Researchers calculate soil freezing depth from satellite data

A team of researchers from the Space Research Institute of the Russian Academy of Sciences (RAS), the Institute for Water and Environmental Problems of the Siberian Branch of RAS, and the Moscow Institute of Physics and Technology (MIPT) has proposed a way to determine soil freezing depth based on satellite microwave radiometry. The findings were published in Studying the Earth From Space, a Russian-language journal of RAS.

Permafrost, sea ice, snow and ice cover, ice sheets, mountain glaciers, and systems of ice clouds are the key components of the Earth's cryosphere. Studying the cryosphere is important for addressing the climate change, permafrost degradation, changing sea levels, and water resource management. However, the regions that hold the cryosphere components, are usually vast, hard to access, and characterized by rough climate conditions.

Satellite microwave radiometry is the best method for remote sensing of poorly accessible and even previously unknown areas on the planet.

"This method has many advantages: gathering data from large areas independently from solar lighting and atmospheric conditions, a high frequency of observation in the high latitudes, sensitivity to subterranean processes, and relative cheapness," said Associate Professor Vasiliy Tikhonov from the space physics department at MIPT, who is also a senior researcher at the Space Research Institute of RAS. "We tested the method's reliability on the Kulunda Plain, a vast steppe in the southeast of Russia's West Siberian Plain. To this end, we compared satellite microwave radiometry data with the actual soil parameters and climate indicators measured on location at weather stations."

It turned out that identical sets of satellite data may correspond to different soil freezing depths. The additional factors at play are soil moisture, salinity, and composition, which can all affect the soil's capacity for microwave emission. The researchers also found that one-time radiometric observations do not produce reliable results, because radio waves may reflect at the interface between the frozen and unfrozen soil.

The team accounted for these findings in their calculations, proposing a method that determines soil freezing depth with a high accuracy based on the data from the Soil Moisture and Ocean Salinity (SMOS) satellite. To remotely determine soil freezing depth, the researchers employed daily series of thermal emission measurements, along with their own emission model that incorporates soil characteristics. The time period considered in the study began with the date of freezing, defined as a spike in thermal radiation picked up by the satellite. It ended with the first thaw day, when the amount of thermal radiation dropped sharply.

The team compared their model predictions with the on-site measurements made in four test areas (fig. 1). The values coincide to an extent that makes the method useful for retrieving soil freezing depths from satellite data.