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

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