Permafrost Dynamics Observatory (PDO): 2. Joint Retrieval of Permafrost Active Layer Thickness and Soil Moisture From L‐Band InSAR and P‐Band PolSAR

Abstract

In permafrost regions, the active layer is the top soil layer subject to seasonal freeze and thaw. The active layer is differentiated from the seasonal frost zone in temperate regions by the presence of underlying water-impermeable permafrost that maintains the active layer soils in a cold and wet condition and reduces decomposition rate (Woo, 2012). As a result, a large amount of organic carbon has accumulated in the active layer and underlying permafrost since the Pleistocene (Ping et al., 2015). The swift rise in air temperature in the northern circumpolar region has enhanced active layer thawing, which subsequently promotes the decomposition of soil organic matter (OM) and the release of carbon in the form of carbon dioxide or methane into the atmosphere (Schaefer et al., 2014). This positive feedback can threaten the already vulnerable permafrost carbon cycle and it is therefore crucial to have a means to monitor active layer thaw progression and permafrost degradation across the northern circumpolar region.Since 2015, National Aeronautics and Space Administration (NASA)'s Arctic-Boreal Vulnerability Experiment (ABoVE) has initiated a decade-long research campaign aiming to better understand the vulnerability and resilience of social-ecological systems to the permafrost environment in Alaska and western Canada. As an effort to link field-based studies with satellite remote sensing data, ABoVE has conducted a series of airborne campaigns starting in 2017 to acquire synthetic aperture radar (SAR), LiDAR, and imaging spectrometer observations (Miller et al., 2019). The flight transects span major vegetation (tundra to boreal forest), permafrost (continuous to sporadic), and climate (polar Arctic to boreal) gradients, and encompass a diverse range of disturbance conditions, including thermokarst, human infrastructure developments, and wildfire burns. NASA/Jet Propulsion Laboratory (JPL)'s Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) was deployed to fly its Land P-band radars two times each during the thaw season of 2017. The L-and P-band SAR data are sensitive to

Maps of active layer thickness in northern Alaska by upscaling P-band polarimetric synthetic aperture radar retrievals

“…Thus, the upscaling does a reasonably good job of predicting PolSAR-derived ALT values but is unable to capture the full range of CALM in situ measurements. Our maps exhibit minimal bias and high levels of accuracy comparable to the PolSAR-derived ALT retrievals [1] used for model training, and relative to more comprehensive ALT retrievals derived from combined L-band InSAR/P-band PolSAR retrievals over Alaska [44].…”

Fine-Resolution Measurement of Soil Moisture From Cumulative InSAR Closure Phase

“…In most InSAR applications today, two complex SAR images are interfered from repeated passes over the same area, and the phase difference between the images is often interpreted as deformation of the ground surface [26], [27]. InSAR deformation measurements can be used to study geophysical processes such as earthquakes [28], volcanoes [29], [30], landslides [31], permafrost freeze/thaw cycles [32], [33], and underground fluid transport [34]- [36].…”

A Modification to the Remotely Sensed Active Layer Thickness (ReSALT) Algorithm to have Self-Consistency

“…ReSALT has been used in several studies since its conception, such as in [6,[8][9][10]. While it has demonstrated promising results, there is room for improvement.…”

“…While it has demonstrated promising results, there is room for improvement. For example, the Permafrost Dynamics Observatory project reported a root-mean-square-error (RMSE) of 15.18 cm in ReSALT predicted vs in-situ ALTs [10]. The in-situ data had an average ALT of around 55 cm, making the RMSE significantly large with respect to the unit of measure.…”