Simulating global dynamic surface reflectances for imaging spectroscopy
spaceborne missions - LPJ-PROSAIL
Abstract
Imaging spectroscopy is a remote-sensing technique that retrieves
reflectances across visible to shortwave infrared wavelengths at high
spectral resolution (<10 nm). Spectroscopic reflectance data
provide novel information on the properties of the Earth’s terrestrial
and aquatic surfaces. Until recently, imaging spectroscopy missions were
limited spatially and temporally using airborne instruments, such as the
Next Generation Airborne Visible InfraRed Imaging Spectrometer
(AVIRIS-NG), providing the main source of observations. Here, we present
a land-surface modeling framework to help support end-to-end
traceability of emerging imaging spectroscopy spaceborne missions. The
LPJ-wsl dynamic global vegetation model is coupled with the canopy
radiative transfer model, PROSAIL, to generate global, gridded, daily
visible to shortwave infrared (VSWIR) spectra. LPJ-wsl variables are
cross-walked to meet required PROSAIL parameters, which include leaf
structure, Chlorophyll a+b, brown pigment, equivalent water thickness,
and dry matter content. Simulated spectra are compared to a boreal
forest site, a temperate forest, managed grassland, and a tropical
forest site using reflectance data from canopy imagers mounted on towers
and from air and spaceborne platforms. We find that canopy nitrogen and
leaf-area index are the most uncertain variables in translating LPJ-wsl
to PROSAIL parameters but at first order, LPJ-PROSAIL successfully
simulates surface reflectance dynamics. Future work will optimize
functional relationships required for improving PROSAIL parameters and
include the development of the LPJ-model to represent improvements in
leaf water content and canopy nitrogen. The LPJ-PROSAIL model can
support missions such as NASA’s Surface Biology and Geology (SBG)
and higher-level modeled products.