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The cloud processing scheme APOLLO (AVHRR Processing scheme Over cLouds,Land and Ocean) has been in use for cloud detection and cloud propertyretrieval since the late 1980s. The physics of the APOLLO scheme still buildthe backbone of a range of cloud detection algorithms for AVHRR (AdvancedVery High Resolution Radiometer) heritage instruments. TheAPOLLO_NG (APOLLO_NextGeneration) cloudprocessing scheme is a probabilistic interpretation of the original APOLLOmethod. It builds upon the physical principles that have served well in theoriginal APOLLO scheme. Nevertheless, a couple of additional variables havebeen introduced in APOLLO_NG. Cloud detection is no longerperformed as a binary yes/no decision based on these physical principles. Itis rather expressed as cloud probability for each satellite pixel.Consequently, the outcome of the algorithm can be tuned from being sure toreliably identify clear pixels to conditions of reliably identifyingdefinitely cloudy pixels, depending on the purpose. The probabilisticapproach allows retrieving not only the cloud properties (optical depth,effective radius, cloud top temperature and cloud water path) but also theiruncertainties. APOLLO_NG is designed as a standalone cloudretrieval method robust enough for operational near-realtime use and forapplication to large amounts of historical satellite data. The radiativetransfer solution is approximated by the same two-stream approach which alsohad been used for the original APOLLO. This allows the algorithm to beapplied to a wide range of sensors without the necessity of sensor-specifictuning. Moreover it allows for online calculation of the radiative transfer(i.e., within the retrieval algorithm) giving rise to a detailedprobabilistic treatment of cloud variables. This study presents thealgorithm for cloud detection and cloud property retrieval together with thephysical principles from the APOLLO legacy it is based on. Furthermore acouple of example results from NOAA-18 are presented

APOLLO_NG – a probabilistic interpretation of the APOLLO legacy for AVHRR heritage channels