Aerosol Models and Model Selection for R2009
Prior to this reprocessing, the aerosol models and model selection methodology were based entirely on the work of Gordon and Wang (1994). The aerosol models were derived from aerosol size distributions cataloged by Shettle and Fenn (1979) from aircraft measurements. Over the past decade, the Aerosol Robotic Network (AERONET) has accumulated an extensive catalogue of in situ aerosol data, including retrievals of size distributions and single-scattring albedos, from a global network of autonomous CIMEL sun photometer measurements. Some of the CIMELS are in coastal and island locations to provide measurements of maritime atmospheres.
Aerosol Model Suite
For this reprocessing, a new set of aerosol models has been developed for atmospheric correction based on the range of single-scattering albedos and aerosol size distributions retrieved from maritime AERONET sites. The size distributions were stratified by relative humidity (RH) and catalogued by fine and course-mode radii with fixed modal width (Figure 1). Eight RH-specific families of models were then constructed, each family spanning ten fine-to-coarse mode size fractions from 0.0 to 0.95. The 80 aerosol size distributions and associated albedos were then processed through a vector radiative transfer code to produce the aerosol phase functions, single-scattering epsilons, single-to-multi-scattering relationships, and transmittance coefficients, as decribed in Gordon and Wang (1994), for the range of sensor wavelengths and observed viewing and solar radiant-path geometries. 80 look-up tables were then generated (one per size distribution) in an hdf format consistent with previous reprocessings.
Figure 1: Aerosol Size Distribution Definitions
Note: The primary difference between the old Shettle and Fenn size distributions (Current Models) and the new AeroNET-based models is the typical width of the fina and coarse modes. Examples are shown in Figure 2.
Figure 2: Typical Change in Aerosol Size Distribution Definitions
All radiative transfer simulations were performed using the Ahmad and Fraser (AF) code, as recently updated by Z. Ahmad. AF is a vector radiative transfer code with full accounting for polarization. For consistency, the same code was used to reproduce the Rayleigh-scattering look-up tables. The AF simulation process was integrated into the distributed data processing system of the OBPG, thereby making it possible to produce a full set of tables for 80 models for each sensor in less than one day.
Aerosol Model Selection
The aerosol model selection process was also modified to take advantage of the RH stratification and the RH information associated with each observation (from ancillary NCEP meteorological inputs). The ancillary RH information is now used to select the two RH-specific families of aerosol models which bound the observation. The model selection process is then executed independently on each family of 10 models, and the resulting aerosol reflectance retrievals are interpolated to the RH of the observation.
The primary impact of the updated aerosol model suite is improved agreement between satellite retrieved and in situ observations of aerosol type and aerosol optical thickness. This can be seen in SeaWiFS match-ups against the global deep-water archive of microtops aerosol optical depth measurements available in SeaBASS, where satellite to in situ mean ratios decreased from 1.4 to 0.9 and median absolute percent difference decreased from 42 to 23%. Satellite to in situ match-ups in coastal regions also showed substantial improvement against AeroNET measurements of aerosol optical thickness and Angstrom exponent. Figures 3 and 4 show the time-series of match-ups from the upper reaches of Chesapeake Bay, before and after the Reprocessing 2009 changes.
Figure 3: Upper Chesapeake Bay Time-Series of Aerosol Properties Retrieved by SeaWiFS (blue) and MODIS/Aqua (red) Relative to AeroNET Measurements (black), Before This Reprocessing.
Figure 4: Upper Chesapeake Bay Time-Series of Aerosol Properties Retrieved by SeaWiFS (blue) and MODIS/Aqua (red) Relative to AeroNET Measurements (black), After This Reprocessing.