Using Stable Water Isotopes in Cloud Hydrometeors to Identify the Growth Pathway of Ice in Mixed-Phase Arctic Clouds: Airborne In-Situ Measurements from CAESAR 2024

Identifying the active microphysical processes within observed mixed-phase clouds is difficult due to the limited amount of process-level information that can be measured. For example, imaging probes mounted on airborne platforms provide a wealth of information about cloud hydrometeors at the moment that they are imaged, but lack quantitative information about the ongoing microphysical evolution. Here, we use stable water isotope ratios, measured during the CAESAR 2024 (Cold Air Outbreak Experiment in the Sub-Arctic Region) airborne field campaign, to help elucidate the processes that may have been responsible for ice production and evolution within mixed-phase clouds over the Norwegian sea.

The stable isotopes deuterium (D) and oxygen-18 (O¹⁸) are found in trace amounts in Earth’s water. Because the saturation vapor pressures of water containing these heavy isotopes differ from light (or normal) water, condensation and deposition results in predictable changes in the heavy-to-light isotope ratios in cloud hydrometeors and the surrounding vapor. The heavy-to-light isotope ratio in a cloud hydrometeor is dependent on the temperature at which it formed, the amount of vapor saturation that it experienced, and the phase of the hydrometeor (ice or liquid) during its evolution. By analyzing the isotope ratios in cloud hydrometeors at various heights within a cloud, information about water phase transformation history can be revealed.

In this study, we used a Counterflow Virtual Impactor (CVI) inlet aboard the NSF NCAR C-130 aircraft to ingest cloud droplets, ice particles, and precipitation within and below clouds, while excluding the surrounding water vapor. The cloud water then enters an analyzer inside the aircraft which measures isotope ratios with high precision using cavity ring-down spectroscopy. At the same time, the surrounding water vapor is ingested and analyzed through a separate inlet and isotope analyzer.

Paired with a suite of in-situ cloud probes, including HOLODEC (Holographic Detector for Clouds) and optical array probes, the CAESAR field campaign provides ideal conditions to investigate what this isotope measurement can reveal about mixed-phase microphysical processes in cold air outbreak clouds. We present and interpret the isotopic signatures throughout the vertical extent of clouds containing supercooled liquid regions, mixed-phase regions, and precipitating ice; focusing on identifying the formation pathway of the in-cloud ice and precipitating ice below the cloud.