CALIPSO IIR Version 2 Level 1b calibrated radiances: analysis and reduction of residual biases in the Northern Hemisphere

Abstract

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.

CALIPSO (IIR–CALIOP) retrievals of cirrus cloud ice-particle concentrations

“…As mentioned, the cirrus clouds sampled in this study are optically thick (0.3 < OD < 3.0) and contribute a relatively small fraction of total cirrus cloud coverage according to some cirrus cloud climatologies (e.g., Kienast-Sjögren et al, 2016;Goldfarb et al, 2001;Immler and Schrems, 2002). Thus, it is possible that the cirrus cloud properties retrieved here are not representative of cirrus clouds in general.…”

author comments to Referee #1

“…As mentioned, the cirrus clouds sampled in this study are optically thick (0.3 < OD < 3.0) and contribute a relatively small fraction of total cirrus cloud coverage according to some cirrus cloud climatologies (e.g., Kienast-Sjögren et al, 2016;Goldfarb et al, 2001;Immler and Schrems, 2002). Thus, it is possible that the cirrus cloud properties retrieved here are not representative of cirrus clouds in general.…”

“…Version 3 CALIOP cloud extinction coefficient profiles are used for some of the corrections. IIR calibrated radiances are from the recently released Version 2 Level 1b products (Garnier et al, 2018). These improvements will be implemented in the next Version 4 of the IIR Level 2 products.…”

The thermal infrared optical depth of mineral dust retrieved from integrated CALIOP and IIR observations

“…Recent updates of the calibration corrected the residual systematic bias in the northern hemisphere. A detailed introduction of the IIR instrument and its calibration process is shown in Garnier et al (2018). The noise equivalent differential temperature and calibration accuracy was assessed by the Centre 180 National d'Etudes Spatiales (CNES) (Garnier et al 2012).…”