Variability in seasonal freeze-thaw in the terrestrial high latitudes and relationships with land-atmosphere CO2 exchange: Characterization with spaceborne microwave remote sensing
Kyle
C.
McDonald, Jet Propulsion Lab, kyle.mcdonald@jpl.nasa.gov
(Presenting)
John
S.
Kimball, University of Montana, johnk@ntsg.umt.edu
Steven
W.
Running, University of Montana, swr@ntsg.umt.edu
Landscape transitions between seasonally frozen and thawed conditions occur each year over roughly 50 million square kilometers of Earth’s Northern Hemisphere. These relatively abrupt transitions represent the closest analog to a biospheric and hydrologic on/off switch existing in nature, affecting surface meteorological conditions, ecological trace gas dynamics, energy exchange and hydrologic activity profoundly. We utilize time series satellite-borne microwave remote sensing to examine spatial and temporal variability in seasonal freeze/thaw cycles for the pan-Arctic basin and Alaska. Regional measurements of spring thaw and autumn freeze timing are derived using daily brightness temperature measurements from the Special Sensor Microwave Imager (SSM/I), the Advanced Microwave Scanning Radiometer on EOS (AMSR-E), and the SeaWinds-on-QuikSCAT scatterometer. We examine relationships between freeze/thaw timing as related to sensor, satellite overpass time, and landcover, and in relation to regional biospheric activity indicated by atmospheric CO2 measurements. Spatial and temporal patterns in regional freeze/thaw dynamics show distinct differences between North America and Eurasia, and boreal forest and Arctic tundra biomes. Annual anomalies in the timing of thawing in spring also correspond closely to seasonal atmospheric CO2 concentration anomalies derived from NOAA CMDL arctic and subarctic monitoring stations. Classification differences between AM and PM overpass data average approximately 5 days for the region, though both appear to be effective surrogates for monitoring annual growing seasons at high latitudes. Timing of the primary spring thaw event determined from early evening acquisitions generally precedes that determined from early morning data acquisitions for arctic tundra and boreal forest landscapes. Grasslands in the southern margins of the pan-Arctic watershed show opposite patterns for active and passive sensors. This difference in day/night thaw timing observed by radars vs. radiometers may arise from differences in the influence of vegetation on the surface energy budget across biomes.
This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, and at the University of Montana, Missoula, under contract to the National Aeronautics and Space Administration.