Impact of profile-averaged soil ice fraction on passive microwave brightness temperature Diurnal Amplitude Variations (DAV) at L-band

The dynamic change of frozen soil is crucial to land-surface modeling, carbon feedback studies, ground engineering (e.g., constructions), and microwave remote sensing. L-Band satellite missions Soil Moisture and Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) are currently exploited to characterize soil into freeze or thaw (FT) states. However, brightness temperatures (T_B) at L-band contain more information besides the FT state, particularly over permafrost or seasonally frozen soil, which has not been explored via current retrieval algorithms. To examine the potential for L-band T_B observations, we define an index called Profile-Averaged Frozen Soil Fraction (F_f) related to Diurnal Amplitude Variation (DAV) of T_B (i.e., ΔT_B) based on the optical depth of the frozen soil column. We evaluated F_f inferred from the 0th-order microwave transfer model with the SMAP L1c product, the ground-based European Space Agency L-Band Radiometer III (ELBARA-III) T_B observations, and temperature profiles collected at the Maqu station in northeastern Tibet. While there is a clear relationship between F_f and ΔT_B, no apparent link exists with the ice content fraction (F_fi) within a fixed-depth soil column. The proposed model certifies that the profile-averaged soil ice content F_f relates to the dynamic microwave penetration depth by math and field measurement. The model reproduces well ΔT_B in Period Freezing but has problems in Period Thawing when melted surface water obstruct the microwave signals. Our findings can be used to exploit ΔT_B between 6 am and 6 pm, as a typically overpassing time by the SMOS and SMAP satellites, for estimating F_f, which can be further applied in weather/climate forecasting and for improving land-surface modeling.