## 2 Data and Methodology
### Profiles with Warm Cloud Layers  
The warm cloud layers, either in the form of a single-layered cloud system or as an individual layer in a multilayered cloud system, are thus extracted to ascertain their occurrence frequency, characteristics, and geometric features. In most investigations involving warm clouds, in particular those that are  
Figure 1: Occurrences of cloud profiles with diverse cloud layers over global oceans. based on radiometer measurements, a cloud top temperature of 273 K [24] is a standard criterion for isolating warm clouds that are only composed of liquid-phased hydrometeors. However, in the presence of multilayered clouds, it is hard to correctly interpret the derived cloud top temperature, since a single-layer assumption is generally adopted to make the cloud retrieval feasible. It is plausible to take the retrieved cloud top temperature as representing the uppermost cloud layer in an atmospheric column. However, this treatment inevitably causes ambiguities that would deteriorate as the optical depth of the upper cloud layer gets smaller [27]. Even though the retrieved cloud top temperature is sometimes right for the uppermost cloud layer, the cloud top temperature of the lower cloud layers is always unknown. Therefore, such a cloud top temperature-based scheme cannot capture all warm cloud layers and many samples would be ignored. Many warm cloud layers would be lost due to the overlap of an upper cloud layer with cloud top temperature lower than 273 K. This is actually one of the several motivations for the present study, which is designed to investigate warm clouds that are embedded in multilayered structures. The freezing level data are derived from reanalysis meteorological fields by the European Centre for Medium-Range Weather Forecasts (ECMWF) and are used to distinguish warm cloud layers from mixed-phase and ice layers. It is straightforward to compare the top height of a cloud layer with the altitude of freezing level at the pixel scale. Such a screening scheme of warm clouds is, in principle, rigorous and able to capture all warm cloud layers, despite the possible multilayered situations. The cloud profiles were divided into two parts, according to the presence or absence of warm cloud layers. The first one includes those without any warm cloud layer, while the other includes those containing at least one warm cloud layer. Within the latter group, there is a distinctive subgroup that includes profiles containing only warm cloud layers. It is this subgroup that corresponds to the warm clouds that are derived from the cloud top temperature criterion of 273 K. In other words, the ratio of these two kinds of profiles reflect, to some extent, the possible biases in warm cloud sampling from the cloud top temperature scheme. The global distributions of the relevant ratios are shown in Figure 2 to clarify the quantitative relation among these categories. Figure 2a shows the ratio of cloud profiles among the total profiles. For each grid, this quantity reflects the averaged cloud cover that could be also derived from the radiometer-based cloud mask data. As for the overall oceanic clouds, their occurrences show a general increase from low latitudes to middle and high latitudes in both hemispheres. In the tropics and subtropics, most regions have cloud cover below 40%, except for two distinct zones, the intertropical convergence zone (ITCZ) and the South Pacific convergence zone (SPCZ). From 30\({}^{\circ}\) N and 30\({}^{\circ}\) S poleward, the quantities of cloud cover are universally higher than 50%. The highest values reach 75% and they mostly occur around 60\({}^{\circ}\) S and 60\({}^{\circ}\) N, representing the most overcast regions on the earth. This basic spatial pattern of total cloud occurrences over global oceans is largely determined by large-scale meteorological conditions. Although warm clouds are an important component in the cloud ensemble, their spatial distributions are a bit different.