An isotopic approach to evaluate ice particle growth in mixed phase-clouds

Ice particle growth influences key characteristics, such as the lifetime, of mixed-phase clouds but remains difficult to study observationally and thus represent accurately in numerical simulations. Here I present a possible new approach to address this challenge based on the theoretical assumption that riming and vapor deposition will impart distinct signatures on the hydrogen and oxygen isotope ratios of ice. During winter 2022, three water vapor isotopic analyzers were deployed at the Storm Peak Laboratory (SPL) in Colorado. Using Purdue University’s prototype 3-phase separating inlet SPIDER, we measured simultaneously, for the first time, the isotopic composition of water in vapor, supercooled liquid, and ice within orographic clouds. Here, I compare our observations of the condensed phases to isotopic predictions based on the vapor measurements. The results from SPL suggest a strong correlation in ice isotope ratios with the supersaturation of the environment but little variation in ice deuterium excess, suggesting a limited role for vapor deposition in growing the particles during our measurement period. Based on these findings, I discuss opportunities for refining real-time water isotopic measurements in mixed-phase clouds and for extending this observational approach to recent missions, including CAESAR, which measured water isotope ratios in mixed-phase clouds in the Arctic during cold-air outbreaks. For both the SPL and CAESAR experiments, I also place ice growth within the context of larger-scale moisture transport to create a framework for linking microphysical processes with the notion of “moisture source.”