The MODIS Atmosphere Daily Global HDF product is produced in a multi-step process. Because of memory constraints and system limitations, the 1 by 1 degree global lat-lon grid (180 by 360 pixels) must be split into 36 five-degree (latitude) tiles (5 by 360 pixels) for processing. This tiling procedure is accomplished through the MOD_PR08TC and MOD_PR08T process. Finally, the global grid is generated and populated using the MOD_PR08DC and MOD_PR08D process. Detailed descriptions of the programs are given below. Note that Aqua processes use the prefix MYD instead of MOD; otherwise the two are identical.

Tile Structure File Creation (MOD_PR08TC)

The MOD_PR08TC process creates an HDF structure file that contains the global attributes, 400 (empty) Scientific Data Sets (SDS's) and their associated local attributes for a particular five-degree latitude band (tile). This process must be run 36 times to produce HDF structure files for each five-degree tile that span the globe. Note that after the creation of the 36 tile structure files, this process never needs to be rerun, since the tile structure files can be saved and reused.

Tile File Aggregation and Population (MOD_PR08T)

The MOD_PR08T process reads all the Level-2 input granule files that intersect a particular tile, the appropriate statistics are computed, and the data in placed in the appropriate Scientific Data Set (SDS) grid cell in the tile structure file of interest. This process has to be executed once for each tile, each data day. Also note that this process is, by far, the most lengthy and complex of the four.

Global Structure File Creation (MOD_PR08DC)

The MOD_PR08DC process creates a global HDF structure file that contains the global attributes, 400 (empty) Scientific Data Sets (SDS's) and their associated local attributes. The global structure file and the tile structure files are almost identical, the only differences are the Scientific Data Set (SDS) arrays are 180 by 360, instead of 5 by 360 pixels.

Global File Population (MOD_PR08D)

The MOD_PR08D process copies the 36 five-degree tile files to the global HDF structure file.

Shared Source Code

Finally, some of the subroutines and functions are shared between all of the Level-3 processes.