Simultaneous inversion of multiple land surface parameters from MODIS optical–thermal observations
Introduction
Land surface parameters from remote sensing observations are critical for monitoring and modeling land surface processes and global climate change. Identified as essential climate variables, Leaf Area Index (LAI), which is widely used in agriculture and ecology, Fraction of Absorbed Photosynthetically Active Radiation (FAPAR), which expresses the energy absorption capacity of vegetation and plays a critical role in the carbon cycle, and surface albedo, which regulates energy exchange between the land surface and the atmosphere, are three essential climate variables recognized by the UN Global Climate Observing System (GCOS, 2006). Land surface emissivity, Land Surface Temperature (LST), and Upwelling Longwave radiation (LWUP) are also key parameters for studying the energy and water balance between the atmosphere and land surfaces (Z.-L. Li et al., 2013, Liang et al., 2010). Recently, many algorithms have been developed to retrieve these parameters from remote sensing datasets, including parametric or non-parametric regression methods, physically-based methods and hybrid methods (Verrelst et al., 2015), and the corresponding global land surface products are already routinely produced by various countries and organizations. Since 2000, the MODIS land team has produced global LAI and FAPAR, surface albedo, and LST products from Terra and Aqua satellite data (Justice et al., 2002). The CYCLOPES project has generated global albedo, LAI, vegetation cover fraction, and FAPAR products (1999–2007) from SPOT/VEGETATION data (Baret et al., 2007). The long-term Global LAnd Surface Satellite (GLASS) products are based on both AVHRR and MODIS data, spanning from 1981 to the present (Liang et al., 2013). In addition, many other typical land surface parameters have been produced from sensors, such as Advanced Very-High-Resolution Radiometer (AVHRR), Polarization and Directionality of the Earth's Reflectance (POLDER), Multi-angle Imaging Spectroradiometer (MISR), Medium-Resolution Imaging Spectrometer (MERIS), and so on (Liang et al., 2012). Although significant progress has been made in generating global land products from satellite observations, two main problems remain.
Firstly, due to cloud contamination and insufficient observations, current products are often incomplete and discontinuous in time and space, since they are generally produced by using only single-phase remote sensing data based on instantaneous physical models. Kobayashi et al. (2010) and Fang et al. (2012) reported that MODIS C5 LAI shows temporal gaps and unrealistically strong variability, especially for forest type during the growing season. Many researchers have pointed out that the MODIS albedo product contains data gaps (20%–40% for the MCD43B3 product) and fails to capture the events of ephemeral snow because of weather or abrupt surface changes (Fang et al., 2007, T. He et al., 2012, Qu et al., 2015). Data assimilation, which combines observations and models, provides a solution by utilizing the multi-temporal signatures of satellite data (Lewis et al., 2012b, Liang and Qin, 2008). Xiao et al. (2011) developed a sequential assimilation method for real-time estimation of LAI from MODIS time series reflectance data, and Liu et al. (2014) extended this to multiple satellite data. The LAI values were updated recursively by combining predictions from dynamic models and MODIS reflectance data using an Ensemble Kalman Filter (EnKF) technique. Lewis et al., 2012a, Lewis et al., 2012b developed a weak constraint variational Earth Observation Land Data Assimilation System (EO-LDAS) to improve mapping of land surface biophysical parameters by exploiting the full information content provided by observations from satellite constellations. By involving the measured LAI in the assimilation procedure, Li et al. (2017) produced high-accuracy LAI of three subtropical forest in China from MODIS data based on the integrated EnKF and PROSAIL model, which is significant for the study of subtropical forest ecosystems. Data assimilation methods were also adapted to retrieve LAI/FAPAR or surface fluxes (Jiang et al., 2014, Qin et al., 2007, Stöckli et al., 2008).
Secondly, current methods for estimating land surface variables usually focus on individual parameters separately, even from the same satellite observations, resulting in inconsistent products. For example, the MODIS land surface products are produced separately by different teams; the main algorithm for MODIS LAI/FAPAR products is based on the three-dimensional (3D) radiative transfer model (Knyazikhin et al., 1998), while the main algorithm for the MODIS BRDF/albedo product is based on the semi-empirical kernel-based model (Schaaf et al., 2002). Garrigues et al. (2008) found that MODIS C4 LAI performs well for grass and crop types, while Fang et al. (2012) and De Kauwe et al. (2011) both found that MODIS C5 LAI underestimates the upper range of in situ LAI measurements for forest type. However, by comparing MODIS albedo retrievals and in situ measurements across the global FLUXNET network, Cescatti et al. (2012) concluded that MODIS albedo is systematically lower than in situ measurements for non-forest sites (grasslands, savannas, croplands), whereas the seasonal pattern of MODIS albedo matches extremely well for forest sites. To address this situation, Xiao et al. (2015b) developed a framework for consistent estimation of LAI, FAPAR, and albedo from MODIS time-series data, and demonstrated that the framework can estimate temporally complete land-surface parameters even if some of the reflectance data are contaminated by residual clouds or missing, and that the retrieved parameter values are physically consistent. Shi et al. (2016) extended this work by consistent estimation of land surface (LAI, FAPAR, albedo, PAR) and atmospheric (aerosol optical depth) parameters from MODIS TOA reflectance data. However, none of these studies explored thermal observations, which are suitable to estimate land surface parameters like LST and land surface longwave radiation. In addition, remote sensing observations in the optical- to Thermal InfraRed (TIR) spectrum are usually used separated in different physical or empirical models. Wan and Li (1997) proposed the MODIS day/night algorithm to retrieve LSE and LST from paired day/night observations of seven MODIS MIR and TIR bands. Other studies used only the reflected component of MIR. Libonati et al. (2010) retrieved MODIS MIR reflectance for burned area mapping in tropical environments. MIR observations remain under-utilized for environmental studies due to the complexity of the radiometric signal, which includes both reflected and emitted fluxes (Boyd and Petitcolin, 2004).
This study presents an approach for simultaneous estimation of six land surface parameters (LAI, FAPAR, surface albedo, LSE, LST, and LWUP), by exploiting visible-to-TIR observations from MODIS data, based on a unified radiative transfer model and a data assimilation framework. Section 2 describes the construction of the unified radiative transfer model and the methods used to retrieve the land surface parameters. Section 3 presents the data used in this study, including the field sites and remote sensing products, Section 4 gives the performance of the proposed algorithm, and Section 5 provides discussions and conclusions.
Section snippets
Methods
The flowchart in Fig. 1 illustrates the proposed unified inversion algorithm. At first, VISible/ShortWave InfraRed (VIS/SWIR) cloud-free reflectances from MODIS daily reflectance products were extracted using cloud mask information contained in the MODIS cloud product. An EnKF technique was used to estimate LAI from the unified optical–thermal radiative transfer model by combining predictions from the dynamic model with MODIS cloud-free daily reflectance data. The retrieved free parameters were
Study sites and field measurements
To evaluate the accuracy and consistency of the six parameters retrieved by the unified inversion algorithm, six representative sites from the AmeriFlux network (WWW1), consisting of Bondville, Brooks Field Site 11- Ames, Fort peck, Santa Rita Mesquite, Walker Branch Watershed, and Chestnut Ridge, were selected based on the principles of the concurrent land surface parameter observations during the survey period, typical land-surface biomes, and the landscape homogeneity for representativeness
Results analysis
A validation exercise was conducted for the selected sites with sufficient field measurements. The presently distributed MODIS, GLASS, and GlobAlbedo products were also used for comparison with the proposed method. Detailed validations of the six land surface parameters are discussed in the following sections.
The Bondville site is an agricultural site in the Midwestern United States, near Champaign, Illinois. The field was continuous no-till with alternating years of soybean and corn crops (
Discussion and conclusion
A new, unified inversion algorithm has been proposed for simultaneous estimation of six land surface parameters, by exploiting the visible-to-TIR spectral observations from MODIS sensor, based on a unified optical–thermal soil–canopy–leaf radiative transfer model and a data assimilation framework. LAI was first estimated using a data assimilation method that combines MODIS daily reflectance data and a LAI phenology model. The estimated LAI values were then input to the unified model to produce
WWW sites
WWW1: AmeriFlux Site and Data Exploration System. http://ameriflux.ornl.gov/
WWW2: ASTER spectral library. http://spclib.jpl.nasa.gov/
WWW3: USDA soil taxonomy. http://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/survey/class/
WWW4: MODIS UCSB spectral library. http://www.icess.ucsb.edu/modis/EMIS/html/em.html
Acknowledgements
This research was financially supported by the National Natural Science Foundation of China under grant nos. 41331173 and 41171264, the Chinese Grand Research Program on Climate Change and Response under grant 2016YFA0600103. The authors thank the AmeriFlux network for providing ground validation data. We are grateful for two anonymous reviewers for their constructive comments and suggestions.
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