Introduction   

The history, theory and strategy of the aerosol retrieval over the ocean is described in in Tanré et al., (1997), Levy et al., (2003), Remer et al., (2005) and the C6 updates are described in Levy et al. (2013). The mechanics of the ocean algorithm are shown in the flowchart below. The algorithm is based on a ‘look-up table’ (LUT) approach, i.e., radiative transfer calculations are pre-computed for a set of aerosol and surface parameters and compared with the observed radiation field. The algorithm assumes that one fine and one coarse lognormal aerosol modes can be combined with proper weightings to represent the ambient aerosol properties over the target. Spectral reflectance from the LUT is compared with MODIS-measured spectral reflectance to find the ‘best’ (least-squares) fit. This best fit, or an ‘average’ of a set of the best fits is the solution to the inversion.

Below is a flow chart of the MODIS dark target ocean algorithm for the 10 Km product.  The algorithm is also explained below the flow chart in text which is largely excerpted from the MODIS Algorithm Theoretical Basis Document (ATBD).

Pixel Masking and Selection

The algorithm has the arduous task of separating 'good' pixels from 'cloudy' pixels.  The standard M?D35 cloud mask includes using the brightness in the visible channels to identify clouds.  This procedure will mistake heavy aerosol as 'cloudy', and miss retrieving important aerosol events over ocean.  On the other hand, relying on IR-tests alone permits low altitude, warm clouds to escape and be misidentified as 'clear', introducing cloud contamination in the aerosol products. Thus, our cloud mask over ocean combines spatial variability tests (e.g. Martins et al., 2002) along with tests of brightness in visible and infrared channels. Underwater sediments have proved to be a problem in shallow water (near coastlines) as the sediments can easily have land-like surface properties. Thus, the sediment mask of Li et al. (2003) is used in addition to the cloud mask.

The algorithm sorts the remaining pixels that have evaded all the cloud masks and the sediment mask according to their r_0.86 value, discards the darkest and brightest 25%, and thereby leaves the middle 50% of the data.  The filter is used to eliminate residual cloud contamination, cloud shadows, or other unusual extreme conditions in the box. For the 10 km retrieval, at least 10 of the 400 pixels in the original box must remain after masking and filtering. If the minimum number of pixels is not met, no retrieval is attempted and fill values are given for the aerosol products in the retrieval box.

The ocean algorithm was designed to retrieve only over dark ocean, (i.e. away from glint). Glint angles of < 40˚ are masked. There is a special case where heavy dust can be retreived over glint, and the retrieval will be assigned a poor quality.

Ocean Aerosol Models

The ocean algorithm aerosol models were derived mainly from data gleaned from sun/sky photometers and from analysis of errors in the products from previous versions of the MODIS algorithm. In the C6 ocean algorithm there are four fine modes and five coarse modes described in the following tables.

Table 1: Refractive Indices, Number Median, Standard Deviation and Effective Radius for the aerosol modes used in the MODIS Lookup Table for the ocean algorithm.  Models 1-4 are fine modes and Models 5-9 are coarse modes.

Table 2: Spectral Extinction Coefficients for the aerosol modes used in the MODIS Lookup Table for the ocean algorithm.  Models 1-4 are fine modes and Models 5-9 are coarse modes.

Table 3: Spectral Single Scattering Albedos for the aerosol modes used in the MODIS Lookup Table for the ocean algorithm.  Models 1-4 are fine modes and Models 5-9 are coarse modes.

Table 4: Spectral Asymmetry Parameters for the aerosol modes used in the MODIS Lookup Table for the ocean algorithm.  Models 1-4 are fine modes and Models 5-9 are coarse modes.