Isotopic Fractionation in Wintertime Orographic Precipitation

The sensitivity of mixed-phase orographic clouds, precipitation and their isotopic content to changes in dynamics, thermodynamics and microphysics is explored in idealized simulations of two-dimensional flow over a mountain barrier. These simulations use the Weather Research and Forecasting (WRF) Model with stable water isotopologues (HDO and H₂¹⁸O), which have been integrated into the Thompson microphysics scheme within WRF as part of the present project. In order to understand how the isotopic composition of precipitation (δ¹⁸O_precip) is fixed, the mountain height, temperature, and the prescribed cloud droplet number concentration (CDNC) have been varied in a series of simulations. For the given range of values explored in this work, changes in mountain height and temperature induce stronger changes in domain-averaged δ¹⁸O_precip than do changes in CDNC by a factor of approximately 10. The strongest response to changing CDNC in the present study leads to local variations of δ¹⁸O_precip of about 3 ‰, though those occur in regions of weak precipitation (<0.1 mm hr^-1) and would likely not be visible in snow accumulated over multiple storms. Changes in δ¹⁸O_precip can be understood through the microphysical pathways by which precipitable hydrometeors are formed and by the isotopic signature associated with each pathway. The decrease in δ¹⁸O_precip with increasing mountain height is not a simple function of decreasing temperature, but also reflects the changing contributions from riming of cloud liquid and vapor deposition onto snow, the leading sources of precipitating hydrometeors in these simulations. These two processes have distinct isotopic signatures with a δ¹⁸O difference of 3-8 ‰.

We will also present preliminary simulations of conditions at Storm Peak Laboratory in Steamboat Springs, Colorado (USA) during the IFRACS experiment of Jan-Feb 2014. During IFRACS, measurements of the isotopic composition of vapor, cloud liquid and snowfall at Storm Peak Lab were gathered along with meteorological data. These data, and satellite observations of the isotopic composition of water vapor aloft, are used to evaluate the model simulations. The simulations, in turn, inform the large- and meso-scale conditions that produce the obervations at Storm Peak.