Passive microwave freeze/thaw classification for wet tundra regions

Several climate change scenarios have predicted that the greatest changes would occur at high latitudes. In the Arctic, long-term changes in temperature would be reflected, for example, in the growth or retreat of permafrost regions and in the response of the vegetation. Tundra-covered areas are a major terrestrial reservoir of carbon and changes in temperature and moisture will affect the storage and release of carbon by the tundra. The most important annual hydrological events for tundra regions are the thaw in the spring and freeze-up in the fall. These regions are often cloud-covered for extended periods, especially during these key times of the year. A remote freeze/thaw classification technique based on microwave satellite measurements has the potential for providing more consistent observations compared with one based on visible observations. The authors begin by developing a classification technique from ground-based data (i.e., no intervening atmosphere) collected during the authors' Radiobrightness Energy Balance Experiment 3 (REBEX 3) on the North Slope of Alaska from September, 1994 until September, 1995. Radiobrightness measurements were made at 19.35, 37.0, and 85.5 GHz using a tower-mounted Special Sensor Microwave/Imager (SSM/I) simulator. Co-located surface energy balance measurements included: thermal IR surface brightness, solar and net radiation, soil and snowpack temperatures, soil heat flux, sensible heat, latent heat, soil moisture, and other related meteorological variables. REBEX 3 was conducted in conjunction with other interdisciplinary experiments under the Arctic System Science (ARCSS) program of the U.S. National Science Foundation. Using the spectral gradient, changes in the spectral gradient on a diurnal time scale, polarization effects, and absolute radiobrightnesses, a technique for classifying tundra as snow-free frozen, snow-free thawed, or snowcovered is described. Supporting measurements of subsurface temperature and moisture are used to understand the physical basis for the differing radiobrightness signatures.