## 4 Concluding Remarks and Discussion  
The overlapping of multiple cloud layers is rather frequent on the global scale, which has substantial impacts on the estimation of cloud radiative forcing [17]. In addition, these distinctive cloud appearances can be also used as effective indices for evaluating numerical models [21]. The low-level warm clouds act as a major component in generating the multilayered cloud system and are thus very likely to be accompanied by upper clouds, either mixed-phase or ice ones. The warm clouds are generally hidden from the top-down view of satellite since only the topmost cloud layer can be detected in spaceborne passive sensors, leading to incomplete sampling of warm cloud layers by satellite remote sensing. By using CloudSat radar data with fine vertical resolution, this study takes into account all the warm cloud layers, with special attention to those embedded in multilayered systems. The occurrences of warm cloud events overlapping were quantified and the geometric features of the several kinds of warm cloud layers were examined. It is found that there are three types of cloud systems that involve warm cloud layers, including warm single layer clouds, cold-warm double layer clouds, and warm-warm double layer clouds. The double layer clouds account for 23%, while in the double layer occurrences warm-warm double layer subsets contribute about 13%. Three oceanic regions are found to have very few occurrences of cold-warm double layer clouds, i.e., central Indian Ocean, southeastern Pacific Ocean, and southern Atlantic Ocean, where the occurrence frequencies of low-level warm clouds are notable. However, warm single layer clouds are frequent in these three domains. The warm-warm double layer clouds are widespread throughout the tropical and subtropical oceans, with the highest occurrences found in central Indian Ocean. Single-layer warm clouds and the lower warm clouds in the cold-warm double layer system have nearly identical geometric parameters. If the upper cold clouds in the double layer system are ignored, there remains no discrepancy of cloud appearance between the two kinds of warm cloud layers. Such a  
Figure 8: Sample frequency in the two-dimensional space of upper cloud thickness and separation distance (**a**), lower cloud thickness and separation distance (**b**), lower cloud thickness and upper cloud thickness (**c**), and the variation of separation distance depending on lower and upper cloud thickness (**d**), for warm-warm double layer clouds. similarity suggests that the warm cloud layer that is embedded in cold-warm double layer system is hardly impacted by the upper cloud layer. Although they constitute a double layer system in the perspective of observation, these two cloud layers tend to be uncorrelated. The warm clouds at lower levels have their own formation and evolution regimes, independent of the mixed-phase or ice clouds aloft. The upper and lower layer warm clouds in the warm-warm double layer system have features that are distinct from the previous two. Their vertical extents are small. Meanwhile, the cloud top and base height tend to be elevated for the upper cloud layer, while they are depressed for the lower one. The separation distance between the two cloud layers in the warm-warm double layer system is found to be weakly dependent on the cloud layer thickness. For these double layer systems, most cases have a separation distance within 1 km, even though the cloud thickness can be small. The separation distance exceeds 600 m in many cases, which occur for those with moderate thickness of upper and lower cloud layers. As the two warm cloud layers get extremely thin, the cloudless gap between them tends to have a moderate thickness. In contrast to the independence of the two cloud layers in cold-warm double layer system, the two kinds of warm cloud layers in the warm-warm double layer system are likely to be coupled with each other.