Analyzing AVHRR thermal calibration using Sensor Stability for SST (3S) system

Recently, the National Oceanic and Atmospheric Administration (NOAA) performed sea surface temperature (SST) reanalysis (RAN1) from seven AVHRR/3s onboard NOAA-15 to -19 and Metop-A and -B, from 2002-present using operational L1b input. The time series of clear-sky ocean brightness temperatures (BTs) and derived SSTs were found to be unstable. While the SSTs were empirically stabilized against in situ SSTs, the measured BTs were left intact. To better understand the AVHRR calibration and stabilize its BTs, the Sensor Stability for SST (3S) system was established at NOAA, which monitors orbital statistics of the sensor measured blackbody temperatures (BBTs), blackbody counts (BCs), and the space counts (SCs), along with the derived calibration gains and offsets. Other factors affecting the calibration are also monitored, including the Sun and Moon position relative to the sensor, local equator crossing time, and duration of the satellite night. All AVHRRs show long-term and band-specific smooth changes in the calibration gains and offsets, which are occasionally perturbed by spurious non-monotonic anomalies. We argue that the operational quality control (QC) and calibration procedures are suboptimal and should be improved. More stringent QC should be applied, and scan lines with bad calibration should be filled in by interpolation from the best parts of orbit or satellite lifetime. Work is underway to redesign the AVHRR QC and calibration algorithms, reprocess L1b, and use in the subsequent ACSPO SST RANs.