Jeffrey L. Privette

Invertibility of a 1-D discrete ordinates canopy reflectance model☆

The invertibility of an accurate discrete ordinates canopy reflectance model is investigated through a series of experiments. Effects of different canopy types, noise levels, spectral ranges, and sampling geometries [including the NOAA Advanced Very High Resolution Radiometer (AVHRR) and the proposed Multi-angle Imaging SpectroRadiometer (MISR) satellite sampling schemes] are considered. Both error-free synthetic bidirectional reflectance data and empirical field reflectance data are utilized. Results suggest that the model can retrieve soil and canopy parameters with reasonable accuracy in most cases, and surface state parameters (absorbed radiation, spectral albedo and photosynthetic efficiency) with high accuracy in all cases. The efficiency of several commonly used optimization algorithms is also assessed, and a model sensitivity study is conducted.

Effects of orbital drift on advanced very high resolution radiometer products: Normalized difference vegetation index and sea surface temperature

Abstract Although orbits of the NOAA TIROS-N satellites are designed to be sun-synchronous, epheremis data shows that the afternoon, ascending node satellites currently cross the equator hours later than they did upon launch. This delay results in different illumination conditions for measurements made by the Advanced Very High Resolution Radiometer (AVHRR). The effects of illumination on two standard AVHRR products—normalized difference vegetation index (NDVI) and sea surface temperature (SST)—are modeled here. Combining orbital data with model results, the effects of the NOAA-11 orbital drift on NDVI are quantitatively assessed for three earth targets: an equatorial Africa site (0° N), the First ISLSCP Field Experiment (FIFE) site (39° N), and the Boreal Ecosystem-Atmosphere Study (BOREAS) site (55° N). Top-of-atmosphere NDVI corrections for solar zenith angle are developed for a dense, deciduous forest. Orbital drift effects on SST are given for an equatorial site. Although results vary with season, latitude, atmosphere and time since launch, NDVI differences of up to 0.23 and SST differences of up to 0.5 K may occur due strictly to orbital drift.

Optimal sampling conditions for estimating grassland parameters via reflectance

The sensitivity of grassland bidirectional reflectance to soil, vegetation, irradiance, and sensor parameters is assessed. Based on these results, a vegetation bidirectional reflectance distribution function (BRDF) model is inverted with ground reflectance data from the First ISLSCP Field Experiment (FIFE). Results suggest leaf area index (LAI) is most accurately retrieved from data gathered in near-infrared bands at low solar zenith angles (SZA), and leaf angle distribution is best retrieved from data gathered in near-infrared bands at SZA. Generally, leaf optical properties are more accurately estimated from data acquired at high SZA. Canopy albedo and fraction of absorbed photosynthetically active radiation (fAPAR) are also estimated and compared to measured values. Albedo estimates are accurate to about /spl plusmn/0.01 (4% relative) when model parameters are determined from reflectance data gathered under preferred conditions. Estimates of fAPAR are less accurate. These results provide a guide for efficiently sampling surface reflectance and accurately retrieving parameters for use in climate ecosystem models.