First Measurement of Soil Freeze/Thaw Cycles in the Tibetan Plateau Using CYGNSS GNSS-R Data

Abstract

The process of soil freezing and thawing refers to the alternating phase change of liquid water and solid water in the soil, accompanied by a large amount of latent heat exchange. It plays a vital role in the land water process and is an important indicator of climate change. The Tibetan Plateau in China is known as the “roof of the world”, and it is one of the most prominent physical characteristics is the freezing and thawing process of the soil. For the first time, this paper utilizes the spaceborne GNSS-R mission, i.e., CYGNSS (Cyclone Global Navigation Satellite System), to study the feasibility of monitoring the soil freeze-thaw (FT) cycles on the Tibetan Plateau. In the theoretical analysis part, model simulations show that there are abrupt changes in soil permittivities and surface reflectivities as the soil FT occurs. The CYGNSS reflectivities from January 2018 to January 2020 are compared with the SMAP FT state. The relationship between CYGNSS reflectivity and SMAP soil moisture within this time series is analyzed and compared. The results show that the effect of soil moisture on reflectivity is very small and can be ignored. The periodic oscillation change of CYGNSS reflectivity is almost the same as the changes in SMAP FT data. Freeze-thaw conversion is the main factor affecting CYGNSS reflectivity. The periodical change of CYGNSS reflectivity in the 2 years indicates that it is mainly caused by soil FT cycles. It is feasible to use CYGNSS to monitor the soil FT cycles in the Tibetan Plateau. This research expands the current application field of CYGNSS and opens a new chapter in the study of cryosphere using spaceborne GNSS-R with high spatial-temporal resolution.

The Potential of Spaceborne GNSS Reflectometry for Soil Moisture, Biomass, and Freeze–Thaw Monitoring: Summary of a European Space Agency-funded study

“…They were obtained from microwave radiometer products, i.e., the SMAP L3 daily products, version 5, projected onto the Equal-Area Scalable Earth (EASE) Grid, and the SMOS Level 3 product. It must be pointed out that VOD is not a vegetation parameter that can be measured on the ground, but it is rather an electromagnetic parameter that is correlated with the vegetation biomass and height [30]. MODIS/Terra Normalized Difference Vegetation Indices (NDVIs) generated every 16 days were also collected.…”

“…Some data collected by the airborne GLORI sensor [12], [17] in France and the CORTO sensor [18] in Spain were also analyzed and compared to the spaceborne case to understand at what extent the platform height can affect both sensitivity and retrieval capability. Another promising application of GNSS-R over land was also investigated, that is the monitoring of freezethaw soil conditions in subarctic areas [28], which has attracted scientific interest from different groups [29], [30], [31], [32].…”

Retrieving Freeze/Thaw Surface State From CYGNSS Measurements

“…More recently, some pioneering studies have shown a promising sensitivity to F/T surface state transitions over high latitude regions, including SMAP GNSS-R mode [34], [39] and TechDemoSat-1 [40]; and also CYGNSS [41]. CYGNSS [30] was originally proposed for ocean surface winds speed estimation over tropical cyclones using reflected L1 Global Positioning System (GPS) signals at left-handed circular polarization (LHCP).…”

A New Coherence Detection Method for Mapping Inland Water Bodies Using CYGNSS Data

“…Although spaceborne GNSS-R has been extensively used to estimate the wind speed on oceans [11,12], its land application is challenging. At present, most of the applications of spaceborne GNSS-R on the land surface used DDM (delay-Doppler map) measurements from the CYGNSS platform and focused on the hydrology variables such as soil moisture [13,14], lake water level [15], wetland extent [16], soil freezingthawing [17], and flooding area [18][19][20]. GNSS-R has strong responses to the inland water caused by coherent forward scattering, leading to studies on its ability for permanent inland water detection.…”