Ent/ACTS: Canopy Albedo Predictions With An Analytical Geometric-Optical Radiative Transfer Model for Fusing Lidar Remote Sensing and Demographic Dynamic Global Vegetation Models Coupled to Earth System Models

Abstract:

Earth System Models (ESMs) that simulate the carbon cycle require computationally parsimonious dynamic global vegetation models (DGVMs) that can account for the effect of canopy structure on the individualistic ecological dynamics of plant populations while at the same time quantifying the structural and spectral aspects of vegetation that can be observed by remote sensing: albedo, the bidirectional reflectance distribution function, foliage clumping, vertical foliage profiles. We present evaluations of simulated field site canopy albedos using a geometric-optical radiative transfer (GORT) model, the Analytical Clumped Two-Stream (ACTS) model, coupled with a demographic DGVM (dDGVM), the Ent Terrestrial Biosphere Model (Ent TBM), that offers capabilities to be consistent with both lidar remote sensing of vegetation structure and simulation of vegetation populations and carbon dynamics. We analyze diurnal and seasonal broadband and some hyperspectral simulated albedos compared to observations in different ecosystem types, including boreal forest, broadleaf deciduous forest, oak savanna, grasslands, and croplands. We examine sensitivity of albedo and simulated fluxes of CO₂ and water vapor to uncertainty in crown dimension allometric relations, end member spectra, and direct/diffuse radiation partitioning. The model captures well the diurnal variation of canopy albedo with solar zenith angle. While it simulates albedo variation with seasonal leaf area index, leaf end member near-infrared seasonality is a significant driver to capture fully the temporal variation in deciduous and annual plant types. Global scale simulation of surface albedo can make use of satellite lidar vertical foliage profiles to define canopy structure. Model codes output diagnostics of vertical foliage profiles, the foliage clumping factor, and black sky and white sky albedo in flexible spectral bands. The fortran95 codes are available for community use through the NASA Goddard Institute for Space Studies ESM ModelE online repository, with drivers for site-specific and global simulations.