Comparisons of Evening and Morning SMOS Passes Over the Midwest United States

This study investigates differences in the soil moisture product and brightness temperatures between 6 p.m. and 6 a.m. local solar time Soil Moisture Ocean Salinity (SMOS) passes for a region in the Midwest United States. This region has uniform land cover, consisting largely of maize and soybean row crops. The comparison was restricted to periods with no rainfall. There were 19 days available for analysis of the soil moisture product. It was found that there was a significant difference in the soil moisture product for all 19 days, with lower soil moisture for most mornings. The difference between the soil moisture products on some days exceeded the allowable error of 0.04 m³ m^-3. In-situ and model results indicate that there should be virtually no change in soil moisture between the evening and morning. In order to investigate this discrepancy, measured brightness temperature was converted to a polarization index (PI), and evening and morning values were compared. Investigation of the measured brightness temperature was limited to five days where a large range in incidence angles was available. Large differences between evening and morning passes were found for incidence angles less than 40° that could not be explained with radiative transfer theory but may be attributed to technical issues. There was also a difference in the PI values between the evening and morning passes for incidence angles greater than 40°. This can be caused by a decrease in soil moisture from evening to morning or could be attributed to an increase in the volumetric water content of the vegetation.