Filtering cold outliers in SSTs retrieved from early AVHRRs for the second AVHRR GAC reanalysis

Global long-term SST record is being created under the AVHRR GAC Reanalysis (RANs) project, by historical reprocessing 4 km data of the AVHRR/2 and /3 instruments flown onboard multiple NOAA satellites from 1981 – present. The AVHRR data are reprocessed with the NOAA Advanced Clear-Sky Processor for Oceans (ACSPO) system. During RANs, the ACSPO algorithms are being adjusted to mitigate various issues intrinsic to AVHRR sensors, especially AVHRR/s on the earlier NOAA missions in 1980s and 1990s. The focus of the latest interim release of RAN2, Beta 02, is the contamination of retrieved SSTs with massive cold biases, originating from two sources. First, in all AVHRR missions, periodic cold SST outliers occur at night due to solar impingement on the black body calibration target, when the satellite orbit approaches the terminator. Second, multiple cold outliers appear in the NOAA-7, -11 and -12 SSTs following three major volcanic eruptions of Mt. El Chichon (1982), Mt. Pinatubo (1991) and Mt. Hudson (1991). The current mitigation algorithm exploits the fact that in both cases, the spatial densities of the cold outliers exhibit well-expressed latitudinal dependencies. The algorithm identifies 5° latitudinal bands with abnormally high density of outliers and makes the cloud mask more conservative within those bands. This improves filtering cold SST outliers in the contaminated areas without increasing the false cloud detection rate in the unaffected parts of the ocean. We also discuss the ongoing development to mitigate cold SST biases by correcting AVHRR L1b calibration (rather than eliminating the affected SST data).