Abstract
This study presents the first systematic comparison of MAIAC Collection 6 MCD19A1 daily surface reflectance (SR) product with standard MODIS SR (MOD/MYD09). The study was limited to four tiles located in mid-Atlantic United States (H11V05), Canada (H12V03), central Amazon (H11V09), and North-Eastern China (H27V05) and used over 5000 MODIS granules in 2018. Overall, there is a remarkable agreement between the best quality pixels of the two products, in particular in the Red and NIR bands. Over selected tiles, the evaluation found that MAIAC provides from 4 to 25% more high-quality retrievals than MOD09 annually, with the largest difference in tropical regions, confirming results of the previous studies. The comparison of spectral characteristics showed a systematic MAIAC-MOD09 difference increasing from NIR to Blue, typical of biases of a Lambertian assumption in MOD09 algorithm. Over the North-Eastern China, MCD19A1 SR is found more stable at wide range of aerosol optical depth (AOD) variations, whereas MOD09 SR shows a consistent positive bias increasing with AOD and at shorter wavelengths. The observed SR differences can be attributed to differences in cloud detection, aerosol retrieval and in atmospheric correction which is performed using an accurate BRDF-coupled radiative transfer model in MAIAC and a Lambertian surface model in MOD09. While this study is not representative of the global performance because of its limited geographical coverage, it should help the land community to better understand the differences between the two products.
Highlights
Compared with MOD09 QA structure using a total of 10 flags, Multi-Angle Implementation of Atmospheric Correction (MAIAC) MCD19A1 offers a very simple QA with only three values: “aerosol optical depth (AOD) level”, “Adjacency Mask” and “Algorithm Initialize Status” (Table 1)
It is worth mentioning that MAIAC reports surface reflectance only for the cloud-free pixels (QA flag Cloud Mask 001) whereas MOD09 reports SR for all pixels including clouds
This study is the first systematic comparison of MCD19A1 SR product with standard Moderate Resolution Imaging Spectroradiometer (MODIS) MOD09 SR over four MODIS tiles located in three different continents
Summary
Since the launch of Terra in 1999 and Aqua in 2002, the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard the Terra and Aqua satellites has greatly enhanced our understanding of the Earth’s climate and the effects of human activity and natural disasters on ecosystems through a suite of MODIS-based land products, including surface reflectance (SR), vegetation indices (VI), biophysical parameters [e.g., leaf area index (LAI) and fraction of absorbed photosynthetically active radiation (FPAR)], bidirectional reflectance distribution function (BRDF) and albedo and others (e.g., Huete et al, 2002; Myneni et al, 2002; Schaaf et al, 2002; Vermote and Kotchenova, 2008). In the current MODIS Collection 6 (C6), two independent SR products are available, MOD09 (Vermote and Kotchenova, 2008) and the Multi-Angle Implementation of Atmospheric Correction (MAIAC) MCD19A1 (Lyapustin et al, 2018) Both are level 2 products derived from MODIS L1B calibrated top of atmosphere (TOA) reflectance though with different cloud/cloud shadow detection, aerosol retrieval and atmospheric correction strategies. MAIAC avoids empirical assumptions: it works with gridded top of atmosphere (TOA) MODIS measurements and uses the time series approach for detailed surface characterization. Accumulation of such information, including spectral BRDF, spatial and thermal contrasts etc. The operational Collection 6 MODIS processing by the MODAPS (MODIS Adaptive Processing System) used a buffer 1-year period to initialize MAIAC, and process the entire MODIS Terra and Aqua record starting in year 2000
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