CALIPSO IIR Version 2 Level 1b calibrated radiances: analysis and reduction of residual biases in the Northern Hemisphere
Version 2 of the Level 1b calibrated radiances of the Imaging Infrared Radiometer (IIR) on board the Cloud-Aerosol Lidar and Infrared Satellite Observation (CALIPSO) satellite has been released recently. This new version incorporates corrections of small but systematic seasonal calibration biases...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-04-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/11/2485/2018/amt-11-2485-2018.pdf |
| Summary: | Version 2 of the Level 1b calibrated radiances of the Imaging Infrared
Radiometer (IIR) on board the Cloud-Aerosol Lidar and Infrared Satellite
Observation (CALIPSO) satellite has been released recently. This new version
incorporates corrections of small but systematic seasonal calibration biases
previously revealed in Version 1 data products mostly north of
30° N. These biases – of different amplitudes in the three IIR
channels 8.65 µm (IIR1), 10.6 µm (IIR2), and
12.05 µm (IIR3) – were made apparent by a striping effect in
images of IIR inter-channel brightness temperature differences (BTDs) and
through seasonal warm biases of nighttime IIR brightness temperatures in the
30–60° N latitude range. The latter were highlighted through
observed and simulated comparisons with similar channels of the Moderate
Resolution Imaging Spectroradiometer (MODIS) on board the Aqua spacecraft. To
characterize the calibration biases affecting Version 1 data, a
semi-empirical approach is developed, which is based on the in-depth analysis
of the IIR internal calibration procedure in conjunction with observations
such as statistical comparisons with similar MODIS/Aqua channels. Two types
of calibration biases are revealed: an equalization bias affecting part of
the individual IIR images and a global bias affecting the radiometric level
of each image. These biases are observed only when the temperature of the
instrument increases, and they are found to be functions of elapsed time
since night-to-day transition, regardless of the season. Correction
coefficients of Version 1 radiances could thus be defined and implemented in
the Version 2 code. As a result, the striping effect seen in Version 1 is
significantly attenuated in Version 2. Systematic discrepancies between
nighttime and daytime IIR–MODIS BTDs in the 30–60° N latitude
range in summer are reduced from 0.2 K in Version 1 to 0.1 K in Version 2
for IIR1–MODIS29. For IIR2–MODIS31 and IIR3–MODIS32, they are reduced from
0.4 K to close to zero, except for IIR3–MODIS32 in June, where the
night-minus-day difference is around −0.1 K. |
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| ISSN: | 1867-1381 1867-8548 |