Freeze/Thaw Retrieval Over High Altitude Areas with CYGNSS.

Soil Freeze/Thaw (F/T) transition monitoring is essential for quantifying climate change and hydrologic dynamics over cold regions. In this work, we investigated the use of Earth-reflected Global Positioning System (GPS) L-band signals as collected by the CYclone Global Navigation Satellite System (CYGNSS) mission for F/T retrieval over high altitude areas. CYGNSS can potentially detect the freezing and thawing of soil surface due to changes of dielectric constant when frozen, at an improved spatio-temporal sampling as compared to traditional microwave radiometry missions. A feasibility study has been performed over the Andes Mountains to further advance on this topic. Sensitivity in CYGNSS data to F/T transitions was demonstrated by comparing reflectivity differences between Summer and Winter to soil and snow temperature as provided by the European Centre for Medium-Range Weather Forecast (ECMWF) ERA 5-Land numerical reanalysis model. Finally, F/T retrievals with CYGNSS showed a better performance than those derived with the Soil Moisture Active Passive (SMAP) mission.