Physical analysis and recalibration of MetOp HIRS using IASI for cloud studies

Abstract

The HIRS long‐wave CO2 channels (702 cm−1, 716.5 cm−1, 731.7 cm−1, and 748.8 cm−1) have been used by scientists for estimating cloud height and amount. However, spectral and radiometric biases in these HIRS channels significantly affect the accuracy and consistency of the level‐1b data used. This study utilizes the MetOp IASI measurements to calibrate the long‐wave CO2 channels of the HIRS on the same satellite. The radiance bias characteristics are displayed for a sample MetOp orbit. Detailed analysis is performed in an attempt to find the causes of the bias. The analysis consists of three steps. First, a thermal‐equilibrium analysis of blackbody using the HIRS window channel reveals the blackbody of MetOp HIRS has a 0.14 K bias. Then, the effective Spectral Response Function (SRF) corrections are estimated for the three long‐wave CO2 channels by minimizing the orbital bias variation, which is −0.13 cm−1 for channel 4, 0.09 cm−1 for channel 5, and −0.15 cm−1 for channel 7. Channel brightness temperature dynamic range is found to be important for separating SRF versus nonlinearity effects. Finally, the nonlinearity effect is evaluated and the nonlinearity term is derived to reduce the orbital mean biases toward zero. The improved radiometric and spectral recalibration parameters are validated using 16 months of independent MetOp data, with very positive results, showing consistent performance. It is found that the HIRS biases relative IASI are generally within 0.1 K after the recalibration.