Frost Degree-Day Mapping over Permafrost Soils in the Arctic Boreal Zone from Multifrequency Passive Microwave Radiometry

The terrestrial biomes of the Arctic-Boreal Zone (ABZ) are key feedbacks on sources and sinks of atmospheric carbon - feedbacks that are largely based on seasonal freeze/thaw (F/T) controls. The future trajectory of the ABZ as a carbon (C) sink or source is of global importance due to vast quantities of C stored in permafrost and frozen soils. Substantial emissions of methane are linked to the "zero curtain" period when soil temperatures are poised near 0 °C and freezing progresses downward, and from depth, retaining a thinning thawed and actively metabolizing soil horizon towards the frozen season. Observations of refreeze in the active layer are vital for understanding of greenhouse gas exchange in the ABZ. Common observational records of F/T from remote sensing instruments are binary and representative of a depth and landscape constituents (e.g. vegetation, snow, soil) that is determined by the landcover and frequency of the radar or radiometer. The Soil Moisture Active Passive (SMAP) radiometer operates at L-Band and is optimized for sensitivity to soil conditions, especially in sparsely vegetated ecosystems of the ABZ. Using SMAP along with multi-frequency radiometer observations from Advanced Microwave Scanning Radiometer (AMSR2) we determine the F/T state of upper soil and the start of the "zero curtain" period and the subsequent frost-degree day (FDD) forcing at the surface during refreeze. The spatially-resolved FDD product is directly related to the active layer depth during refreeze. The uncertainty in FDD from SMAP, and corresponding frost depth, are determined at automated weather stations that are part of the Snow Telemetry (SNOTEL) and Circumpolar Active Layer Monitoring (CALM) network as well as from the surface component temperature records along our Alaska Ecological Transect (ALECTRA). The physical basis of FDD observations are demonstrated using radiometric modelling of vegetation, snow and soil. We present observations of FDD timing and progression over permafrost soils in the Arctic Boreal Zone during the SMAP satellite record (2015-2019).

Portions of this work were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.