A consistent record of vegetation optical depth retrieved from the AMSR-E and AMSR2 X-band observations

Mengjia Wang (1, 2, 3) , Jean-Pierre Wigneron (1) , Rui Sun (2, 3) , Lei Fan (4) , Frédéric Frappart (1, 5) , Shengli Tao (6) , Linna Chai (7) , Xiaojun Li (1) , Xiangzhuo Liu (1) , Hongliang Ma (1) , Christophe Moisy (1) , Philippe Ciais (8)

1 UMR ISPA - Interactions Sol Plante Atmosphère
2 State Key Laboratory of Remote Sensing Science [Beijing]
3 BNU - Beijing Normal University
4 Southwest University [Chongqing]
5 LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales
6 EDB - Evolution et Diversité Biologique
7 ESPRE - State Key Laboratory of Earth Surface Processes and Resource Ecology
8 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]

Vegetation optical depth (VOD) retrieved from microwave remote sensing techniques has been used as an important proxy for monitoring the vegetation dynamics at large scales. In a first study we developed a new X-VOD indice from inversion of the X-MEB model from the Microwave Scanning Radiometer 2 (AMSR2, 2012-present) observations. The Advanced Microwave Scanning Radiometer for EOS (AMSR-E, 2002–2011) and AMSR2 share many characteristics, such as local crossing time, incidence angle, frequency at X-band, etc, which makes it possible to produce a consistent long-term X-VOD product over a long period spanning from 2002 to present. The objective of this study was to merge the AMSR-E and AMSR2 observations in order, in a second step, to produce a consistent long-term X-VOD product spanning over the combined periods of AMSR-E and AMSR2. The main challenges in retrieving a consistent X-VOD data set from AMSR-E and AMSR2 are that (i) there is a bias between observations from the two sensors; and (ii) the lack of overlapping observations between the two sensors makes it impossible to achieve a direct inter-calibration. Here, to overcome this problem, we used AMSR-E slow rotation data (AMSR-E L1S), which has similar characteristics as AMSR-E and provided observations concurrently with AMSR2, as a bridge to calibrate the AMSR-E brightness temperature (TB) observations with AMSR2 TB. As our main objective in this TB calibration study was to produce a consistent long-term AMSR-E/AMSR2 X-VOD product, we evaluated the retrieved X-VOD product based on different inter-calibration, either global-based or pixel-based, methods. Both AMSR-E and AMSR2 X-VOD were evaluated against the Aboveground Biomass (AGB), Leaf Area Index (LAI) and the Normalized Difference Vegetation Index (NDVI). The results suggest that global-based inter-calibration methods using homogeneous and temporally stable land covers (Evergreen Broadleaf Forests and Snow and Ice) as reference calibration data sets provided the best results. For instance, the spatial relationships between X-VOD and AGB/LAI/NDVI are highly consistent over the AMSR-E and AMSR2 periods after the calibration work. This study laid a solid foundation for monitoring the dynamics of X-VOD, as a proxy of AGB, over the combined periods of AMSR-E and AMSR2 sensors (almost 20 years).

Mots clés: Vegetation optical depth; IB X-VOD; AGB; AMSR-E; AMSR2

Domaines: Sciences du Vivant [q-bio]

Mengjia Wang, Jean-Pierre Wigneron, Rui Sun, Lei Fan, Frédéric Frappart, et al.. A consistent record of vegetation optical depth retrieved from the AMSR-E and AMSR2 X-band observations. International Journal of Applied Earth Observation and Geoinformation, 2021, 105, pp.1-16. ⟨10.1016/j.jag.2021.102609⟩. ⟨hal-03438966⟩