Data from the Advanced Scatterometer (ASCAT) on the Metop series of satellites can provide a new perspective on global vegetation dynamics, and specifically the role of vegetation in the carbon and water cycles. Here we will illustrate the benefits of ASCAT data for monitoring vegetation dynamics and constraining parameters in the terrestrial carbon balance. The theory behind the ASCAT slope will be introduced to demonstrate its direct value as a microwave observable. Results will be presented to illustrate how ASCAT slope dynamics vary over a range of land cover types, revealing vegetation dynamics across temporal scales. Radar backscatter data are sensitive to the dielectric properties of vegetation, as well as the number, distribution and geometry of scatterers in the vegetation layer. The relationship between backscatter and incidence angle is determined by the degree to which total backscatter is due to surface scattering, volume scattering and multiple scattering. Temporal variations in the incidence angle dependence of backscatter therefore provide information on vegetation dynamics, specifically changes in above ground biomass, structure and moisture content. Over deciduous evergreen forests in Amazonia, slope dynamics follow changes in radiation, corresponding to the limiting factor of radiation on vegetation activity for this region. Over the more heterogeneous vegetation of the Cerrado, slope dynamics vary per land cover type and can be explained by the variability in limiting factors to vegetation activity. Over croplands, slope follows seasonality in precipitation and Equivalent Water Thickness from GRACE. For shrubs and forests, slope has its peak during the dry season simultaneously with a peak in radiation. The deeper rooting depth enables them to increase photosynthesis and leaf development slightly before or at the onset of increasing radiation. For the 2010 and 2015 Amazonia droughts, no clear impact of droughts were found over deciduous evergreen forests. In the Cerrado, the response in drought varied between 2010 and 2015 due to the timing of the drought. The 2015 drought occurred during the wet season, and positive anomalies in slope were observed over natural vegetation, as increased radiation allowed for increased photosynthesis. In 2010 the drought occurred during the dry season. Similar to what was found by Liu et al. (2018) in passive microwave VOD, positive anomalies were observed before and during the beginning of the 2010 drought, whereas negative anomalies occurred at the end and after the drought. An analysis of the ASCAT slope, compared to model simulations and ground data from the Sodankyla ICOS site, has been conducted within the scope of the ESA Land Carbon Constellation project. Results from this boreal forest region in Northern Finland show that slope dynamics are influenced by freezing temperatures and snow, revealing the transition in seasonality as well as phenological changes during the summer months. During the 2018 drought, positive anomalies in slope were found. This is consistent with results found by Bastos et al., (2020), who demonstrated that increased temperature, drier than average conditions and increased radiation led to increased vegetation growth as modelled with several vegetation models and observed with SMOS L-VOD. To benefit terrestrial carbon cycle modelling and science, ASCAT slope can be assimilated directly into land surface models to constrain states and parameters related to the fast and slow water and carbon fluxes. Results from the ESA Land Carbon Constellation project will be presented to demonstrate that the measurement operator required for assimilation can be determined using several approaches, and is analogous to that which would be used for VOD. In addition, it will be shown that a machine learning approach can be highly effective in capturing indirect relationships between model states and the ASCAT slope, e.g. when vegetation water content is not directly modelled. The availability of ASCAT data since 2007, and the guaranteed availability of data from the SCA instrument on Metop-SG means that the ASCAT slope series will provide a uniquely continuous, consistent dataset spanning several decades. This is essential to study terrestrial carbon processes at scales relevant for climate science. Bastos, A., Ciais, P., Friedlingstein, P., Sitch, S., Pongratz, J., Fan, L., Wigneron, J.P., Weber, U., Reichstein, M., Fu, Z., Anthoni, P., Arneth, A., Haverd, V., Jain, A.K., Joetzjer, E., Knauer, J., Lienert, S., Loughran, T., McGuire, P.C., Tian, H., Viovy, N., Zaehle, S., n.d. Direct and seasonal legacy effects of the 2018 heat wave and drought on European ecosystem productivity. Science Advances 6, eaba2724. https://doi.org/10.1126/sciadv.aba2724 Liu, Y.Y., van Dijk, A.I.J.M., Miralles, D.G., McCabe, M.F., Evans, J.P., de Jeu, R.A.M., Gentine, P., Huete, A., Parinussa, R.M., Wang, L., Guan, K., Berry, J., Restrepo-Coupe, N., 2018. Enhanced canopy growth precedes senescence in 2005 and 2010 Amazonian droughts. Remote Sensing of Environment 211, 26–37. https://doi.org/10.1016/j.rse.2018.03.035
