Surface Roughness Effect on L-Band Multiangular Brightness Temperature Modeling and Soil Liquid Water Retrieval of Frozen Soil

A less explored aspect of L-band microwave emission is the surface roughness effect on the multiangular brightness temperature $(T_{\mathrm {B}}^{\mathrm {p}})$ modeling and retrieval of liquid water content $(\theta _{\mathrm {liq}})$ in frozen soil. This letter investigates this effect based on a data set of multiangular ETH L-band radiometer (ELBARA-III) $T_{\mathrm {B}}^{\mathrm {p}}$ measurements performed in a seasonally frozen Tibetan meadow ecosystem. The surface roughness effect is first investigated via comparing the performance of the HQN model and Shi’s parameterization of the integral equation model in simulating the measured $T_{\mathrm {B}}^{\mathrm {p}}$ signatures. Overestimations are noted for the two model simulations at the vertical polarization in comparison to the ELBARA-III measurements, and Shi’s model with Gaussian autocorrelation function shows better performance especially for the vertical polarization at large incidence angles. The HQN model is then calibrated with the multiangular $T_{\mathrm {B}}^{\mathrm {p}}$ measurements, which leads to $h = 0$ and $Q > 0$ that depends on the polarization. As such, the noncoherent emission contribution to cross-polarization mixing can be accounted for by the HQN model. Furthermore, $\theta _{\mathrm {liq}}$ retrievals are derived from the multiangular $T_{\mathrm {B}}^{\mathrm {p}}$ measurements using the default and optimized HQN model and Shi’s model and compared to the in situ $\theta _{\mathrm {liq}}$ . Unbiased root-mean-square errors of less than 0.025 m3 m−3 are obtained for these retrievals, which are well within the satellite mission requirements. This study demonstrates that the multiangular $T_{\mathrm {B}}^{\mathrm {p}}$ modeling can be enhanced with site-specific calibration of the HQN model that also leads to the improvement of the $\theta _{\mathrm {liq}}$ retrievals, and consideration of polarization mixing is necessary for L-band emission modeling.