Friday, 7 June 2002: 9:15 AM
Mapping surface reflective properties over the ARM SGP CART area by combining multi-platform observational datasets
Surface albedo is a key variable determining the disposition of solar radiation between the surface and the atmosphere. Adequate knowledge of surface albedo in terms of spectral, spatial, and temporal variations is required to specify accurately the surface boundary conditions in modeling and remote sensing studies. The primary goal of our research project is to develop a comprehensive database of spectral reflective properties over the Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) Cloud And Radiation Testbed (CART) site. It is achieved by using all available datasets primarily from polar orbiting and geostationary satellite observations. Ground and aircraft measurements are used for validation of the remote sensing products and for case studies. Size of the mapping area is approximately 1000x1000km2, which encompass entire ARM SGP CART site. A new method for atmospheric correction of satellite observations is presented. It is based on the results of detailed radiative transfer computations by the latest MODTRAN radiative transfer model, which includes an updated HIRTRAN 2001 spectroscopic database. It is found that for some narrowband intervals especially in the near infrared part of the solar spectrum, the water vapor absorption in earlier models was underestimated. This may lead to substantial biases up to 0.02-0.03 or more in retrieved surface reflectance. Some results of the validation of derived surface albedo products against ground field measurements and aircraft observations will be presented. Impact of aerosol model on the retrieval of surface reflectance and surface radiation budget is also investigated. Difficulties and consequences of satellite retrievals under high solar zenith angles and low scattering angles are discussed. The effects of surface inhomogeneity and directionality on surface radiation budget and satellite retrievals are evaluated, together with scaling-up issues using a hierarchy of satellite imagery (TM, AVHRR, MODIS, CERES). Multi-year datasets of narrowband and broadband surface reflective properties will be presented and compared to model results. Potential impact of an improved knowledge on the spatial and temporal variability of surface albedo over the ARM SGP CART site on the solar radiation budget, cloud and aerosol radiative forcing and climate is assessed.