Microphysical controls on the isotopic composition of wintertime orographic precipitation

Abstract

The sensitivity of mixed-phase orographic clouds, precipitation, and their isotopic content to changes in dynamics, thermodynamics, and microphysics is explored in idealized two-dimensional flow over a mountain barrier. These simulations use the Weather Research and Forecasting (WRF) model with stable water isotopologues (HDO and math formulaO), 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 ( math formula) 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 responses in domain-averaged math formula than do changes in CDNC by a factor of approximately 10. The strongest response to changing CDNC leads to local variations of math formula of about 3‰, though those occur in regions of weak precipitation (<0.1 mm h−1). Changes in math formula 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 math formula with increasing mountain height is not just a function of decreasing temperature but also reflects the changing contributions and distinct isotopic signatures of riming of cloud liquid and vapor deposition onto snow, the leading sources of precipitation in these simulations. The changes in math formula with mountain height, temperature, and CDNC are governed in part by the microphysical pathways through which precipitating hydrometeors are formed and grow.