Collaborative Research: Isotopic Fractionation in Snow (IFRACS)

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This collaborative project seeks to improve understanding of precipitation processes in mixed phase orographic clouds and the variation in the isotopic signature of snowfall that is associated with aerosol effects on cloud microphysics. The study will use measurements and modeling of the isotopic content of water vapor, cloud water, and snow at the Desert Research Institute's (DRI's) Storm Peak Laboratory (SPL, altitude 3210 m) to test the hypotheses that: 1) variations in the isotopic composition of snow associated with riming and riming inhibition are significant; and 2) the isotopic composition and chemistry of cloud droplets near cloud base and falling snow are sufficient to resolve the relative roles of vapor deposition and riming in snow growth. Model simulations with a new isotope-enabled version of the Weather Research and Forecasting (WRF) model will be used to test observation-based inferences on snow formation by riming and vapor deposition, the altitudes at which they occur, and the effects of variation of aerosol concentration on the isotopic composition of snowfall. The sensitivity of the isotopic composition of snowfall to aerosol-induced changes in the rimed mass fraction will be evaluated with model simulations over a range of aerosol concentration based on observational case studies representing a variety of meteorological and synoptic conditions during wintertime storms at SPL.Intellectual Merit: Riming of ice crystals in wintertime orographic clouds significantly enhances their water content and the amount of water deposited in snowfall. The isotopic composition of precipitation at a given location reflects local cloud processes as well as the history of upwind condensation, evaporation and mixing. In the work, the impact of aerosol-induced changes in mixed phase orographic clouds on the isotopic content of snowfall will be explored. Inhibition of snow growth by riming due to aerosol-induced reduction of cloud droplet size has been observed in wintertime storms at SPL. Broader Impact: The contribution of isotope-enabled microphysical models to the WRF community during this project will allow for broader application of these methodologies in studies of general circulation and the water cycle. The isotopic signal recorded in glacial ice has been used as a proxy record of past climates. Low-level riming over glacial ice may contribute to a component of the isotopic signal that is independent of global temperature. This project involves the participation and training of graduate students from DRI and Harvard in the field program and modeling studies. During the field experiment season, SPL will host atmospheric science field courses from the University of Wisconsin, North Carolina State University, and the University of Colorado for undergraduate and graduate students. As part of this work, a series of lectures will be presented to each of these classes on the use of isotopic signatures in environmental science. Reports on SPL research projects have appeared on CNN, NBC, National Public Radio, National Geographic Explorer, local television stations, in newspapers across the U.S., and the NCAR EOL Channel (http://tinyurl.com/ISPAscience). SPL is integrally involved in the Steamboat Weather Summit of meteorologists from television stations around the country. These activities will coincide with the winter field study at SPL and a talk on this work will be presented at the Weather Summit. The investigators will also present a community lecture in Steamboat Springs and an "open house" at SPL to share the scientific motivation of this research. Finally, a curriculum developed to provide middle school students with hand-on learning focused on meteorology and climate will continue with the project and have classes observe weather balloons launches during the field study.