foursail2b: R implementation of the foursail2 model with 2 canopy layers.

In MarcoDVisser/ccrtm: Coupled Chain Radiative Transfer Models

R implementation of the foursail2 model with 2 canopy layers.

Description

The foursail2b model is a two layer implementation of the foursail model described in Zhang et al (2005). Layers are assumed identical in hotspot, but may differ in the relative density, inclination and types of particles. In comparison to foursail, the background (soil), can now be non-Lambertain, having it own 4-stream BDRF (not implemented here but may be input by the user). There are two types of particles, generalized to primary and secondary (originally termed "green" and "brown" particles). The realtive abundance of the secondary particle in the top canopy is regulated by the dissociation paramerter. The model 4SAIL2 combines with prospect, libery or procosine for the reflectance and transmittance of the particles, and with the the foursail or Hapke elements for the background reflectance. If run alone, these require direct inputs which could be measured leaf reflectance.

Usage

foursail2b(
  rhoA,
  tauA,
  rhoB = NULL,
  tauB = NULL,
  bgr,
  rsobgr = NULL,
  rdobgr = NULL,
  rsdbgr = NULL,
  rddbgr = NULL,
  param
)

Arguments

rhoA	primary particle reflectance from 400-2500nm (can be measured or modeled)
tauA	primary particle transmittance from 400-2500nm (can be measured or modeled)
rhoB	secondary particle reflectance from 400-2500nm (can be measured or modeled)
tauB	secondary particle reflectance from 400-2500nm (can be measured or modeled)
bgr	background reflectance. Usual input is soil reflectance spectra from 400-2500nm (can be measured or modeled)
rsobgr	: background bidirectional reflectance (rso)
rdobgr	: background directional hemispherical reflectance (rdo)
rsdbgr	: background hemispherical directional reflectance (rsd)
rddbgr	: background bi-hemispherical diffuse reflectance (rdd)
param	A named vector of 4SAIL2 parameter values (note: program ignores case): 1 = Mean leaf angle for first (top) layer (LIDFa) 2 = Mean leaf angle for second (bottom) layer (LIDFb) 3 = Leaf angle distribution function type (ignored, only value 2 allow) 4 = Total Leaf Area Index (LAI), including primary and secondary particles (brown and green leafs). 5 = fraction secondary particles ("brown leaf fraction", fb) 6 = Canopy dissociation factor for secondary particles ("diss") 7 = Hot spot effect parameter (hspot). Often defined as the ratio of mean leaf width and canopy height. 7 = vertical crown coverage fraction (Cv), models clumping in combination with parameter zeta. 7 = tree shape factor (zeta), defined as the ratio of crown diameter and height. 6 = Solar zenith angle (tts) 7 = Observer zenith angle (tto) 8 = Sun-sensor azimuth angle (psi)

Details

Leaf inclination angles: leaf angles in 4SAIL2b are set for each layer and only the Cambell leaf angle distribution model is allowed. This means that each layer has a single parameter that defines leaf angles.

Value

spectra matrixwith 4 reflectance factors and canopy transmission for wavelengths 400 to 2500nm:

• 1 = bi-hemispherical reflectance (rddt). White-sky albedo: the reflectance of the canopy under diffuse illumination. The BRDF integrated over all viewing and illumination directions. Diffuse reflectance for diffuse incidence.

• 2 = hemispherical directional reflectance (rsdt). Black-sky albedpo: reflectance of a surface under direct light without a diffuse component. It is the integral of the BRDF over all viewing directions. Diffuse reflectance for direct solar incidence.

• 3 = directional hemispherical reflectance (rdot). Diffuse reflectance in the vieweing direction.

• 4 = bi-directional reflectance (rsot). The ratio of reflected radiance in the viewing direction to the incoming radiant flux in the solar direction.

References

Zhang, Q., Xiao, X., Braswell, B., Linder, E., Baret, F., Moore, B. (2005). Estimating light absorption by chlorophyll, leaf and canopy in a deciduous broadleaf forest using MODIS data and a radiative transfer model. Remote Sens. Environ. 99, 357-371.

Examples

## see ?foursail for lower-level implementations
fRTM(rho~prospectd+foursail2b)

MarcoDVisser/ccrtm documentation built on Feb. 19, 2025, 1:15 p.m.