A 2-SOURCE APPROACH FOR ESTIMATING TURBULENT FLUXES USING MULTIPLE-ANGLE THERMAL INFRARED OBSERVATIONS

A recently developed two-source model for computing the surface energy balance with directional radiometric surface temperature observations can be simplified with the use of radiometric surface temperature obs ervations at two substantially different view angles. Such observation s have been available from the Along Track Scanning Radiometer (ATSR) aboard the First European Remote Sensing Satellite (ERS-1) and therefo re could be used operationally with this simplified model. Compared to the original model formulation, the new version, 2ANGLE, does not req uire an estimate of the fraction of ''green'' or active vegetation, f( g), and use of the Priestley-Taylor parameterization for partitioning of net radiation absorbed by the canopy into latent, LE, and sensible, H, heat in order to obtain a solution. However, if one uses the Pries tley-Taylor parameterization with two radiometric observations at diff erent view angles, then the model, 2ANGLE_PT, does not require a measu rement of air temperature. The model-derived fluxes using multiple ang le radiometric surface temperature observations collected during the F irst International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE), which was conducted in the tall grass; prair ie in eastern Kansas during the summers of 1987 and 1989, are compared to measured surface fluxes. Differences between predicted and measure d H using the 2ANGLE and 2ANGLE_PT models were generally significant ( i.e., > 50%), while the original version, 1ANGLE_PT, yielded differenc es of 45-50%, on average. The 2ANGLE and 2ANGLE_PT model predictions o f LE yielded differences of 25-40% with measured values, while the 1AN GLE_ PT model produced differences of 15-20%, on average. Using a simp le technique to estimate daytime total LE from the ''instantaneous'' v alues with the three versions of the model resulted in differences gen erally between 20 and 25%. A review of studies analyzing FIFE flux mea surement techniques indicates that differences in half-hourly to hourl y observations of H and LE can reach 50% or greater, but typically are between 20 and 30%. Therefore the performance of the models in predic ting H ranged from marginal to poor, but was generally satisfactory in estimating LE because in many cases LE was significantly larger than H. The performance of the 2ANGLE_PT model in predicting H and LE was s ignificantly better than the 2ANGLE model, but not as good as the orig inal 1ANGLE_PT model. A sensitivity analysis with the three models ind icated that not only were the 2ANGLE_PT model predictions unaffected b y a 3 K uncertainty in the estimation of air temperature using satelli te data, but also showed little sensitivity to a 1.5 K uncertainty ass ociated with deriving an atmospherically corrected surface temperature from ATSR observations. Therefore the 2ANGLE_PT model has potential f or computing regional-scale LE operationally with a satellite-based se nsor such as the ATSR.