## 5 Conclusions  
The \(i\)Quam QC, as well as other in situ QC systems, relies on comparing individual in situ SST measurements with reference SST obtained from L4 analyses. The challenge with such comparisons is that true significant deviations from reference SST, caused by diurnal warming or degraded feature resolution of the reference SST in the dynamic zones of the World Ocean, can be screened out. The Diurnal Reference Check (DRC), proposed in this study, mitigates the overscreening problem for drifting and tropical moored buoys, whose sampling frequency is sufficient for identifying and estimating the diurnal signal. To optimize the DRC performance in the presence of a significant diurnal signal in in situ SSTs, the SST records from individual buoys were subdivided into 24-h daily segments, and each segment was characterized by the median nighttime SST and the amplitude of a diurnal signal. This allowed separate optimization of filtering individual SST outliers from filtering daily segments affected by significant systematic errors or extraneous factors, such as the overheating of the buoys' hulls. The main DRC features and thresholds have been determined from the analyses of the statistics of deviations of in situ SSTs from reference (L4) SST, median nighttime SSTs, and satellite 'Subskin' SSTs averaged over the period 2012-2018. The matchups with satellite 'Subskin' SST, used in this analysis, came from long-term full-mission historical reprocessings (RANs) of the data of satellite radiometers AVHRR GAC onboard NOAA-15/18/19, AVHRR FRAC onboard Metop-A/B/C, VIIRS onboard S-NPP/NOAA-20, and MODIS onboard the Terra/Aqua satellites with the NOAA ACSPO system. Based on the analysis of the statistics of in situ SST versus 'Subskin' SST, the upper threshold for the amplitude of the diurnal signal in the water temperature was set at 5 K. The diurnal segments, in which the amplitudes of the diurnal signals exceed the threshold, are rejected, assuming that these unrealistic amplitudes are mainly caused by extraneous factors. It was found that in the presence of significant diurnal signals, the statistics of deviations of in situ SST from reference SST are less stable than the corresponding statistics of deviations from median nighttime SST. Therefore, the individual in situ SSTs are checked for outliers by comparison with the median nighttime SSTs rather than reference SSTs. This stabilizes the results of screening the individual outliers. The reference L4 SST is used in the DRC only to identify the daily sets affected by significant systematic errors (calibration trends, 'gross' errors, etc.) by comparison with median nighttime SST. The thresholds used in this comparison are geographically dependent, being set proportional to the local spatial/temporal variability of the reference SST estimated on a monthly basis. The coefficient of proportionality was derived from the matchups with 'Subskin' SST such that to limit the maximum systematic error at \(\pm 1\) K.  
The previous studies [5; 9] suggested that the overscreening caused by comparing in situ SST with reference SST during diurnal warming events can be mitigated using models or reference SST data sets that capture the diurnal warming cycle. The DRC does not require such information because the reference SST is used only for comparison with the median nighttime SST, which, in turn, is used as a reference in filtering individual in situSST outliers. The DRC would rather benefit from the availability of the L4 analysis derived solely from nighttime data because assimilation of daytime SSTs, even after excluding those most affected by the diurnal warming, introduces additional 'noise' in the deviations between the median nighttime and reference SSTs. We compared the results of the experimental QC, which includes the DRC along with a number of the heritage iQuam QC checks, with the ones of the current iQuam QC.