MODIS LWIR CO2 Band Radiometric Performance

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MODIS LWIR CO2 Band Radiometric Performance

• Terra MODIS L1B validation
• Aqua MODIS L1B validation
• L1B radiometric bias application in MOD06
• What are we learning?

Moeller1, Frey1, Tobin1, MCST2 1.Univ.
Wisconsin 2. MODIS Characterization Support
Team January 4, 2006
Presented at MCST Calibration breakout meeting,
January 04, 2006
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MAS and SHIS data sets collected on the NASA ER-2
aircraft have been key for directly assessing
Terra and Aqua MODIS L1B accuracy.
Terra MODIS 11 um 04/01/01
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The Lake Tahoe 2004 field activity evaluated
Terra MODIS radiometric performance.
3.7um
3.9um
CO2
11 12
4
wv
CO2
11um
12um
Detector averaged
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SAFARI-2000 field experiment objectives included
Terra MODIS radiometric performance.
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Possible Sources of Radiometric Bias(Cold Scenes)

• PC band optical leak 0.5 - 1 K
• Spectral characterization up to 1 K
• Terra MODIS RSR in ambient
• Aqua MODIS RSR in T/V
• Calibration coefficients 0.5 K
• Set A00 for Terra LWIR CO2 bands
• Scan Mirror Reflectance lt 0.3 K
• DSM characterization
• External Cal/Val standard lt 0.3 K

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Optical leak region
Normalized Response
Position on Focal Plane
Courtesy of MCST
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PC Band Optical Leak Crosstalk Impact Simulation
Simulated B35
Simulated Radiometric Impact on B35
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Courtesy of MCST
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B36 transmittance from sfc to TOA is effectively
0.
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Baja
11um
14.3um
Pre launch correction
A known optical leak at 11um caused the image of
the Baja peninsula to be present in MODIS 14.3um
data. Through testing, the pre-launch correction
coefficients were revised, removing the
contamination.
14.3um
Post launch correction
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MODIS RSR PFM solid lines FM-1 dashed
Response or Transmittance
Wavelength (um)
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PFM FM-1 (K)
Wavelength (um)
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TX-2002 Experiment Assess Aqua MODIS Cal/Val
Detector dependent (atmospheric bands)
Detector dependent (window bands)
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Mean brightness temperature differences and
uncertainties for 6 September 2002 (blue) and 18
February 2004 (red).
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Radiometric Bias and L2

• Level 2 MOD06 product relies on LWIR CO2 bands
• Test Radiometric Bias Correction on Cloud Top
  Pressure (CTP) product of MOD06

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Aqua MODIS B36
Aqua MODIS B31
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Aqua MODIS Collect 5 CTP
Before subtracting Radiometric Bias
After subtracting Radiometric Bias
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Aqua MODIS B31
Aqua MODIS B36
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MODIS Band Pair Representing the Best CTP
Retrieval
Pink, cyan, and green are 36/35, 35/34, 34/33,
respectively
Chosen band pair for retrieval is more
physically consistent
Before subtracting Radiometric Bias
After subtracting Radiometric Bias
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Terra MODIS B31
Terra MODIS B36
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Terra MODIS Collect 5 CTP
Before subtracting Radiometic Bias
After subtracting Radiometric Bias
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What Are We Learning

• MODIS LWIR CO2 bands have shown out-of-spec
  radiometric behavior.
• Aqua MOD06 Cloud Top Pressure results are
  qualitatively improved by applying a radiometric
  bias based on AIRS and ER-2 based comparisons to
  MODIS L1B radiances.
• Early testing of Terra MOD06 CTP suggests that it
  also benefits from applying a radiometric bias.
• We are left with a question of why the
  radiometric bias apparently exists in MODIS.
  There are suspects, but no conviction.

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Backup
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CO2-slicing equation
Rcld(?1) - Rclr(?1) N?1
?pcld ?(?1)dB(?1) dp LHS ----------------------
- ------------------------------- RHS
Error Rcld(?2) - Rclr(?2)
N?2 ?pcld ?(?2)dB(?2) dp where
?1and ?2 are MODIS bands 34/33, 35/34, or 36/35
Traditionally, HIRS, GOES products use
observations on LHS RHS is calculated from NCEP
1-degree T(p), q(p), and forward radiance
model. For MODIS, Rclr(?1), Rclr(?2) are
calculated, adding an error term on the LHS. To
mitigate this, we apply clear-sky radiance
biases, as functions of geographic region and
band (Collection 5). Work on the CHAPS
(Collocated HIRS and AVHRR Products) algorithm in
the mid-1990s indicated close agreement between
CTP results when calculated clear-sky radiances
were used with bias corrections.