Mapping of anomalous C-band backscatter signals caused by subsurface scattering and their correlations with land surface characteristics over the Tibetan Plateau

Abstract

Subsurface scattering is a rarely recognized phenomenon, which may cause abnormal backscatter (σ^o) signal and deteriorate soil moisture (SM) retrieval. Although this phenomenon has been recently detected by Wagner et al. (2024a) over the global scale, the cold regions such as the Tibetan Plateau (TP) widely covered by frozen soil were excluded for analysis. This study aims to fill the research gap to detect and map the σ^o anomalies and subsurface scattering on the TP, discuss their impacts on SM retrievals and explore their correlations with land surface characteristics in the warm seasons when surface soil is thawed. Pearson correlation coefficients (R) obtained between ASCAT or Sentinel-1A σ^o observations and in-situ or ERA5-Land SM data at local scale show that anomalous C-band σ^o signals are detected in the arid area of TP as indicated by negative R values. This directly leads to poorer performance of ASCAT SM retrievals over barren areas than over grasslands in the humid and semi-humid regions. By fitting two backscatter models without and with subsurface scattering term to above collocated σ^o and SM data pairs, the detected anomalous σ^o signals are found to be dominated by subsurface scattering. The results obtained with three indicators proposed to characterize the subsurface scattering further confirm above findings, which show that notable σ^o anomalies and strong subsurface scattering are predominately detected in the western part of TP and Qaidam Basin. Along with the dryness of SM, high level of soil pH index and sparse vegetation cover especially bare land are important factors that favor their occurrences. Moreover, the maps of above indicators with certain thresholds show great potential for masking the regions with high probability of anomalous σ^o signals caused by subsurface scattering and thus inaccurate ASCAT SM retrievals. These results demonstrate for the first time the potential of detecting σ^o anomalies and subsurface scattering and alleviate their effects on SM retrievals in cold and arid regions.