Aerosol Retrievals from Individual Avhrr Channels: I. Retrieval Algorithm and Transition from Dave’ to 6s Radiative Transfer Model

The present 2 generation aerosol retrieval algorithm over oceans at NOAA/NESDIS separately retrieves two values of aerosol optical depth, 1 and 2, from AVHRR channels 1 and 2 centered at 1=0.63 (operational) and 2=0.83 μm (experimental), respectively. From these, an effective Angstrom exponent, , related to particle size, can be derived as =-ln( 1/ 2)/ln( 1/ 2). The single-channel look-up-tables, relating reflectance to optical depth in the retrievals, have been precalculated with the Dave’ (1973) scalar radiative transfer (RT) model. This first part of a two-part paper describes the retrieval algorithm, with emphasis on its RT-modeling related elements, and documents the transition to the Second Simulation of the Satellite Signal in the Solar Spectrum (6S; 1997) RT model. The new 6S RT model has the capability to account for reflection from windroughened sea surface; offers a wide choice of flexible aerosol and gaseous absorption models; and allows easy convolution with the sensor’s spectral response. The value of these new features for aerosol remote sensing from AVHRR is discussed in detail. The transition effect is quantified by directly applying the Dave’ and 6S based algorithms to four large datasets of NOAA14/AVHRR measurements, collected between February 1998 May 1999 over the latitudinal belt of 5-25 S. Statistics of the differences ( = Dave6S and = Dave6S) are as follows: averages < 1><1×10 , < 2> -4×10 , and < > +8×10; and standard deviations are 1~6×10 , 2~4×10 , 9×10. These are found to be well within a few percent of typical values of and , and their respective ranges of variability, thus ensuring a smooth transition and continuity in the operational aerosol retrieval. On the other hand, the 6S model leaves us with a much more flexible RT modeling tool compared to the previously used Dave’ code.