MODIS Land Products
This page will provide a brief description of the MODIS land product parameters followed by a description of how the parameters are named within the MODIS aerosol file. The MOD04 Level 2 product contains data from Dark Target (DT) and Deep Blue (DB) land algorithms as well as a merged DT - DB product. The DB product is described on the Deep Blue website. The merged product parameters are described on our Merged Products page.
The most important and frequently used parameter within the land product is the aerosol optical depth (AOD). Fine mode AOD over land is no longer included in the collection 6 product. We still include fine mode fraction in Collection 6 but it is not recommended for use. See details below.
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 full 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 land AOD are reported in seven wavelengths: 0.47, 0.55, 0.65 µm. AOD is a unitless value.
For collection 5 and 6 the estimated error for the land AOD product is +/-(0.05 τ + 15 %). 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. The Collection 6 Terra product will be analyzed and error estimates made shortly after its release. We expect it to be the same as Aqua.
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 land products we advise using only QA 3.
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.
For the land product Fine Mode Fraction is considered a diagnostic parameter for how the algorithm is performing. It does not give a useful measure of the proportion of fine mode aerosol. We recommend strongly that this parameter not be used to determine the fine mode fraction of aerosol over land.
Fine Mode Aerosol Optical Depth
This parameter is no longer included in the colleciton 6 product for the land retrieval
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 collection 5.1 and 6.0 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 collection 5.1 file spec page or collection 6 file spec page of the Modis-atmos website.
Scientific Data Sets For Ocean
Unless otherwise noted these parameters are the same for the 10 km and 3 km products.
Corrected_Optical_Depth_Land - The AOD over ocean in 0.47, 0.55, 0.67 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.
10 km SDS dimensions 203 (along track) x 135 (across swath) x 3 (0 - .47, 1 - 0.55, 2 - 0.67 µm)
3 km SDS dimensions 676 (along track) x 451 (across swath) x 3 (0 - .47, 1 - 0.55, 2 - 0.67 µm)
Land_Ocean_Quality_Flag - An indicator of the quality of the aerosol optical depth retrieval.
Values range from 0 - 3.
10 km SDS dimensions 203 (along track) x 135 (across swath)
3 km SDS dimensions 676 (along track) x 451 (across swath)