Impact of surface roughness, vegetation opacity and soil permittivity on L-band microwave emission and soil moisture retrieval in the third pole environment

Abstract

The brightness temperature (TBp) observed by the Soil Moisture Active Passive (SMAP) satellite mission is significantly affected by the soil permittivity (εs), surface roughness and vegetation opacity (τp). This study assesses the impact of these factors on simulating the SMAP horizontally (p = H) and vertically (p = V) polarized TBp measurements and retrieving the liquid soil water content (θliq) for both frozen and thawed soils in the typical Tibetan desert and meadow ecosystems. For this investigation, the zero-order approximation of the radiative transfer equations, i.e., τ-ω emission model, is configured with surface roughness and τp parameterizations adopted by current SMAP soil moisture retrieval algorithms, and the εs is computed with the four-phase dielectric mixing model that is applicable for both frozen and thawed soils.For the Tibetan desert site, the τ-ω emission model with above configurations underestimates year-round the SMAP TBH measurements (bias > 20 K), while TBV are underestimated during the cold season. Implementation of a new surface roughness parameterization reduces the TBH underestimation, and the improved TBp simulations lead to better θliq retrievals produced by the single channel algorithm (SCA) using the TBV as well as TBH measurements. The remaining TBH and TBV underestimations are removed by further adopting a new εs parameterization.For the Tibetan meadow site, the τ-ω emission model overestimates both TBH and TBV during the warm season and underestimates TBH during the cold season when the vegetation is sparse. Implementation of the new surface roughness parameterization reduces the TBH underestimation, and further the TBp overestimation is mitigated by adopting a new τp parameterization derived from a discrete radiative transfer model previously developed and tested for the same site. The in-situ measured θliq dynamics are better captured by corresponding retrievals for both frozen and thawed soils with implementation of the new surface roughness and τp parameterizations, which reduces the unbiased RMSEs by more than 40%. The parameterizations developed in this study are useful to provide consistent and reasonable TBp simulations and θliq retrievals over the Tibetan Plateau for both frozen and thawed soils based on both SMAP TBH and TBV measurements.