Uncertainty and Validity of Aerosol Radiative Forcing Determinations

Research Staff: Peter Pilewskie, Warren Gore, and Larry Pezzolo

In February and March 1999, we participated in an experiment EOPACE (Electro Optical Propagation Around Coastal Environments) in Duck, North Carolina to characterize the radiative effects of boundary layer marine aerosol near the Outer Banks region of the coast of North Carolina (Figure 1). The NASA Ames Solar Spectral Flux Radiometer (SSFR) was integrated on the CIRPAS (Center for Interdisciplinary Remotely-Piloted Aircraft Studies) Twin Otter to measure zenith irradiance (Figure 2) and nadir radiance. Data have been used to characterize the sea surface reflectance in a coastal region and to compare with open ocean reflectance for purposes of improving aerosol optical depth retrievals from the AVHRR satellite instrument. The Twin Otter was also equipped with microphysical sensors to measure the extinction of the aerosol. We determined the net solar radiative forcing of the aerosol and examined dependencies of radiative parameters on wind speed and surface roughness. This research represents an important test of our previous discovery of a bias between measured and modeled spectral irradiance that is highly correlated with water vapor, and increases at a rate of 8 Wm-2 per cm of water vapor.

[SeaWiFS Image] [Downwelling SSFR Spectra]

Another aspect of this research is to modify the radiative transfer model used in our model to incorporate new spectral bandpasses that match that of the SSFR. The newly generated k-distributions will be compiled using the line by line code LBLTRM developed by Atmospheric and Environmental Research (AER). Testing began in summer 1999 and we anticipate that the completed radiative transfer model to be operational by the first half of FY00. Analysis will be applied to the previous aerosol/water vapor study described above and to the EOPACE Duck data set. Estimates of solar spectral radiative forcing will be generated from both studies.

Collaborators: Robert Bergstrom, Bay Area Environmental Research Institute; Maura Rabbette, National Research Council Associate: John Pommier, Symtech

Point of Contact: Peter Pilewskie, (650) 604-0746, ppilewskie@mail.arc.nasa.gov