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Cloud Mask Issues (35_L2)

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.




Cloud Mask 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 Anchor Link
#1. False Clouds over Ocean (Band 29)

H

 

 

 

Link

#2. Detection Prob. w. Antarctica Clouds

L

Link

#3. Band 27 anomalies impact CldMask

L

 

 

 

Link

Cloud Mask 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 Anchor Link
#1. Missing Granules Aug 2017

Q

 

 

 ✔

 

Link

#2. GDAS Change Impacts

Q

Link




Issue #1: MOD35 (Terra Cloud Mask) False Clouds over Ocean (Impact of the MODIS Terra Band 29 (8.6 µm) radiometric calibration drift on cloud products)

Description (Updated Information): The Terra spacecraft and instruments went into safe hold from 2016-02-18 through 2016-02-22. On 2016-04-08, Terra MODIS Collection 5 (C5) and Collection 6 (C6) Atmosphere forward processing and distribution via LAADS was restarted from data day 2016 059 (Feb 28th 2016). The MODIS Characterization and Support Team (MCST) has made post-safe mode radiance calibration adjustments to the L1B (MOD02) code. However, degraded detector performance in Terra MODIS Band 27 (6.7 um) and Band 29 (8.6 um) necessitated algorithm changes to the L2 cloud mask (MOD35) and the L1B de-striper code used in the production of the L2 atmospheric profile (MOD07/05) and cloud-top (MOD06) products. Specifically, cloud mask thresholds were adjusted to remove false positives for the 6.7 um brightness temperature (BT) test for high clouds and the 8.6-11 um BTD test over the ocean, while reference detectors in the de-striper code were modified to avoid noisy detectors.  Users are advised that further testing is needed to re-validate the current forward stream for the above products (and also down stream products, e.g., MOD06 cloud optical properties and Level-3 Daily, Eight-Day, and Monthly) and to document the impact (if any) of these combined sensor and algorithm changes on the use of MODIS Terra atmosphere team products for climate studies. We previously noted that Band 29 detector crosstalk degradation resulted in artificially high cloud cover over tropical oceans in recent years. The post-safe mode adjusted threshold for the 8.6-11 um BTD test is expected to reduce the impact on cloud fraction. Therefore, the climate continuity record is broken, specifically cloud mask (i.e., fractions) and to a varying extent the downstream products such as cloud top, IR phase, and optical properties.

Description (Initial Detection): In early 2015, it was noticed that recent Terra Collection 6 (C6) cloud top pressure products (MOD06) contained ocean regions with an anonymously high incidence of low clouds. This was traced to false clouds being reported by the cloud mask (MOD35). In turn, this was found to be the result of errors in the b29-b31 brightness temperature difference (BTD) high (ice phase) cloud test. As a result, a substantial number of clear pixels were being flagged as cloudy by this test. The text that follows is a summary of the impact of the MODIS Terra Band 29 (8.6 µm) radiometric calibration drift on cloud products. MOD35 includes a spectral test using the brightness temperature difference (BTD) between band 29 (8.6 µm) and band 31 (11 µm), referred to here as the b29-b31 BTD test. This test is used only over water surfaces. The root problem is a gradual warming in the Terra MODIS b29 channel over the years, that is not being captured by the on-board calibration systems and apparently related to crosstalk issues. Monthly time series trend analysis of Terra MODIS cloud products indicates that the significance of the impact may extend back to 2010. In lieu of a successful fix to the L1B algorithm for b29, an adjustment to the 8.6-11 µm MOD35 test threshold was delivered to the production test system to quantify the impact. Testing of this potential fix in the data is ongoing as of September 2015. A fix should eventually be implemented, at which time affected data will be rerun (corrected) and made available. See the PDF document linked below for for detailed information and imagery.

MODIS Terra Band 29 (8.6 micron) impact of calibration drift on cloud products (PDF)

PGE Versions & Production Dates of Problematic & Corrected Data:

Problematic Data Corrected Data
PGE Versions Production Dates PGE Versions Production Dates
All ≥ 2010 001 (1/1/2010) Correction Testing Underway Correction Testing Underway

Data Dates Affected: January 2010 and on (but worse 2014 and on)

Affected Platform: Terra Only

Collections Affected: Collection 6.0

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Issue #2: Cloud Mask not detecting high thick clouds over Weddell Sea (Antarctica) when ice covered

Description: In collection 006, the cloud mask is failing to detect high thick clouds over the Weddell Sea area in Antarctica when it is ice-covered. This represents a small area over very dry conditions. This problem might be fixed in a future PGE03 delivery, until then users should be aware of this shortcoming.

PGE Versions & Production Dates of Problematic & Corrected Data:

Problematic Data Corrected Data
PGE Versions Production Dates PGE Versions Production Dates
≤ PGE03 v6.0.40 All Not Corrected Not Corrected

Data Dates Affected: July 2002 to present

Affected Platform: Aqua Only

Collections Affected: Collection 6.0 and Collection 6.1

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Issue #3: Occasional Band 27 Anomalies impact Cloud Mask Parameter

Description: An anomaly was noted sometime after the calibration change of Terra C6.1 (noted by Chris Moeller, UW). The cloud mask uses band 27 in a high cloud test that inadvertently flags many of the pixels at the end of scan (1349-1354) as being cloudy because of anomalous BTs.  (Note that intermittent edge-of-scan anomalies in Band 28 and Band 30 have also been recently noted. The anomalies in Band 28 match the location of anomalies in Band 27; however in band 30 the anomalies seem to switch to the first few pixels at the beginning of a scan 1-5 instead of at the end of a scan 1349-1354).  This band 27 issue propagates into the Cloud Mask and then into the Cloud Fraction from Cloud Mask and then into many Cloud Top Property parameters in L2.  Its difficult to see in L3 due to coarser resolution and orbit overlap.  It does not appear that this issue propagates into Cloud Optical Property parameters at L2.  Figure 9a below shows an unmapped granule image of b27 BTs and the b27 cloud mask test.  Figure 9b shows mapped impacts in downstream Cloud Top Property products (Date Imaged 2018 148).

PGE Versions & Production Dates of Problematic Data:

Data Dates Affected: Shows up intermittently in all time periods (from launch)

Affected Platform: Terra

Collections Affected: Collection 6.1 only (Cloud Mask and Cloud Top Property parameters at L2)

Additional Documentation:

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Quality Statement #1: Missing Terra Data Granules between 5 Aug 2017 and 6 Aug 2017

Description: Following the Terra Deep Space Calibration on August 5th, 2017 (2017 217) there were some faulty C6 Terra MODIS Atmosphere data as the focal plane temperatures did not return to nominal values until the day after - August 6th, 2017 (2017 218) 1420 UTC. So the granules for the time period between 2017 217 23:00 UTC and 2017 218 14:15 UTC were removed before the downstream Atmosphere processing. Data users will notice missing MODIS Atmosphere Data during this period for all Level-2 (L2) and Level-3 (L3) Daily atmosphere products.

Data Dates Affected: Note there will be Missing Terra Data between 2017 217 (5 Aug 2017) at 23:00 UTC and 2017 218 (6 Aug 2017) at 14:15 UTC for all MODIS Atmosphere L2 and L3 Daily products.

Here is a sample L3 animation of Scattering Angle for 6 Aug 2017, which shows the missing data issue fairly clearly.  The animation loops between the C6.0 data and the C6.1 data. The C6.1 data is improved showing less missing data, however a gap still exists in C6.1


  C6.0 vs C6.1 comparison of missing data gap (slight improvement shown in C6.1)

Affected Platform: Terra Only

Collections Affected: Collection 6.0 and Collection 6.1 (some improvement noted in C6.1)

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Quality Statement #2: GDAS Change Impacts

Description: The GDAS Model changed in January 2015, so for both Aqua and Terra MODIS products users should note:

  1. For most daytime granules, there will be no changes in the cloud mask product (MOD35) due to the new GDAS model output. There will be more changes in nighttime data due to surface temperature and total precipitable water differences but we anticipate no biases due to the new GDAS inputs.
  2. We also anticipate no significant changes to the 5-km and 1-km cloud top pressure product (MOD_PR06CT). Most changes found in the Level 2 cloud top pressures are one atmospheric pressure level that seem to be randomly distributed.

Affected Platform: Aqua and Terra

Collections Affected: Collection 6.0 and Collection 6.1

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