Abstract
The operational Moderate Resolution Imaging Spectroradiometer (MODIS) Leaf Area Index (LAI) and Fraction of Photosynthetically Active Radiation absorbed by vegetation (FPAR) algorithm has been successfully implemented for Visible Infrared Imager Radiometer Suite (VIIRS) observations by optimizing a small set of configurable parameters in Look-Up-Tables (LUTs). Our preliminary evaluation showed reasonable agreement between VIIRS and MODIS LAI/FPAR retrievals. However, there is a need for a more comprehensive investigation to assure continuity of multi-sensor global LAI/FPAR time series, as the preliminary evaluation was spatiotemporally limited. In this study, we use a multi-year (2012–2016) global LAI/FPAR product generated from VIIRS and MODIS to evaluate for spatiotemporal consistency. We also quantify uncertainty of the product by utilizing available ground measurements. For both consistency and uncertainty evaluation, we account for variations in biome type and temporal resolution. Our results indicate that the LAI/FPAR retrievals from VIIRS and MODIS are consistent at different spatial (i.e., global and site) and temporal (i.e., 8-day, seasonal and annual) scales. The estimate of mean discrepancy (−0.006 ± 0.013 for LAI and −0.002 ± 0.002 for FPAR) meets the stability requirement for long-term LAI/FPAR Earth System Data Records (ESDRs) from multi-sensors as suggested by the Global Climate Observing System (GCOS). It is noteworthy that the rate of retrievals from the radiative transfer-based main algorithm is also comparable between two sensors. However, a relatively larger discrepancy over tropical forests was observed due to reflectance saturation and an unexpected interannual variation of main algorithm success was noticed due to instability in input surface reflectances. The uncertainties/relative uncertainties of VIIRS and MODIS LAI (FPAR) products assessed through comparisons to ground measurements are estimated to be 0.60/42.2% (0.10/24.4%) and 0.55/39.3% (0.11/26%), respectively. Note that the validated LAI were only distributed in low domains (~2.5), resulting in large relative uncertainty. Therefore, more ground measurements are needed to achieve a more comprehensive evaluation result of product uncertainty. The results presented here generally imbue confidence in the consistency between VIIRS and MODIS LAI/FPAR products and the feasibility of generating long-term multi-sensor LAI/FPAR ESDRs time series.
Highlights
Leaf Area Index (LAI) and Fraction of Photosynthetically Active Radiation absorbed by vegetation (FPAR) are well-known as two essential variables to describe the exchange of fluxes of energy, mass and momentum between the surface and atmosphere [1].LAI, which is generally defined as one-sided green leaf area per unit ground area in broadleaf canopies and as the projected needle leaf area in coniferous canopies [2], is extensively used to characterize the structure and function of vegetation [3]
The evaluation of continuity between Visible Infrared Imager Radiometer Suite (VIIRS) and Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals is prioritized in this study to build a long time series of LAI/FPAR Earth System Data Records (ESDRs) that is independent of sensor observation
It is critical that LAI/FPAR products are generated with high accuracy and precision, but more importantly, they must be produced with consistent algorithms across different sensor platforms in order to maintain a continuous and well-characterized data record
Summary
Leaf Area Index (LAI) and Fraction of Photosynthetically Active Radiation absorbed by vegetation (FPAR) are well-known as two essential variables to describe the exchange of fluxes of energy, mass (e.g., water, nutrients, and carbon dioxide) and momentum between the surface and atmosphere [1]. Spectroradiometer (MODIS) sensors onboard Terra and Aqua satellites have provided an opportunity for opening a new horizon of global LAI/FPAR products [2]. Land Processes Distributed Active Archive Center (LP DAAC), and is widely used by the scientific, public, and private user communities [5,6,7] Both Terra and Aqua MODIS have far exceeded their design life, and they will be likely terminated in the early 2020s [8]. Products; (2) to quantify the uncertainty of the VIIRS and MODIS products using available ground measurements; and (3) to understand the observed inconsistency between LAI/FPAR retrievals from two sensors.
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