Any observed AVHRR pixel has been said to be cloud-contaminated or fully cloudy, if a sufficiently large number of bits are added in the second run (see Saunders and Kriebel, 1988 Kriebel et al., 2003). The succession of the respective tests fed into a bit- adding scheme. Two of these tests were run twice with updated information. Moreover water clouds can be identified by their reflectance ratio between two solar channels (being close to 1) as well as by differential absorption at two wavelengths in the infrared window. The physical ideas behind the five APOLLO cloud tests are that cloud tops are cold, bright or inhomogeneous or a combination thereof. The five tests include the infrared gross temperature test, the dynamical visible cloud test, the spatial coherence test, the reflectance ratio test and the brightness temperature difference test. The original APOLLO cloud detection is based on five consecutive threshold tests, for which the thresholds are determined dynamically from the analyzed scene. 5, a discussion of the algorithms and corresponding results in Sect. 4 describes the retrieval of physical cloud properties subsequent to the cloud detection. Section 3 deals with the detection of snow and its discrimination from clouds while Sect. Section 2 introduces the APOLLO_NG cloud detection algorithm. Re- sults from different sensors will thus be at least comparable, although they are not absolutely consistent. But 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. 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.
The AVHRR channel terminology is used, i.e., channel numbering from 1 to 5 with channel 1 referring to a red channel centered at 0.6 μm and channel 5 referring to an IR channel centered at 12 μm (see Kriebel et al., 2003). Consequently, the scheme will be called APOLLO_NextGeneration (or APOLLO_NG throughout this article).
Log in to apollo cloud update#
Consequently, we feel it is justified to still call the method APOLLO although it is not exactly an update of the existing algorithm but rather a new approach using the same physical ideas.
Log in to apollo cloud full#
The scheme is specifically designed to be applied to the full AVHRR series, which lacks the information of additional channels.
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