PhD Proposal: Mumin Abdulahi
TITLE: Diurnal and Seasonal Variation of Vegetation Reflectance and Indices: Physically-based Modeling Analysis and Implication for Multisource Application
ABSTRACT: The top of canopy reflectance factor and corresponding vegetation indices (VIs) show a diurnal and seasonal variation, whose shape depends on the canopy properties, leaf optical characteristics and external factors which vary diurnally, such as atmospheric condition, illumination and observation geometry, and environmental condition (temperature, vapor pressure deficit) as observed in our study sites. This diurnal variability is problematic as it prevents the direct comparison of satellite data from different sources, hinders the transferability of vegetation indices (VIs) across different sites and different solar zenith angle for vegetation biophysical and biochemical properties retrieval. The diurnal variation in VIs can overshadow the direct influence of the plant physiological condition on TOC bidirectional reflectance factor (BRF) and VIs. The goals of the proposed research are to: (1) modify a publicly available one dimensional canopy radiative transfer code to account for horizontal heterogeneity in radiation regime in the vegetation canopy, so as to be able to simulate bidirectional distribution function, from which other remote sensing products can be derived; (2) apply coupled physically based radiative transfer modelling approach to simulate and explain the observed diurnal and seasonal variability in the context of observed hyperspectral reflectance, satellite data, biophysical and biochemical properties of vegetation measured across the study sites; (3) derive relationships between VIs (NDVI, PRI and NDWI), canopy structures, canopy optics and other external factors with explicit inclusion of multiple scattering contributions, and (4) analysis of satellite broadband reflectance data (multisensory data application, using NOAA and NASA observations such as the harmonized Landsat 8 and Sentinel 2, HLS) for sensitivity to the influence of the diurnal and seasonal variability. The proposed research will advance our understanding and ability to use interchangeably normalized difference VIs acquired at different time of day and solar-sensor geometries, from the multiple NASA, NOAA and International optical satellite missions, including Landsat, MODIS, VIIRS, GOES-16, DESIS/ISS, Sentenial-2/ESA, the forthcoming NASA Surface Biology and Geology (SBG) and NASA Plankton, Aerosol, Cloud, ocean Ecosystems (PACE) missions. It will contribute for developing approaches for using data at high temporal resolution for diagnostic understanding to achieve better interpretation of remote sensing data and improving sensitivity of vegetation indices and canopy biophysical parameters relations.
Proposal will be held using WebEx.