Simulating global dynamic surface reflectances for imaging spectroscopy spaceborne missions - LPJ-PROSAIL

Imaging spectroscopy at high spectral resolution (<10 nm) across the visible to shortwave infrared spectrum can provide novel information on the properties of the Earth's terrestrial and aquatic surfaces. These spectroscopic reflectances can be integrated with numerous modeling approaches to infer various chemical and physical constituents of vegetation that, when scaled up to the canopy level, can greatly improve our understanding of terrestrial carbon, water, and energy cycling. Historically, imaging spectroscopy missions were dependent on airborne instrumentation, which limits observation in the spatial and temporal domains, yet soon there will be numerous spaceborne spectroscopy missions. To provide end-to-end support for the traceability of these spaceborne missions, we have coupled the LPJ-wsl dynamic global vegetation model with the canopy radiative transfer model, PRO4SAIL, to generate globally gridded spectra across the visible to shortwave infrared (VSWIR) range (400-2500 nm) at a daily time-step. LPJ-wsl variables are modified to estimate required PROSAIL parameters, which include leaf structure, Chlorophyll a+b, brown pigment, equivalent water thickness, and dry matter content. Using reflectance data from canopy imagers mounted on towers and air and spaceborne platforms, we compare simulated spectra with a boreal forest site, a temperate forest, managed grassland, and a tropical forest site. We also compare simulated normalized differenced vegetative index (NDVI) with the commonly used NASA MODIS NDVI product. 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 the continued development of LPJ-PROSAIL to represent improvements in leaf area index, 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.