Tracking ice growth pathways in mixed-phase Arctic clouds using stable water isotopes: Airborne in-situ measurements from CAESAR 2024.

In-situ cloud measurement techniques, particularly those collected by airborne platforms, capture microphysical characteristics of mixed-phase clouds but are unable to directly measure ice formation mechanisms and particle growth histories. Addressing this observational gap, we demonstrate that in-situ measurement of stable water isotopes can be used to quantify ice growth processes more directly. By analyzing the isotopic composition of ice hydrometeors, we can identify their dominant growth pathway: direct vapor deposition, riming, or through Wegener-Bergeron (WBF) conditions.

Stable water isotopologues are water molecules which contain deuterium (D) or oxygen-18 (O¹⁸). They are present within water everywhere, and are termed “heavy” due to their larger molecular mass. The concentration of heavy water isotopes found in atmospheric ice particles is dependent on the thermodynamic conditions experienced during growth. Specifically, the in-situ temperature, relative humidity, and thermodynamic phase (liquid or ice) dictate the amount of heavy isotopes that enter the cloud condensate.

Water isotopes within mixed-phase clouds were measured in-situ during the CAESAR 2024 (Cold Air Outbreak Experiment in the Sub-Arctic Region) airborne field campaign. We first provide an overview of stable water isotopes and their dependence on environmental conditions. Then, we present the use of isotopic measurements to identify vapor deposition, riming, and WBF growth conditions inside mixed-phase clouds from CAESAR. A suite of in-situ cloud probes (PHIPS, HOLODEC, and Optical Array Probes) is used to validate the results of the isotopic analysis. This observational technique can be leveraged to study each ice growth mechanism’s influence on cloud properties.