Evaluation of SMOS Frozen Ground Flags

English Abstract:

The Soil Moisture Ocean Salinity (SMOS) L2 soil moisture product is provided with science flags to indicate the reliability of the soil moisture estimates based on the surface conditions which may impede the ability to accurately estimate the soil moisture for a region. During periods of soil freezing, the soil dielectric decreases as the water in the soil freezes, resulting in an increase in measured brightness temperature by passive microwave sensors. Inaccurate estimates of frozen soil conditions could result in an assumption that there is a decrease in soil moisture. SMOS uses modelled data from the European Centre for Medium-Range Weather Forecasts (ECMWF) soil temperature level 1 (0-7cm) (STL1) depth as a flag for frozen soil conditions. The flag is set when the STL1 indicates a temperature below 270K. These flags were evaluated against data collected at the Kenaston soil moisture network, south of Saskatoon, Saskatchewan. For this analysis, data were collected during 2012, with data used from January to April, 2012 to monitor the spring thaw, and data from October to December 2012 to monitor the winter freeze. Flags were examined for both ascending and descending SMOS pass times. It was found that there was greater agreement between the in-situ probe installed horizontally that 5cm than the vertically installed probe (which provides soil surface temperature) with greater agreement occurring for descending passes. When using the threshold of 270K to indicate frozen soil conditions (as is used for the SMOS frozen ground flag), the SMOS flag indicated a false positive (SMOS flag indicates thawed soils when the network indicates frozen) rate of 32.9% for ascending passes (compared to the 5cm sensor) and 18.8% for descending passes. If the threshold were increased to 273.15K, the false negative rate would be reduced considerably, to 15% for ascending passes and 2.8% for descending passes.