## 1 Introduction  
Clouds play a crucial role in modifying the energy budget of the Earth-Atmosphere system, through their interaction with solar radiation and longwave radiation [1; 2; 3]. Both microphysical properties and macrophysical features of clouds determine cloud radiative effects. The primary microphysical parameter of clouds, the droplet size distribution, determines cloud optical properties, and in turn affects scattering and absorption. The horizontal cover and vertical structure of clouds result in additional impacts on cloud radiative effects. These two macrophysical features represent geometric properties of cloud systems on the large scale. A number of numerical simulations and satellite observations have proved that diverse cloud vertical structures greatly affect regional and the global radiation budget, and thus act to modify atmospheric circulation [4]. Accurate knowledge regarding the vertical structure of cloud systems and their global distribution is vital for quantitatively understanding cloud radiative effects. For single-layer clouds, the vertical structure simply manifests as the vertical distribution of liquid/ice water content or droplet size distribution inside clouds [5]. However, for overlapping cloud layers in the same atmospheric column, known as multilayered clouds, the vertical distribution of multiple cloud layers forms another structural feature. When compared with the varying cloud microphysical properties within a single cloud layer, these vertical structures originate from disconnected cloud layers and cloudless gaps. In such a multilayered cloud system, the altitude aspects of each cloud layer is described by the individual top and bottom height of each cloud layer, as well as their geometrical thickness [6]. Multilayered clouds are common in routine weather observations and they have been frequently recorded in many field experiments. The radiosonde is the major approach that was traditionally employed in ground-based meteorological sites to identify the overlapping appearances of multiple cloud layers [7]. Relying mostly on the vertical gradient of relative humidity, the top and bottom boundary of a cloud layer can be detected, and thus the occurrence of multilayered clouds is straightforwardly derived. Many facts regarding multilayered clouds have been revealed by these ground-based observations. By using data from weather observations, both over land and ocean, [PERSON] et al. [8] found that the existence of cirrus is often accompanied by alostratus at lower levels on a global scale, forming a very common appearance of a double-layered cloud system. On the contrary, there are rather few occurrences where alostratus and cumulus coexist. The occurrences of multilayered clouds are also found to be highly dependent on geographic regions [9]. For instance, the coexistence of upper cirrus and lower cumulonimbus occurs frequently in the tropics, while it infrequently occurs in the extratropical regions. [PERSON] et al. [9] conducted a comprehensive investigation on 20-years of radiosonde measurements and revealed that multilayered clouds account for about 42% of total cloud occurrences. Double-layered clouds were found to be the predominant form in multilayered systems, regardless of geographic regions. However, the representative domain by a radiosonde measurement is very limited. The drift of sounding balloon driven by upper strong winds may induce false overlapping of multiple cloud layers, and in turn lead to biases in the sampling of multilayered clouds. Moreover, the meteorological sites that deploy radiosondes are limited in number, only covering a very small area globally. Such a deficiency inevitably affects global-scale cloud observations, especially for the identification of multilayered cloud systems over oceans. Satellite remote sensing is suitable for spatially consistent cloud observations on the global scale [10]. For most spaceborne meteorological observations, scene classification or cloud detection is the first step in data processing flow.