MPI2019: Mixed Phase Isotopes 2019

Summary

Scientific relevance

Ice crystals and supercooled liquid droplets exist simultaneously in mixed-phase clouds. As a result, precipitation may form by distinct pathways. Water vapor in the atmosphere can deposit directly onto tiny particulates called ice nuclei, or supercooled droplets can freeze onto existing ice crystals, resulting in riming. Environmental conditions (such as temperature) and the presence of different particulate types are both thought to influence the balance of ice and liquid content in mixed-phase clouds, which has important implications for weather and climate. For instance, liquid droplets tend to be more efficient than ice crystals at scattering sunlight back to space, and riming is thought to increase the efficiency of precipitation, resulting in more intense storms. Understanding the processes that control mixed-phase cloud characteristics is thus critical for improving numerical predictions of weather and climate.

Description of scientific experiment

Because of their sensitivity to the temperature, efficiency, and microphysical pathway of precipitation formation, the stable isotopes of water provide insight into the processes controlling ice crystal growth. Theory suggests, for example, that ice crystals formed by vapor deposition should be more depleted (i.e., have lower isotope ratios) than ice crystals formed by riming. Knowing the isotope ratios of water vapor, liquid, and ice in mixed-phase clouds should thus provide a means to estimate the mass fraction of precipitation formed by riming. However, isotopic measurements of distinct cloud particle types are challenging to obtain.

During January 2019, a new observational strategy was tested for making simultaneous measurements of isotope ratios in mixed-phase cloud particles and water vapor at the Desert Research Institute’s (DRI) high-elevation Storm Peak Laboratory (SPL, 3220 m) in northwestern Colorado, USA. Two water vapor isotopic analyzers were deployed: one targeting ice crystals and the other targeting interstitial vapor.

Interstitial vapor was measured by attaching a stainless steel backwards-facing inlet to a 1000 liter per minute ambient sampling stack. Vapor was drawn through the inlet to the isotopic analyzer via ¼-inch (outer diameter) copper sample tubing.

Ice crystals were separated from supercooled liquid and vapor using a new phase-separating inlet known as SPIDER, or the phaSe seParation Inlet for Droplets, icE crystals, and aeRosols, developed by researchers at the Massachusetts Institute of Technology and Purdue University. SPIDER separates cloud liquid and ice crystals from the interstial vapor by pulling air through two counterflow virtual impactors (CVIs), positioned on either end of a temperature-controlled cylinder. During passage through the first CVI, dry air flowing counter to the sample excludes molecules in the gas phase while allowing condensate to pass. Once in the temperature-controlled cylinder, the supercooled droplets evaporate while the ice crystals remain intact, due to saturation vapor pressure differences between liquid and ice. The second CVI then separates the crystals from the newly vaporized liquid droplets. These were delivered to the second isotopic analyzer via ¼-inch copper sample tubing.

This dataset describes a number of distinct measurement periods associated with the 2019 Storm Peak deployment.

• Simultaneous measurements of isotope ratios in ice crystals and interstitial vapor were made over a two-hour intensive observational period on January 23, 2019.
• During the remainder of the SPL deployment (January 14-23, 2019), both isotopic analyzers sampled water vapor (in clear and mixed-phase cloud conditions) from the SPL ambient stack.
• One isotopic analyzer remained at SPL sampling water vapor beyond the main deployment period (January 23 - March 15, 2019).
• Extensive instrument characterization, including side-by-side water vapor sampling, was performed prior to deployment at the NCAR Research Aviation Facility (December 20, 2018 through January 7, 2019).