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Description
A recent study demonstrates that closure phases ($\Phi_{123}$), constructed by a circular summation of three interferometric phases, have a geophysical component [1]. Constructing closure phases with consecutive 6-day Sentinel-1 acquisitions that are interferometrically multilooked, we observed consistent positive annual mean closure phases in South Spain agricultural areas. Taking corn fields area as an example, Figure 1 shows area-averaged time series of radar observables with precipitation data and Sentinel-3 Leaf Area Index (LAI) products. The observed closure phases exhibit consistently positive magnitudes during vegetation development, and the evolution of such positive closure phases appear to be related to the phenological stages of plants.
In existing literature, De Zan et al. developed two analytical expressions to model closure phases. One is a dielectric model which considers soil moisture variation induced closure phase, another model accounts for volume scattering in combination with perpendicular baselines[1,2,3]. Modeled closure phases with these known mechanisms show fluctuations in sign and no correlation with the evolution of vegetation development. Therefore, the observed consistently positive closure phases suggest the existence of additional mechanisms.
We try to interpret observed closure phases by proposing two conceptual models that consider vegetation development. Firstly, taking the dielectric variation within plant canopy into consideration and describing the canopy with the dielectric constant of an equivalent medium, we developed a conceptual dielectric closure phase model as an analogy to the preliminary soil moisture model. Secondly, since neither of known mechanisms accounts for the vertical motion of plants, we proposed a skewed motion model considering the line-of-sight (LoS) motion of the scatterers within the vegetation canopy with a skewed velocity distribution. Both of our conceptual models could explain the magnitudes and the consistently positive sign of the observed closure phases.
Our research shows plant growth related positive closure phases with real data examples. Both our conceptual models manage to produce plausible positive closure phases. This study provides further insight into the origins of geophysical closure phases. Thus, it sheds light on a new opportunity to monitor crop-growth with radar satellites.
