Hao, D., Asrar, G. R., Zeng, Y., Zhu, Q., Wen, J., Xiao, Q., and Chen, M.: DSCOVR/EPIC-derived global hourly and daily downward shortwave and photosynthetically active radiation data at 0.1° × 0.1° resolution, Earth Syst. Sci. Data, 12, 2209–2221, https://doi.org/10.5194/essd-12-2209-2020, 2020.
Hao, D.; Asrar, G. R.; Zeng, Y.; Zhu, Q.; Wen, J.; Xiao, Q.; Chen, M. Estimating Hourly Land Surface Downward Shortwave and Photosynthetically Active Radiation from DSCOVR/EPIC Observations. Remote Sens. Environ. 2019, 232, 111320. https://doi.org/10.1016/j.rse.2019.111320.
DSCOVR/EPIC-derived global hourly and daily downward shortwave and photosynthetically active radiation data at 0.1° × 0.1° resolution
Downward shortwave radiation (SW) and photosynthetically active radiation (PAR) play crucial roles in Earth system dynamics. Spaceborne remote sensing techniques provide a unique means for mapping accurate spatiotemporally continuous SW–PAR, globally. However, any individual polar-orbiting or geostationary satellite cannot satisfy the desired high temporal resolution (sub-daily) and global coverage simultaneously, while integrating and fusing multisource data from complementary satellites/sensors is challenging because of co-registration, intercalibration, near real-time data delivery and the effects of discrepancies in orbital geometry. The Earth Polychromatic Imaging Camera (EPIC) on board the Deep Space Climate Observatory (DSCOVR), launched in February 2015, offers an unprecedented possibility to bridge the gap between high temporal resolution and global coverage and characterize the diurnal cycles of SW–PAR globally. In this study, we adopted a suite of well-validated data-driven machine-learning models to generate the first global land products of SW–PAR, from June 2015 to June 2019, based on DSCOVR/EPIC data. The derived products have high temporal resolution (hourly) and medium spatial resolution (0.1∘×0.1∘), and they include estimates of the direct and diffuse components of SW–PAR. We used independently widely distributed ground station data from the Baseline Surface Radiation Network (BSRN), the Surface Radiation Budget Network (SURFRAD), NOAA’s Global Monitoring Division and the U.S. Department of Energy’s Atmospheric System Research (ASR) program to evaluate the performance of our products, and we further analyzed and compared the spatiotemporal characteristics of the derived products with the benchmarking Clouds and the Earth’s Radiant Energy System Synoptic (CERES) data. We found both the hourly and daily products to be consistent with ground-based observations (e.g., hourly and daily total SWs have low biases of −3.96 and −0.71 W m−2 and root-mean-square errors (RMSEs) of 103.50 and 35.40 W m−2, respectively). The developed products capture the complex spatiotemporal patterns well and accurately track substantial diurnal, monthly, and seasonal variations in SW–PAR when compared to CERES data. They provide a reliable and valuable alternative for solar photovoltaic applications worldwide and can be used to improve our understanding of the diurnal and seasonal variabilities of the terrestrial water, carbon and energy fluxes at various spatial scales.
Cite: Chen, M., Vernon, C.R., Graham, N.T. et al. Global land use for 2015–2100 at 0.05° resolution under diverse socioeconomic and climate scenarios. Sci Data 7, 320 (2020). https://doi.org/10.1038/s41597-020-00669-x
Global land use for 2015–2100 at 0.05° resolution under diverse socioeconomic and climate scenarios
Global future land use (LU) is an important input for Earth system models for projecting Earth system dynamics and is critical for many modeling studies on future global change. Here we generated a new global gridded LU dataset using the Global Change Analysis Model (GCAM) and a land use spatial downscaling model, named Demeter, under the five Shared Socioeconomic Pathways (SSPs) and four Representative Concentration Pathways (RCPs) scenarios. Compared to existing similar datasets, the presented dataset has a higher spatial resolution (0.05° × 0.05°) and spreads under a more comprehensive set of SSP-RCP scenarios (in total 15 scenarios), and considers uncertainties from the forcing climates. We compared our dataset with the Land Use Harmonization version 2 (LUH2) dataset and found our results are in general spatially consistent with LUH2. The presented dataset will be useful for global Earth system modeling studies, especially for the analysis of the impacts of land use and land cover change and socioeconomics, as well as the characterizing the uncertainties associated with these impacts.
Cite: Hao, D.; Zeng, Y.; Qiu, H.; Biriukova, K.; Celesti, M.; Migliavacca, M.; Rossini, M.; Asrar, G. R.; Chen, M. Practical Approaches for Normalizing Directional Solar-Induced Fluorescence to a Standard Viewing Geometry. Remote Sens. Environ. 2021, 255, 112171. https://doi.org/10.1016/j.rse.2020.112171.
Practical SIF angular normalization methods
Recent advances in remote sensing of solar-induced chlorophyll fluorescence (SIF) have improved the capabilities of monitoring large-scale Gross Primary Productivity (GPP). However, SIF observations are subject to directional effects which can lead to considerable uncertainties in various applications. Practical approaches for normalizing directional SIF observations to nadir viewing, to minimize the directional effects, have not been well studied. Here we developed two practical and physically-solid approaches for removing the directional effects of anisotropic SIF observations: one is based on near-infrared or red reflectance of vegetation (NIRv and Redv), and the other is based on the kernel-driven model with multi-angular SIF measurements. The first approach uses surface reflectance while the second approach directly leverages multi-angular SIF measurements. The performance of the two approaches was evaluated using a dataset of multi-angular measurements of SIF and reflectance collected with a high-resolution field spectrometer over different plant canopies. Results show that the relative mean absolute errors between the normalized nadir SIF and the observed SIF at nadir decrease by 3–6% (far-red) and 6–8% (red) for the first approach, and by 7–13% and 6–11% for the second approach, compared to the original data, respectively. The effectiveness and simplicity of our proposed approaches provide great potential to generate long-term and consistent SIF data records with minimized directional effects.
Cite: Wu, S.; Zeng, Y.; Hao, D.; Liu, Q.; Li, J.; Chen, X.; Asrar, G. R.; Yin, G.; Wen, J.; Yang, B.; Zhu, P.; Chen, M. Quantifying Leaf Optical Properties with Spectral Invariants Theory. Remote Sens. Environ. 2021, 253, 112131. https://doi.org/10.1016/j.rse.2020.112131.
A simple leaf spectrum model based on spectral invariant theory
Leaf optical spectra reflect the combination of leaf biochemical, morphological and physiological properties, and play an important role in many ecological and Earth system processes. Radiative transfer models are widely used to simulate leaf spectra by quantifying photon transfer processes of reflection, transmission and absorption within a plant leaf. Recent advances in spectral invariants theory offer a unique and efficient approach for modeling the canopy-scale radiative transfer processes, but remain underexplored for applications at the leaf scale. In this study, we developed a leaf-scale optical property model based on the spectrally invariant properties (leaf-SIP) of plant leaves. Similar to the canopy-scale model, the leaf-SIP model decouples leaf-scale radiative transfer process into two parts: wavelength-dependent contribution from leaf chemical components and wavelength-independent contribution from leaf structures, described by two spectrally invariant parameters (i.e., a photon recollision probability p and a scattering asymmetry parameter q). We implemented the leaf-SIP model by parameterizing p and q with a measurable leaf morphological trait, the leaf mass per area (LMA). We evaluated the performance of the leaf-SIP model with two in situ datasets (i.e., LOPEX and ANGERS) and the widely used PROSPECT leaf optical model. The results show that the leaf spectra simulated by the leaf-SIP model agreed well with in situ datasets and the simulations of the PROSPECT model, with a small root mean squared error (RMSE), bias, and high coefficients of determination (R2). Our results also show that the leaf-SIP model can be used with measured leaf spectra to accurately estimate several key leaf functional traits, such as the leaf chlorophyll content, equivalent water thickness, and LMA. The leaf-SIP model provides an efficient and physical way of accurately simulating leaf spectra and retrieving key leaf functional traits from hyperspectral measurements.