive little solar radiation

Summary

INTRODUCTION

Global climate models (GCMs) simulate the evolution of the climate based on physical principles as well as on initial and boundary conditions To do so, these models must represent the exchanges of radiation, heat, momentum and mass between the atmosphere and the underlying surface, in particular in terrestrial environments. Be able to simulate accurately both the flux reflected from the top of the canopy, that is its albedo, and the flux transmitted to the ground underneath the vegetation layer. In this modeling context, the albedo is a prime candidate for validation exercises. We model the radiative transfer using the radiative transfer module of the Integrated Biosphere Simulator (IBIS) and validate the simulations against albedo measurements taken at the tropical rainforest site at the Cuieiras Biological Reserve (K34)

METHODS

RESULTS AND DISCUSSION

CONCLUSIONS