4. Improvement of CRTM Surface TIR Emissivity: In Search of a
Physically-Based Canopy Emissivity Model
Author：
Ming Chen and Fuzhong Weng
Affiliation： IMSG at NOAA/NESDIS
The interaction of electromagnetic radiation with plant leaves depends on a variety
of canopy physical and chemical characteristics such as leaf area index, leaf
biochemical compositions, leaf thickness, leaf water content, and so on. The
radiative transfer modeling of canopy is therefore challenging in practice, yet
desirable for accurate canopy radiation calculation and indispensible for the
inference of vegetation properties from remote sensing data. Theoretically, a
physically-based canopy model consists of two major radiative processes: the
interactions of electromagnetic waves with individual leaf, and the multi-scattering
among canopy leaves. As our initial effort, the PROSPECT leaf optical property
model (Jacquemoud,1990), together with the SAIL (Scattering by Arbitrarily
Inclined Leaves) canopy model (Verhoef, 1984, 1985), were adopted into the NOAA
Community Radiative Transfer Model (CRTM) to simulate the two canopy radiative
processes and to calculate the canopy directional-hemispherical emissivity. Although
being widely used, the original combined PROSPECT and SAIL model, also referred
to as PROSAIL, was mainly applied to the visual and near-infrared domain of
0.4um-2.5um. In our study, the PROSAIL model was expanded to thermal infrared
range with the construction of the essential leaf absorption spectra and the leaf bulk
refractive index over the range of 0.4um-15um. Various sensitivity tests and analyses
were performed with the expanded CRTM-PORSAIL model. The model physical
structures and the performance with respect to various canopy parameters are
discussed in our presentation. In particular, the Kramers-Kronig-constrained
variational function analysis (Kuzmenko, 2005) is introduced for the estimation of
the leaf bulk refractive index (wavelength dependent) from leaf absorption spectra. A
group of leaf \"trait\" parameters are used to characterize different canopy types. And
the biophysical relationship between the original PROSAIL input parameters and the
\"trait\" parameters is formed. Although our tentative leaf absorption spectra data
were collected from literatures, it will be shown that the expanded CRTM-PORSAIL
model could be used to successfully simulate the emissivity of the basic forest and
grass types of the JPL emissivity library. In real applications, the expanded
CRTM-PORSAIL model can be used for global emissivity mapping when combined
with proper soil emissivity model or soil data library, and can be used for land
surface property retrieval when coupled with CRTM.