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

Anne Garnier,Jacques Pelon,Noëlle A Scott,Pascale Ferrage,Thierry Trémas,Nicolas Pascal,Delphine Nobileau,Kam-Pui Lee,Lydwine Gross-Colzy

doi:10.5194/amt-11-2485-2018

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.

Highlights

Since 2006, the Cloud-Aerosol Lidar and Infrared Satellite Observation (CALIPSO) satellite (Winker et al, 2010) has been providing a quasi-3-D description of the atmosphere with vertically resolved cloud and aerosols properties from the Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP), complemented by passive observations in the thermal infrared atmospheric window from the Imaging Infrared Radiometer (IIR) and in the visible spectral range from the Wide Field of view Camera (WFC)
Following a similar approach to that developed for previous infrared instruments, we find that, for each channel, R can be converted to brightness temperature (BT) using the equation
The standard deviation of the inter-channel brightness temperature differences (BTDs) is reduced by 40 % from 0.2 K in Version 1 to 0.12 K in Version 2 for the IIR1-IIR3 pair, and by 30 % from 0.26 to 0.18 K for the IIR2-IIR3 pair

Summary

Introduction

Since 2006, the Cloud-Aerosol Lidar and Infrared Satellite Observation (CALIPSO) satellite (Winker et al, 2010) has been providing a quasi-3-D description of the atmosphere with vertically resolved cloud and aerosols properties from the Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP), complemented by passive observations in the thermal infrared atmospheric window from the Imaging Infrared Radiometer (IIR) and in the visible spectral range from the Wide Field of view Camera (WFC). A. Garnier et al.: CALIPSO IIR Version 2 Level 1b calibrated radiances. The IIR calibrated radiances are reported in the IIR Level 1b product (Vaughan et al, 2017), where they are registered on a 1 km resolution grid centered on the CALIOP ground track, with a 69 km swath. The calibrated radiances are often expressed in terms of equivalent brightness temperatures computed using Planck’s law and the relevant instrument spectral response functions (e.g., Weinreb et al, 1997; EUMETSAT, 2012a)

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Conclusion