| Abstract | Photosynthetically Active Radiation (PAR) absorbed by Canopy (APAR) is essential to the vegetation photosynthesis, net primary productivity and the carbon cycle. Due to the paucity of ground-based
measurements, global monitoring of APAR can only be achieved by means of space-borne remote sensing. So far, APAR has been estimated from the retrievals of the Derivation of downwelling PAR from satellite data entails the determination of cloud
properties which is generally a difficult and error prone task. We proposed a new approach to retrieve APAR that is defined as the product of the PAR absorbed by the surface layer below the top of canopy and the fraction of PAR absorbed by green
foliage only. Since clouds do not absorb solar radiation in the PAR spectral region, it is much easier and more accurate to estimate the PAR absorbed by the surface layer that the incoming PAR at the surface. Comprehensive atmospheric radiative
transfer modelling was conducted to establish the relationship between surface absorbed PAR and the visible albedo at the top of the atmosphere that is measured by satellite. A canopy radiative transfer model was developed to relate the fraction of
the PAR absorbed by be green canopy to the vegetation indices. Some parameterization schemes were obtained on the basis of canopy radiative transfer simulations. In addition, development of a method to retrieve surface PAR albedo is under way using
the clear-sky composite data from visible channels. With the knowledge of the surface absorbed PAR and the PAR albedo, downwelling PAR can be derived which is then combined with APAR to derive FPAR. Therefore, the methodology we are developing is
able to retrieve all the components of PAR. These inversion algorithms will be tested using ground-truth observations collected during the BOREAS field campaigns. After validations, we will apply the algorithms to various satellite datasets including
AVHRR, LANDSAT and SPOT for mapping the distributions of APAR and its components over Canada and the world. |