A Physical Method for the Calibration of the AVHRR/3 Thermal IR Channels. Part II: An In-Orbit Comparison of the AVHRR Longwave Thermal IR Channels on board MetOp-A with IASI

Obtaining stable and accurate satellite radiances for climate change research requires extremely high standards for satellite calibration. Many satellite sensors do not currently meet the accuracy criteria, especially heritage sensors such as the Advanced Very High Resolution Radiometer (AVHRR), which shows scene temperature dependent trends and biases of up to 0.5 K. Recently, however, a detailed study of the AVHRR/3 prelaunch data showed significant problems with both the calibration algorithm and the prelaunch data and indicated that the inherent accuracy of the AVHRR may actually be quite high. A new approach has been suggested that fixed many of the issues with the current (operational) calibration, but has not yet been applied to the in-orbit case. In this paper the behavior of the AVHRR in orbit is examined and compared to the operational AVHRR radiances from the Meteorological Operation (MetOp)-A with those based on the new calibration to radiances derived from the Infrared Atmosphere Sounding Instrument (IASI). It is shown that the current AVHRR calibration does indeed introduce large (0.5 K) biases, but these biases are remarkably stable. It is further shown that, with some modification related to differences between the prelaunch test environment and the in-orbit environment, a physically based AVHRR calibration can match IASI to better than similar to 0.05 K, which is an order of magnitude better than what is currently available. Finally, it is shown that, while the new calibration is capable of providing accurate and stable radiances for the nadir view, off-nadir biases of up to 1.5 K still exist at the largest zenith angles and at the coldest scene temperatures (similar to 210 K). For surface temperature determination, however, the scan angle bias is very small (<0.02 K), implying that the new AVHRR calibration will provide a significant improvement to, for example, sea surface temperature measurements, one of the Global Climate Observing System (GCOS)-designated essential climate variables.