Comparative analysis of passive microwave satellite products for soil moisture and freeze–thaw dynamics

Abstract

Surface soil moisture (SM) and freeze–thaw (FT) state are key components of the terrestrial system and are recognized as Essential Climate Variables due to their influence on hydrological, ecological, and energy exchange processes. While in situ observation networks provide accurate point-based measurements, their sparse spatial distribution limits the ability to capture large-scale variability. Satellite-based passive microwave remote sensing offers a consistent, long-term, and spatially continuous alternative for monitoring SM and FT dynamics. This study evaluates and compares Level 3 SM and FT products derived from two L-band passive microwave missions: NASA’s Soil Moisture Active Passive (SMAP) and ESA’s Soil Moisture and Ocean Salinity (SMOS). Using data spanning April 2015 through December 2024, period-of-record statistics were generated to assess seasonal variability and differences in FT state classification between the two datasets. Despite operating at similar frequencies, SMAP and SMOS employ different retrieval algorithms, particularly in their use of normalized polarization ratios and thresholding approaches for FT classification. Results indicate general agreement in autumn freeze timing between the datasets; however, notable discrepancies are observed during the spring transition. SMAP tends to detect earlier thaw onset, while SMOS suggests a more rapid progression through the thaw period and, in some cases, delayed autumn freeze-up. These differences are attributed to variations in algorithm design, including SMAP’s binary classification approach and SMOS’s incorporation of a partially frozen state and ancillary environmental data. Overall, SMAP and SMOS soil moisture products show broadly consistent spatial and seasonal patterns, although SMAP generally exhibits a slight positive wet bias relative to SMOS, particularly in persistently wet and densely vegetated regions such as tropical forests and high-latitude boreal environments.