The SMAP level 4 carbon product for monitoring ecosystem land-atmosphere CO2 exchange

Authors: JONES, L.A. - University Of Montana; KIMBALL, J.S. - University Of Montana; REICHE, R.H. - National Aeronautics And Space Administration (NASA) - Johnson Space Center; MADANI, N. - University Of Montana; GLASSY, J. - University Of Montana; ARDIZZONE, J. - National Aeronautics And Space Administration (NASA) - Johnson Space Center; COLLIANDER, A. - Jet Propulsion Laboratory; CLEVERLY, J. - University Of Technology Sydney; EAMUS, D. - University Of Technology Sydney; EUSKIRCHEN, E. - University Of Alaska; HUTLEY, L. - Charles Darwin Research Station; MACFARLANCE, C. - Commonwealth Scientific And Industrial Research Organisation (CSIRO); Scott, Russell - Russ

Submitted to: IEEE Transactions on Geoscience and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2017
Publication Date: 8/1/2017
Citation: Jones, L., Kimball, J., Reiche, R., Madani, N., Glassy, J., Ardizzone, J., Colliander, A., Cleverly, J., Eamus, D., Euskirchen, E., Hutley, L., Macfarlance, C., Scott, R.L. 2017. The SMAP level 4 carbon product for monitoring ecosystem land-atmosphere CO2 exchange. IEEE Transactions on Geoscience and Remote Sensing. 55:6517-6532. https://doi.org/10.1109/TGRS.2017.2729343.
DOI: https://doi.org/10.1109/TGRS.2017.2729343

Interpretive Summary: Soil moisture availability plays a major role in explaining the amount of carbon dioxide that is taken up by the global land surface. However, quantifying global soil moisture and its interactions with the carbon cycle have been obscured by a lack of continuous, accurate soil moisture observations from space. Understanding linkages between the global water and carbon cycles using global estimates of soil moisture is a major objective of the NASA Soil Moisture Active Passive (SMAP) mission. The overarching goals of this work are: 1) to link SMAP soil moisture data to inform ecosystem CO2 exchange and underlying environmental controls on vegetation growth, soil litter decomposition and respiration processes using flux tower observations; 2) to determine net CO2 exchange and component carbon flux sensitivity to global soil moisture variability; and 3) to determine whether SMAP observations provide added value over other sources of information for estimating carbon cycle flux components. The SMAP carbon cycle components provide consistent, operational global daily estimates of ecosystem-atmosphere carbon fluxes, surface stocks and their underlying environmental controls. Our initial global assessment using several independent observation benchmarks indicates that the model accuracy and performance is consistent with product design specifications and target accuracy requirements, and that the product is suitable for a range of science investigations, including drought-related impacts on vegetation growth and the terrestrial carbon cycle. The SMAP product provides a new tool for monitoring global land carbon dynamics informed by model data assimilation of SMAP satellite observations with enhanced sensor sensitivity to soil moisture and temperature conditions.

Technical Abstract: The NASA Soil Moisture Active Passive (SMAP) mission Level 4 Carbon (L4C) product provides model estimates of Net Ecosystem CO2 exchange (NEE) incorporating SMAP soil moisture information. The L4C product includes NEE, computed as total ecosystem respiration less gross photosynthesis, at a daily time step posted to a 9-km global grid by plant functional type. Component carbon fluxes, surface soil organic carbon stocks, underlying environmental constraints, and detailed uncertainty metrics are also included. The L4C model is driven by the SMAP Level 4 Soil Moisture (L4SM) data assimilation product, with additional inputs from the Goddard Earth Observing System, Version 5 weather analysis and Moderate Resolution Imaging Spectroradiometer satellite vegetation data. The L4C data record extends from 31 March 2015 to present with ongoing production and 8-12 day latency. Comparisons against concurrent global CO2 eddy flux tower measurements, satellite Solar Induced Canopy Florescence, and other independent observation benchmarks show favorable L4C performance and accuracy, capturing the dynamic biosphere response to recent weather anomalies. Model experiments and L4C spatiotemporal variability were analyzed to understand the independent value of soil moisture and SMAP observations relative to other sources of input information. This analysis highlights the potential for microwave observations to inform models where soil moisture strongly controls land CO2 flux variability; however, skill improvement relative to flux towers is not yet discernable within the relatively short validation period. These results indicate that SMAP provides unique and promising capability for monitoring the linked global terrestrial water and carbon cycles.