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Joint Atm. Issues (ATML2)

Tracking of known problems and subsequent fixes is an important issue for MODIS data users. This page will act as a repository of all known MODIS Atmosphere Data Product problems, as well as how to determine the problematic version (and the fixed version) of the HDF data -- therefore data users should check this page for updates regularly. Data Users unfamiliar with how to properly track problems and fixes by determining the version of their downloaded HDF files should refer to the documentation in the Introduction sublink (the top link along the LHS of this page).   

Note that a checkmark (✔) in the tables below means Data Issue or Quality Statement applies.  A blank means it's been fixed or does not apply.

 




Joint Atm. Data Issues: Table of Contents & Anchor Links  

(Note: Anchor Links point to a specific place within this page.)

 

Collection/Platform where Issues Apply 
Issue Impact

C6.1/Terra

C6.1/Aqua

C6.0/Terra

C6.0/Aqua

 
#1. Sampling Issue w Paired SSH files

L

✔ 

 
#2. Input L2 Problems propagate downstream

L

 

Joint Atm. Data Quality Statements: Table of Contents & Anchor Links  

(Note: Anchor Links point to a specific place within this page.)

 

Collection/Platform where Quality Statements Apply 
Quality Statement Impact

C6.1/Terra

C6.1/Aqua

C6.0/Terra

C6.0/Aqua

 
#1. C6 5km Across Swath Dimension change

Q

 ✔

 ✔

 
#2. C6 5km Lat/Lon change

Q

 ✔

 ✔

 

 




Issue #1: Issue with Sampling of "potentially paired" SSH files 

Impact Meter showing low levelsDescription: For some scientific studies, some users might wish to pair SSH files with ATML2 files. MOD02SSH files contain subsampled geolocation (latitude and longitude) and solar and sensor angles from the MOD03 files, as well as subsampled Earth view radiance/reflectance and uncertainty indexes from the L1B files. Due to a coding error in the SSH code, the subsampled geolocation in the SSH (which was incorrectly specified) does not match the subsampled geolocation in the ATML2 (which was correctly specified), a problem that affects both Terra and Aqua MODIS.

The ATML2 sampling (correctly) starts at column 3, row 4 in the 1km geolocation data, but the SSH sampling (incorrectly) starts at column 4, row 3. It appears that the sampling changes that were made for C6 to align the SSH and ATML2 sampling, and to sample the working Aqua detectors, did not get applied to the SSH geolocation SDSs. Users should also note that the solar and sensor angles (zenith and azimuth) in the SSH are sampled consistent with the SSH geolocation, and are thus also incorrect.

Advice to Users: When pairing SSH and ATML2 files, users should point to (use) the ATML2 for the correct geolocation (latitude and longitude) information, and ignore or bypass the geolocation in the SSH files. However, the angle SDSs are unfortunately only available in the SSH files. Because the sampling differences are only a single pixel (1km at nadir) in the x and y horizontal directions, this disconnect is not expected to have a significant impact on results obtained by pairing these two datasets, but it does exist and introduces a discontinuity.

PGE Versions & Production Dates of Problematic & Corrected Data:

Problematic Data  Corrected Data

PGE Versions

Production Dates

PGE Versions

Production Dates

All

All

Not Corrected

Not Corrected

Data Dates Affected: All data since launch.

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Issue #2: Problems in input 04_L2, 05_L2, 06_L2, 07_L2, and 35_L2 propagate into downsteam products 

Impact Meter showing low levelsDescription: See the Known Problem pages for the input Level-2 products 04_L2 (Aerosol), 05_L2 (Water Vapor), 06_L2 (Cloud), 07_L2 (Profiles), and 35_L2 (Cloud Mask) by clicking on the links to the left. Any problem in these upstream products will propagate downsteam.  Users should be aware of any issues in the upstream products.

Affected Platform: Aqua and Terra

Collections Affected: All Collections

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Quality Statement #1: 5km Cell-Across-Swath Dimension change from 270 to 271 starting in C6

Data Quality Issue logoDescription: It should be noted that in the native (input) Cloud (06_L2) and Profiles (07_L2) the 5x5 km parameters have the dimension Cell_Across_Swath_5km = 270. This dimension was a matching 270 in the C5 and C5.1 Joint Atmosphere L2 (ATML2) product.  However in the C6 and C6.1 ATML2 Product, that dimension was increased from 270 to 271 in order to match the dimension in the MOD02SSH input files, when information from that new input file was added to the Joint Atmosphere L2 product.  In this case the last element in that 271 dimension will always be fill for 5x5 km Cloud and Profile SDS's.  This is not a bug or an error, but simply a characteristic of the C6 and C6.1 data, which users should note. So the upshot is for any 5x5 km SDS (for example Cloud_Fraction) in the source 06_L2 file will have 270 columns (Cell Across Swath) -- however this "matching" SDS in ATML2 will have 271 columns (Cell Across Swath), with that extra column at the end being set to the Fill Value of "127" for byte SDS's and -9999 for short integer SDS's.

Last 5km scan-direction cell is only 4km wide (Frame #1351-1354). MOD02SSH populates this last 5km cell #271, while MOD06_L2 ends population at 5km cell #270. Note that the gold shaded 1 km cell shows the center geolocation grid, while the blue shaded 1km cell shows where the 5km subsample is taken for MODATML2 and L3 in order to avoid dead detectors.

 

Data Dates Affected: All data since launch.

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Quality Statement #2: 5km Lat/Lon change starting in C6

Data Quality Issue logoDescription: In Collection 6 (C6), a change was made in the Joint Atm L2 product (ATML2) 5x5 km Latitude and Longitude pairs. In C5 and earlier, ATML2 Lat/Lon pairs were copied directly from the 06_L2 product. These Lat/Lon pairs in 06_L2 correspond to the middle of the 5x5 km region, which is denoted by a starting position of column 3 and row 3 with increment 5 (detectors 3 and 8) in the 1x1 km grid space or domain. Beginning in C6 and moving forward, ATML2 Lat/Lon pairs were recalculated from MOD03 input (which contains geolocation at a 1x1 km resolution) to start in column 3 and row 4 with increment 5 (detectors 4 and 9). This allows the ATML2 Lat/Lon pair to match exactly the location where the standard 1x1 km L2 products (Cloud Optical Property SDS's in 06_L2, for example) were sampled (detectors 4 and 9). The upshot of this is -- though the sampled 1 km geophysical or science parameters in both a.) '06_L2 1km input sampled into 08_D3' and b.) sampled 1km science parameters in ATML2 correspond to detectors 4 and 9 and match exactly -- the geolocation (or Lat/Lon pair) is slightly offset between these two products.  In 06_L2, the 5km geolocation corresponds to detectors 3 and 8; while in ATML2, the 5km geolocation corresponds to detectors 4 and 9.  This is of minor consequence and causes only a tiny geolocation offset, but users should be aware of this anomaly when comparing 06_L2 geolocation with ATML2 geolocation. The most pronounced effect in L3 product differences (differences in L3 generated from 06_L2 vs. L3 generated from ATML2) is a.) at high latitudes, where the L3 1x1 deg. grid cells are physically smaller; and b.) in L3 1x1 deg. grids that have low L2 pixel count, where displacement of a few L2 input pixels has a larger relative effect.

A comparison of the Geolocation pixel used for 06_L2 (orange) and that used for ATML2 starting in C6 (blue). Note that the 1km geophysical or science parameter sampling location used for MODIS is detector 4 and 9 (blue), a shift done to avoid dead detectors.

Data Dates Affected: All C6 and C61 data since launch.

Additional Documentation (Sample Imagery):

Left panel shows Cloud Optical Thickness (all phases) generated from ATML2. Center panel shows Cloud Optical Thickness (all phases) generated from 06_L2. Right panel shows where differences are GT 0.2 (red), LT – 0.2 (blue), and within 0.2 tolerance (green).

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