B53G-03:
Seasonal and Diurnal Tropical Forest Greenness Observed and Modeled Using MODIS Terra and Aqua Sensors
Friday, 19 December 2014: 2:10 PM
Alfredo R Huete, University of Technology Sydney, Plant Functional Biology and Climate Change, Ultimo, Australia, Kevin Davies, University of Technology Sydney, Ultimo, Australia, Natalia Restrepo-Coupe, University Technology Sydney, Sydney, Australia, Piyachat Ratana, University of Technology, Sydney (UTS), Sydney, Australia, Qingsong Sun, Boston University, Department of Earth and Environment, Boston, MA, United States, Scott R Saleska, University of Arizona, Tucson, AZ, United States and Crystal Schaaf, University of Massachusetts Boston, Boston, MA, United States
Abstract:
Recent studies on satellite measures of Amazon forest greening suggest that observed seasonalities are optical artefacts resulting from shifting sun- sensor view geometries between solstice and equinox periods. The degree and extent of sun geometry influences on satellite observations have important implications on the utility of multi-sensor time series for generating accurate long-term data records. Here we investigate sun angle interactions on tropical forest greening using Terra- and Aqua-MODIS, and combined Terra-Aqua Nadir BRDF Adjusted Reflectance (NBAR) vegetation index (VI) time series, with distinct seasonal and daily sun angle conditions for 10:30 a.m., 1:30 p.m. overpasses, and local solar noon times, respectively. This was compared with modeled, sun angle corrected data from the MODIS MCD43A1 product for fixed sun angles. The interactions between sun angle and forest greening were analyzed along an equatorial forest transect of constant sun-earth geometry but variable annual rainfall and dry season length, as well as a latitudinal transect ranging from equatorial to dry southern forests. In equatorial forests, seasonality in sun angle geometry was synchronous with drought seasonality and resulted in broad scale, forest greening consistent with the duration of the dry season and light availability. The sun angle corrected data showed a reduction in the magnitude of seasonal greening, but also revealed an extended greening period well beyond the equinox. On the other hand, across the latitudinal gradient there were shifts in the start and duration of the dry season that resulted in greening patterns that were asynchronous with sun angle geometries. Sun angle influences became significant and were more pronounced at greater latitudes, demonstrating need to normalize cross-sensor satellite data for sun geometry effects, especially with the recent and upcoming launches of new satellite systems.
