Abstract
Vegetation cover absorbs and scatters L-band microwave emission measured by SMOS and SMAP satellites. Misrepresentation of this phenomena results in uncertainties when inferring, for instance, surface soil moisture in retrieval algorithms that commonly utilize the tau-omega model which is most applicable for a weakly scattering medium. In this study, we investigate the degree to which multiple-scattering is prevalent over a range of land cover classifications (from lightly vegetated grasslands to dense forests) at the satellite scale by explicitly accounting for multiple-scattering in a first-order radiative transfer model, developed here. Even though the tau-omega model with effective parameters can possibly capture higher-order scattering contributions, deliberately partitioning scattering into different components is required to estimate multiple-scattering properties. Specifically, we aim to determine how one can partition between zeroth and first-order radiative transfer terms within a retrieval algorithm without ancillary information, determine whether this method can detect first-order scattering at the SMAP measurement scale without ancillary information, and quantify the magnitude of detected scattering. A simplified first-order radiative transfer model which characterizes single interactions of microwaves with a scattering medium is developed for implementation within retrieval algorithms. This new emission model is implemented within a recently developed retrieval algorithm, the multi-temporal dual channel algorithm (MT-DCA), which does not require ancillary land use information. Scattering parameters as well as SM and vegetation optical depth (τ) are retrieved simultaneously in Africa and South America using the first year of SMAP brightness temperature measurements on a 36 km grid. Specifically, an introduced time invariant first-order scattering coefficient (ω1) is retrieved representing microwave emission interaction with the canopy. We find that ω1 is typically zero in lightly vegetated biomes and non-zero (~0.06) in 74% of the forest pixels. In forest-dominated pixels, the median first-order emissivity is 0.04, or about 4.3% of a given SMAP radiometer brightness temperature measurement. Additionally, explicitly accounting for first-order scattering terms in the radiative transfer model tends to increase SM and τ retrievals by a median of 0.02 m3/m3 and 0.1, respectively, only in forested regions. This study demonstrates the first attempt to explicitly partition higher-order scattering terms in a retrieval algorithm at a satellite scale and ultimately provides a fundamental understanding and quantification of multiple-scattering from grasslands to forests.
Highlights
Vegetation can play a major role in regulating surface fluxes especially in regions with more active terrestrial water, energy, and carbon cycles
We focus on information obtained from estimates of vegetation scattering parameters and pose the following research questions. (I) How can a higher-order radiative transfer (RT) model which explicitly partitions between zeroth and first-order scattering terms be implemented within a retrieval algorithm without requirement of ancillary land cover information? (II) By explicitly partitioning between zeroth and first order scattering terms, can firstorder scattering be detected from Soil Moisture Active Passive (SMAP) polarized brightness temperature measurements without ancillary information? (III) If so, what is the magnitude of first-order emission across land cover classifications? (II) and (III) address whether we can identify areas where a transition from a zeroth-order to first-order RT model might be important
In order to quantify higher-order scattering, a first-order RT model which explicitly partitions first-order scattering terms from zerothorder emission terms was developed using a ray tracing method of single microwave interactions with the vegetation layer. It is an approximated form of established first-order RT models which conveniently includes the same variables as the zeroth-order RT model, commonly known as the tau-omega model
Summary
Vegetation can play a major role in regulating surface fluxes especially in regions with more active terrestrial water, energy, and carbon cycles. While low frequency microwaves originating from the surface can largely penetrate light vegetation, they are significantly attenuated and subject to multiple-scattering in the presence of denser vegetation including vegetation components of dimensions comparable to the observation wavelength, 21 cm Remote Sensing of Environment 219 (2018) 324–338 for L-band (Ulaby and Long, 2014). This hinders accurate monitoring of SM under moderate to heavy vegetation, especially in forests which comprise approximately 30% of Earth's terrestrial biosphere and significantly influence global biogeochemical cycles (Nemani et al, 2003). We do this by retrieving time-varying soil moisture and vegetation optical depth (τ) as well as scattering parameters (i.e., ω and ω1; discussed later) from SMAP measurements without ancillary information
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