Abstract
Abstract. The cloud processing scheme APOLLO (AVHRR Processing scheme Over cLouds, Land and Ocean) has been in use for cloud detection and cloud property retrieval since the late 1980s. The physics of the APOLLO scheme still build the backbone of a range of cloud detection algorithms for AVHRR (Advanced Very High Resolution Radiometer) heritage instruments. The APOLLO_NG (APOLLO_NextGeneration) cloud processing scheme is a probabilistic interpretation of the original APOLLO method. It builds upon the physical principles that have served well in the original APOLLO scheme. Nevertheless, a couple of additional variables have been introduced in APOLLO_NG. Cloud detection is no longer performed as a binary yes/no decision based on these physical principles. It is rather expressed as cloud probability for each satellite pixel. Consequently, the outcome of the algorithm can be tuned from being sure to reliably identify clear pixels to conditions of reliably identifying definitely cloudy pixels, depending on the purpose. The probabilistic approach allows retrieving not only the cloud properties (optical depth, effective radius, cloud top temperature and cloud water path) but also their uncertainties. APOLLO_NG is designed as a standalone cloud retrieval method robust enough for operational near-realtime use and for application to large amounts of historical satellite data. The radiative transfer solution is approximated by the same two-stream approach which also had been used for the original APOLLO. This allows the algorithm to be applied to a wide range of sensors without the necessity of sensor-specific tuning. Moreover it allows for online calculation of the radiative transfer (i.e., within the retrieval algorithm) giving rise to a detailed probabilistic treatment of cloud variables. This study presents the algorithm for cloud detection and cloud property retrieval together with the physical principles from the APOLLO legacy it is based on. Furthermore a couple of example results from NOAA-18 are presented.
Highlights
The cloud analysis tool APOLLO (AVHRR Processing Over cLouds, Land and Ocean) has been in use for more than 25 years
It has been developed for cloud detection from Advanced Very High Resolution Radiometer (AVHRR) observations (Saunders and Kriebel, 1988)
Cloud detection will first be cross compared with cloud detection results of the original APOLLO scheme, which has already been evaluated with SYNOP data for AVHRR and for Europe (Kriebel et al, 2003; Meerkötter et al, 2004)
Summary
The cloud analysis tool APOLLO (AVHRR Processing Over cLouds, Land and Ocean) has been in use for more than 25 years now. Necessary requirements like the introduction of a cloud droplet effective radius retrieval along with the optical depth estimation (Nakajima and King, 1990), the use of modern representations of ice cloud optical properties (Baum et al, 2014) and the requirement for more flexible cloud detection (Merchant et al, 2005; Holzer-Popp et al, 2013) motivated the development of a new cloud detection and cloud retrieval scheme based on the physical principles of APOLLO. We are fully aware that the scheme will not provide fully consistent results for different sensors due to the varying sensor characteristics of the AVHRR family and the differences in sensor design for other instruments At least it uses a similar mathematical framework for all sensors without introducing specific additional information from one channel or another which is not available from AVHRR.
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