Simulation of forest radiance at top-of-atmosphere level using coupled radiative transfer models

Abstract

Three radiative transfer models were coupled to simulate forest radiances at the top-of-atmosphere (TOA) level: 1) the PROSPECT leaf model, 2) the 4SAIL2 canopy model which includes the hotspot and clumping effects, and 3) the MODTRAN atmospheric radiative transfer model. The output of the coupled model can be compared to radiances measured at satellite level. The study area consisted of two Norway spruce stands in Eastern Czech Republic. The field data were collected in the first half of September 2006 and the CHRIS (Compact High Resolution Imaging Spectrometer) data were acquired on September 22, 2006. The coupled model was applied for the two stands, using a single set of atmospheric parameters for the four available CHRIS viewing directions. The simulated TOA radiances were systematically too high compared to the CHRIS data, for the four viewing directions. However, the simulated spectral and directional trends matched those measured by CHRIS. After investigation, it was found that the reflectance values simulated by the coupled model at the top of the canopy (TOC) were too high and caused the TOA radiances to be too high. Further research will therefore focus on improving the simulation of the TOC reflectances.