Response of L-Band brightness temperatures to freeze/thaw and snow dynamics in a prairie environment from ground-based radiometer measurements

摘要

Land surface freeze/thaw (F/T) dynamics impact the surface energy balance, carbon fluxes, and hydrologic processes. Recent and on-going L-Band (approximate to 1.4 GHz) spaceborne missions have the potential to provide enhanced information on F/T state over large geographic regions with rapid revisit time. However, the low spatial resolution of these spaceborne observations (approximate to 145 km) makes it difficult to isolate the primary contributions to the Fir signal, including the soil, snow, and vegetation states. A ground-based L-Band radiometer measurement campaign was conducted in Saskatchewan, Canada during the winter of 2014-2015 to evaluate brightness temperature sensitivity to F/T processes, snow, liquid water in snow and assess theoretical retrievals of soil permittivity (epsilon(G)), and snow density from experimental data. The ground-based radiometer was run in multiple configurations. First, temporally continuous measurements were conducted through the winter over an agricultural field, with a comprehensive network of reference snow and soil observations characterizing the F/T state of the soils within or adjacent to the radiometer footprint. Secondly, weekly multi-angular L-Band measurements were made at an undisturbed site of naturally accumulating snow cover, over a site that was kept snow free, and a site with artificially compacted snow. Results from the assessment of the land surface F/T retrieval algorithm showed that L-Band measurements are sensitive to the near surface F/T state of the soil, with the highest level of agreement found between the near surface (2.5 cm) F/T reference measurements of soil temperature and epsilon(G) (accuracies of 91.1% and 92.9%, respectively). Several mid-winter melt events with air temperatures (T-air) above 0 degrees, and soil temperatures below 0 degrees C, illustrated that liquid water within the snow dramatically increase the T-B,T- resulting in false retrievals of soil thaw events using existing L-Band F/T retrieval algorithms. However, Tair was also shown to have a high commission errors compared to radiometer observations in detecting snow melt, because of the delay between T-air > 0 degrees C and the onset of melt resulting in a measurable wet snow signal at L-Band. The retrieval of snow density (rho(s)), of the bottom 10 cm of the snowpack tended to underestimate high rho(s) (>400 kg m(-3)), and agreed well for lower rho(s) (<400 kg m(-3)). The paper gives important information on different contributions to the L-Band F/T signal in a prairie environment, which will help improving satellite-based F/T retrieval algorithms. (C) 2017 Elsevier Inc. All rights reserved.