Complex land surface phenologies of moisture status
Abstract
Making cross-scale linkages from experimental plots or flux tower footprints to regional and continental extents is made difficult by disparate spatial and temporal scales between process and observation. While exchanges between the vegetated land surface and the atmospheric boundary layer are continual, sampling and observations are typically intermittent in time and limited across space. Remote sensing of reflected sunlight has proven useful to track ecological dynamics. These observations are, however, restricted to daytime and often obscured by cloud cover, necessitating production of multi-date composites. The current generation of passive microwave radiometers can observe the land surface both day and night regardless of cloudiness, albeit at a spatial resolution coarser than typically used in ecological remote sensing. Datastreams from the AMSR-E (Advanced Microwave Scanning Radiometer-EOS) onboard NASA's Aqua platform are processed daily at the National Snow and Ice Data Center (NSIDC) into various products, including global retrievals of surficial soil moisture and vegetation water content based on microwave brightness temperatures observed at multiple frequencies. Due to sensor orbit and swath width, gaps occur at the lower latitudes in daily products. We have further processed the product-streams from the descending (01:30) and ascending (13:30) orbits into separate smoothed daily composites using an 8-day retrospective moving average. Of particular interest for synoptic ecology is the diel difference in vegetation water content. When the difference between the pre-dawn and the early afternoon values is positive, it suggests that the supply of moisture from the root zone is not able to keep pace with evapotranspiration during the day, but the soil and canopy moisture equalize overnight. Time series of the diel difference show rapid changes in moisture status in response to precipitation events and dry spells. What constitutes the appropriate baseline from which to evaluate moisture stress? How does this baseline differ by ecoregions, land cover and land use? We provide a survey of what temporally-dense microwave image time series can (and cannot) reveal about the seasonality of land surface moisture using data from 2003-2006 at selected herbaceous sites in North America and Northern Eurasia.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.B34C..04H
- Keywords:
-
- 0438 Diel;
- seasonal;
- and annual cycles (4227);
- 0439 Ecosystems;
- structure and dynamics (4815);
- 0480 Remote sensing;
- 0495 Water/energy interactions (1878);
- 1843 Land/atmosphere interactions (1218;
- 1631;
- 3322)