Seasonal and vertical controls on cloud phase partitioning from long-term CALIPSO observations

Cloud phase is often described as a function of temperature, yet whether a single phase–temperature relationship applies across cloud vertical structure and seasons remains poorly constrained by observations. Using 15 years (2008–2022) of CALIPSO lidar observations, we investigate the partitioning of ice and liquid cloud phase as a function of temperature throughout the cloud column. Cloud phase and temperature are collocated at CALIOP’s native vertical resolution, allowing us to distinguish cloud-top and cloud-bulk phase characteristics.We show that cloud phase–temperature relationships differ systematically between cloud tops and cloud interiors, and that these differences are strongly modulated by season and latitude. At low temperatures (below −10 °C), cloud interiors generally exhibit lower liquid fractions than cloud tops, whereas vertical phase differences become small at warmer temperatures. For a given temperature, extratropical clouds in the Northern Hemisphere generally contain more ice than those in the Southern Hemisphere. Seasonal modulation is most pronounced at high latitudes, where clouds during local winter exhibit higher liquid fractions than summer clouds at the same temperature. In contrast, seasonal variations in cloud phase partitioning are relatively weak in the tropics.Vertical phase differences also depend on cloud geometric depth. Shallow clouds tend to be vertically homogeneous in phase, while clouds of intermediate depth exhibit more pronounced vertical phase contrasts, particularly at high latitudes and for colder cloud-top temperatures. These results demonstrate that cloud phase–temperature relationships are not universal, but depend on cloud vertical structure, season, and latitude, with implications for satellite-based cloud phase climatologies and the representation of mixed-phase clouds in climate models.