The Radiative Effects of Aerosol Layers on Solar Spectral Flux Measurements during ACE-Asia

During the ACE-Asia campaign (March/April 2001) detailed moderate resolution solar spectral measurements were made to study the western Pacific radiative energy budget. The NASA Ames Solar Spectral Flux Radiometer (SSFR) were used to measure solar spectral irradiance covering a spectral range 350-1700 nm. The SSFR was flown on the CIRPAS Twin Otter and acquired over 200,000 upwelling and downwelling irradiance spectra. Most of the flights included several horizontal legs above and below various aerosol plumes. For two specific cases April 12th and April 17th the absorption and fractional absorption of the aerosol layers was computed. A K-distribution radiative model specifically designed to analyze the SSFR data is used to model the upwelling and downwelling spectral flux. The aerosol radiative properties are estimated from the comparison of the data with the model results. The aerosol forcing is then computed using the estimated values of the aerosol radiative properties. Principal Component Analysis (PCA) was used to characterize several thousand of the retrieved SSFR spectra and to determine the number of independent pieces of information that exist in the visible to near-infrared solar irradiance spectra. It was found in both the upwelling and downwelling cases that almost 100% of the spectral information (irradiance retrieved from 1350 wavelength channels) was contained in the first few extracted principal components which represent a few fundamental components. This analysis technique is used to quantify the variability in the data and more specifically to try and determine how much of the spectral variance is due to the presence of the aerosol layers.