Space-based passive microwave soil moisture retrievals and the correction for a dynamic open water fraction

摘要

The large observation footprint of low-frequency satellite microwaveemissions complicates the interpretation of near-surface soil moistureretrievals. While the effect of sub-footprint lateral heterogeneity isrelatively limited under unsaturated conditions, open water bodies (if notaccounted for) cause a strong positive bias in the satellite-derived soilmoisture retrieval. This bias is generally assumed static and associatedwith large, continental lakes and coastal areas. Temporal changes in theextent of smaller water bodies as small as a few percent of the sensorfootprint size, however, can cause significant and dynamic biases. Weanalysed the influence of such small open water bodies on near-surface soilmoisture products derived from actual (non-synthetic) data from the AdvancedMicrowave Scanning Radiometer for the Earth Observing System (AMSR-E) forthree areas in Oklahoma, USA. Differences between on-ground observations,model estimates and AMSR-E retrievals were related to dynamic estimates ofopen water fraction, one retrieved from a global daily record based onhigher frequency AMSR-E data, a second derived from the Moderate ResolutionImaging Spectroradiometer (MODIS) and a third through inversion of theradiative transfer model, used to retrieve soil moisture. The comparisondemonstrates the presence of relatively small areas (<0.05) of open waterin or near the sensor footprint, possibly in combination with increased,below-critical vegetation density conditions (optical density <0.8), whichcontribute to seasonally varying biases in excess of 0.2 (m3 m?3)soil water content. These errors need to be addressed, either throughelimination or accurate characterisation, if the soil moisture retrievalsare to be used effectively in a data assimilation scheme.