| Abstract: |
Vegetation optical depth (tau) and effective scattering albedo (omega) are crucial parameters within the widely applied tau-omega model for passive microwave remote sensing. This tau-omega framework is utilized together with L-band data to estimate soil and canopy state variables, like soil moisture or canopy attenuation [1, 2]. In this process the effective scattering albedo is often a fixed input parameter and kept static along time, when time-series retrievals are applied.
In order to enable a single-pass physics-based retrieval of tau and omega, a multi-sensor data integration approach using lidar-based vegetation heights as well as SMAP radar and radiometer data is developed in this research study. Therefore, the definition of tau as the product of canopy extinction coefficient (κ_e) and vegetation height h is combined with the definition of omega, being the ratio of scattering and extinction coefficients (κ_s, κ_e). This enables the physics-based connection of radiometer-based tau with lidar-based h and radar-based κ_s. The crucial step within the retrieval methodology is the calculus of the vegetation scattering coefficient κ_s. This parameter is originally derived from integration of vegetation scattering over the entire half sphere, but typical radar systems solely measure in certain forward or backward scattering directions capturing just a fraction of scattering within this half-sphere. However, when assuming isotropic scattering conditions (e.g. from a randomly oriented vegetation volume), the backscatter measurements of the radar provide a sufficient first order estimate of κ_s and subsequently lead to effective estimates of tau and omega.
In this research study, κ_s is retrieved from radar data of the SMAP mission for the period from April 13th 2015 to July 7th 2015 [1]. Vegetation optical depth (tau) is obtained from a multi-temporal dual-channel (MT-DCA) algorithm using SMAP radiometer data [2]. Lidar-based tree heights were provided by the retrieval algorithm of [3] using ICESat GLAS data and resulting in a global 1-km vegetation height map.
As one option, scattering coefficient κ_s, vegetation optical depth tau and vegetation height h can be used to derive estimates of omega. The temporal dynamics of tau, derived with the MT-DCA approach, allow moving from a static retrieval of omega (currently standard in MT-DCA and in several radiometer-based retrieval approaches [1]) to a dynamic one. This possibility will be tested for different time periods, where seasonal dynamics of vegetation may be identified by change of omega over time.
Moreover, it is planned that also active and passive microwave data of the L-band Aquarius satellite will be investigated using the proposed multi-sensor based retrieval of tau and omega for the time period from September 1st, 2011 to August 31st, 2014 [4].
[1] Entekhabi, D., Das, N.N., Njoku, E.G., Johnson, J., Shi, J., (2012): Soil Moisture Active Passive: Algorithm Theoretical Basis Document L2 & L3 Radar/Radiometer Soil Moisture (Active/Passive) Data Products. JPL Report, Jet Propulsion Laboratory, Pasadena, CA, USA.
[2] Konings A.G., Piles, M., Rötzer K., McColl K.A., Chan K.S., Entekhabi D. (2016): Vegetation optical depth and scattering albedo retrieval using time series of dual-polarized L-band radiometer observations. Remote Sensing of Environment, 172, Pages 178–189.
[3] Simard M., Pinto N., Fisher J.B., Baccini A. (2011): Mapping forest canopy height globally with spaceborne lidar. J. Geophys. Res., 116.
[4] Le Vine, D.M., Lagerloef, G.S.E., Colomb, F.R., Yueh, S.H., Pellerano, F.A. (2007): Aquarius: An instrument to monitor sea surface salinity from space. IEEE Transactions on Geoscience and Remote Sensing, 45(7), 2040–2050.
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