Diurnal Differences in Global ERS Scatterometer Backscatter Observations of the Land Surface

Soil moisture estimates from the European Remote Sensing Satellite (ERS)-1, ERS-2, and Metop scatterometer instruments are available as time series starting in 1991. To better understand the satellite signal backscatter data and the corresponding soil moisture estimates, differences between different overpass times are analyzed. An analysis of more than 15 years of ERS scatterometer data has shown distinct patterns in backscatter between different overpass times. Differences between backscatter data from descending (morning overpass) and ascending (evening overpass) tracks show spatial and temporal patterns that cannot be attributed to soil moisture. Based on regional studies, we highlight the main processes causing the diurnal differences in backscatter data. Data used for this study are based on preprocessed normalized backscatter [σ⁰(40)] and slope [σ^´(40)] data from a modified TUWien WARP 5.0 algorithm. Results show that the diurnal differences in σ⁰(40) between descending and ascending data are systematic and are not artifacts from previous processing steps. Statistically significant diurnal differences [Δσ⁰(40)] are detected over about 30% of the land area, underscoring the potential significance for hydrologic remote sensing on a global scale.