Simulations of tropical rainforest albedo: is canopy wetness important?

Silvia N.M Yanagi,Marcos H Costa

doi:10.1590/s0001-37652011005000047

Silvia N.M Yanagi, Marcos H Costa

Open Access

https://doi.org/10.1590/s0001-37652011005000047

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Abstract

Accurate information on surface albedo is essential for climate modelling, especially for regions such as Amazonia, where the response of the regional atmospheric circulation to the changes on surface albedo is strong. Previous studies have indicated that models are still unable to correctly reproduce details of the seasonal variation of surface albedo. Therefore, it was investigated the role of canopy wetness on the simulated albedo of a tropical rainforest by modifying the IBIS canopy radiation transfer code to incorporate the effects of canopy wetness on the vegetation reflectance. In this study, simulations were run using three versions of the land surface/ecosystem model IBIS: the standard version, the same version recalibrated to fit the data of albedo on tropical rainforests and a modified version that incorporates the effects of canopy wetness on surface albedo, for three sites in the Amazon forest at hourly and monthly scales. The results demonstrated that, at the hourly time scale, the incorporation of canopy wetness on the calculations of radiative transfer substantially improves the simulations results, whereas at the monthly scale these changes do not substantially modify the simulated albedo.

Highlights

Surface albedo is the main factor that affects the land radiation balance controlling the amount of solar energy available for heating the ground and lower atmosphere and for evaporating water (Rowe 1991), and affecting the atmospheric circulation and climate
In this study we have run simulations using three versions of the land surface/ecosystem model IBIS: the standard version with the original calibration used by Delire and Foley (1999), the same version recalibrated to fit the data of albedo on tropical rainforests, and a modified version that incorporates the effects of canopy wetness on calculated surface albedo
We could verify that the DF99 and DC simulation results during the precipitation hours are much worse than the equivalent result for the entire time series, which by itself indicates that a precipitation-related process is missing in the model

Summary

Introduction

Surface albedo is the main factor that affects the land radiation balance controlling the amount of solar energy available for heating the ground and lower atmosphere and for evaporating water (Rowe 1991), and affecting the atmospheric circulation and climate. Berbet and Costa (2003) verified that even a complex, state-of-the-art, land surface scheme coupled to a climate model is unable to correctly reproduce details of the seasonal variability of the albedo of tropical rainforests, it reproduces well the annual mean and some aspects of the seasonal variability. This indicates that there is still much to be learned – and incorporated into models – about the sources of variation of the albedo at both hourly and monthly time scales. The authors reported that the albedo seasonality at these sites is not related to changes on the solar elevation angle nor to cloudiness, but suggested a relationship with soil

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