Recent Advances in Radiative Coupling Between Atmosphere and Land Surface in the WPS Model

Abstract:

A Monte Carlo model was formerly built to combine the photon spread method and the weight reduction concept to develop the weighted photon spread (WPS) model for the simulation of radiation transfer (RT) in three-dimensional (3-D) canopies. The inclusion of simulating RT of sun-induced chlorophyll fluorescence (SIF) of 3-D canopy results into the FluorWPS (Fluorescence model with Weighted Photon Spread method) model. Recently we extended the WPS model with an accurate radiative coupling between atmosphere and land surface using the forward Monte Carlo ray-tracing method. The atmosphere is represented as plane parallel layers with corresponding optical properties, e.g., extinction coefficient, single scattering albedo, and phase function, which are determined by the atmospheric conditions. With the extended atmospheric module, Top-Of-Atmosphere (TOA) radiance both with and without SIF can be simulated under different atmospheric conditions, 3D canopy structures, and sun-viewing geometries.

To evaluate the extension of the WPS model, comparisons for the simulations between WPS and the MODTRAN + SCOPE models were carried out for coupled one-dimensional canopy and atmosphere. The simulations are performed at the 0.15 nm resolution in the range of 400-850 nm (SIF is presented in 640-850 nm). Besides the same final output of the TOA radiance and SIF, the direct comparisons of the atmospheric transmittance, and the bi-directional radiance by the canopy are also provided. From the comparisons and statistical results, the close agreement of the simulation results by WPS and the MODTRAN+SCOPE model can be observed. We can conclude that the extension of the WPS model in radiative coupling between atmosphere and land surface is successful.