Using the Angular Dependence of the Polarization Ratio to Analyze Aircraft Passive Microwave Measurements Over Land

The derivation of geophysical parameters from passive microwave observations over land has always been challenging. Soil conditions, land cover, and the atmosphere affect the measurements to varying degrees, and it is difficult to isolate these individual contributions. In this study, we assess whether multi-angle observations provide additional information that can strengthen existing retrieval algorithms. Between October and November 2024, a series of airborne flights carrying the Advanced Microwave Precipitation Radiometer (AMPR) were conducted over the United States. Three land-based flights with multi-angle observations from 0–45° and dual-polarized measurements at 10.7, 19.35, and 37.1 GHz were analyzed. The data showed a strong linear relationship between the microwave polarization ratio and the incidence angles within the 25–45° range (e.g.R2> 0.9 for 71.2% of all flight scans analyzed at 10.7 GHz). The linear model for the polarization ratio showed a similar performance in terms of Root Mean Square Error (RMSE) to simulations based on a τ −ω radiative transfer model and commonly used assumptions. The observed linearity was further evaluated with satellite observations from the Advanced Microwave Scanning Radiometer (AMSR2). This evaluation confirmed the observed linearity across all three frequencies. The slope of the relationship between the polarization ratio and the incidence angle was calculated for each multi-angle flight scan, which was sensitive to both soil moisture and vegetation. This new parameter, which was derived from multiple observations, appeared to be consistent in time and space, revealing similar patterns along flight lines acquired at different times. The slope was used as input in regression models to derive soil moisture. A model solely based on 10.7 GHz data revealed a strong correlation (R2= 0.81) with Level-3 soil moisture from the Soil Moisture Active Passive (SMAP) mission, demonstrating the potential of multi-angle retrievals with established soil moisture products.