MODIS Ocean Products
This page will provide a brief description of the MODIS ocean product parameters followed by a description of how the parameters are named within the MODIS aerosol file. For an overview of the MODIS aerosol product go to the products page.
The most important and frequently used parameters within the ocean products are aerosol optical depth (AOD), fine mode fraction, and angstrom exponent.
The dimensions of the level 2 files 10 km files are 204 (along swath) x 135 (across swath). This represents the number of 10 km pixels in the array. In actuality the product is 10 km x 10 km near nadir but the pixels expand in the across track direction. The further from nadir the greater the expansion due to the "bow tie" effect.
Aerosol Optical Depth (AOD)
The aerosol optical depth (AOD or τ; also called aerosol optical thickness or AOT) refers to the optical loading of the aerosols in the atmospheric column. It is most closely related to the total surface area of the aerosol. AOD is spectrally dependent, meaning it varies by wavelength. MODIS ocean AOD are reported in seven wavelengths: 0.47, 0.55, 0.65, 0.86, 1.2, 1.6 and 2.1 µm. AOD is a unitless value.
For collection 5 the estimated error for the ocean AOD product is +/- (0.03 τ + 5%). This estimate is for 0.55 µm. These are global estimates from the entire data set. Errors in the product vary from region to region and season to season. For a more complete description of these errors see our “Validation” section. For collection 6 the estimated error is (-0.02 τ - 10%), (+0.04 τ+10%). Quality (QA) flags are an indicator of the algorithm team’s assessment of the quality of the data for AOD. QA flag values range from 0 – 3 where 0 is lowest quality. For ocean products we advise using anything above QA 0.
Aerosol Size Parameters:
Fine Mode Fraction
In general atmospheric aerosols occur in a bimodal distribution and the dark-target algorithm operated under this assumption. The smaller particles are referred to as the fine mode or accumulation mode aerosols. These particles have radii between 0.1 and 0.25 microns. The larger particles comprise the coarse mode. These particles generally have radii between 1.0 and 2.5 microns. The aerosol fine mode fraction is the proportion of fine mode aerosols to the total. This is an optical measurement of the proportion by volume.
In collections 5 and 6 the ocean product contains parameters which report fine and coarse mode AOD separately and the fine mode fraction.
The Angstrom Exponent is often used as a qualitative indicator of mean particle size. As a rough guideline angstrom exponent values in the range of 2 indicate small particles which might be associated with pollution or biomass burning. Values in the range of 1 or less indicate the presence of large particles such as sea salt or dust. For the MODIS algorithm angstrom exponent is not a true measurement but is a derived value.
In Collection 6, the preliminary estimated error for angstrom exponent is 0.45; pixels with an AOD > 0.2 are expected to have a more accurate angstrom exponent.
MODIS File Information
MODIS files are available in HDF format. In this file format the data as well as the meta data (file specificiations that allow for reading and use of the data) are contained in a single file.
Almost all programming languages have the ability to read HDF formatted files.
Each parameter within a MODIS file is referred to as a "Scientific Data Set" (SDS). This section will describe the most often used SDSs for the ocean product.
A full listing of the SDSs for the collection 5.1 files can be found on the Modis-atmos website. The list shows the name as it is written within the file and a brief description of its contents. The parameter names are written as they should be referenced in any software code. A much more detailed description of each SDS including variable type can be found on the file spec pages of the Modis-atmos website.
Scientific Data Sets For Ocean
Some general notes:
Best and Average SDS: The MODIS dark-target ocean algorithm produces theorectical reflectances by iterating through 20 different combinations of aerosol models and amounts of aerosol and compares these to the measured reflectances to see which solutions fall within pre-defined error bounds. All acceptible solutions are averaged and that information is provided in the "Average" SDS. The single best solution with the lowest error is used for the "Best" SDS. We recommend using the Average SDS.
Ocean, Land and Ocean, Land SDS: Several parameters provide data for just ocean, just land or combined land and ocean. There will often be differences in which data is included in the individual ocean or land parameters and the combined land and ocean parameters.
Unless otherwise noted these parameters are the same for the 10 Km and 3 Km products.
Description of Scientific Data Sets (SDS) for Ocean Products
Effective_Optical_Depth_Average_Ocean - The AOD over ocean in 0.47, 0.55, 0.67, 0.87, 1.2, 1.6 and 2.1 µm. Values range from -0.01 to 5. Although it is not physically possible to have less thn 0 AOD negative values are allowed in the retrieval to avoid bias for overall statistical analysis. Data of all QA levels is included in this SDS.
Land_Ocean_Quality_Flag - An indicator of the quality of the AOD retrieval. Values range from 0 - 3
Optical_Depth_Land_And_Ocean - The AOD over land and ocean at 0.55µm. This SDS includes data from only recommended (high quality) QA of 3 for land and 1,2 or 3 for ocean.
Image_Optical_Depth_Land_And_Ocean - The AOD over land and ocean at 0.55µm. This SDS includes data for all QA values from 0 - 3. The suggested use of this SDS is only to produce imagery to fill in the greatest area.
Optical_Depth_Ratio_Small_Ocean_0.55micron - Fraction of AOD at 0.55µm contributed by fine mode aerosol for the average ocean solution. Values range from 0 (all coarse mode aerosol) to 1 (all fine mode aerosol).
Angstrom_Exponent_1_Ocean - Calculated angstrom exponent using the 0.55 and 0.86 µm channels for the average ocean solution.