L-band radiometry soil freeze/thaw state validation using ground measurements vs. air temperature

Assessing the accuracy of remotely sensed freeze/thaw (F/T) soil products requires corroboration with in situ observations. Previous research has used a variety of ground-based observations for these assessments including air temperature observations and ground-based in situ soil moisture or soil temperature networks. Ground based networks, such as those used for the validation of the Soil Moisture Active Passive Mission, utilize soil dielectric and temperature measurements at depths of 5cm below the surface. However, it remains unclear whether the current configurations of these networks are useful for the corroboration or for the development of remote sensing freeze/thaw (F/T) products. Furthermore, numerous previous studies have used air temperature observations for the validation of soil F/T products, however the usefulness of these observations has not been well established. To address this issue, an L-band radiometry study was conducted from November 2014 to April 2015 to capture numerous F/T events. Soil moisture probes measuring soil temperature and real dielectric permittivity were installed within the footprint of a ground-based L-band radiometer. The in-situ soil probes were installed vertically at the surface and horizontally at 2.5, 5 and 10 cm depths. Additional measurements included air temperature and snow pack profile temperatures. A binary soil F/T estimate was derived using radiometer brightness temperatures and was compared to soil F/T classifications using air temperature, soil temperature, and real dielectric permittivity. The results of the study found that the dielectric or soil temperature measurements from the probes installed at shallow depths (vertical and 2.5 cm) more closely correspond to radiometer F/T estimates. Corroboration among in situ and radiometer derived F/T conditions were much higher for the near surface measurements than those observed at the 5 cm depth; a depth currently utilized by numerous soil moisture networks. Air temperature based estimates of the soil F/T state did not correspond as closely to the radiometer based F/T estimate as soil based measurements, particularly during periods of bare soils and under wet snow.