A particle-based microphysics study of isotope exchanges in a single-column rain-shaft model

The aim of this work is to revisit, employing probabilistic particle-based microphysics, the single-column model of isotopic fractionation proposed in Rozanski & Sonntag (Tellus 1982, doi:10.3402/tellusa.v34i2.10795). The original formulation conceptualizes the rain shaft as a counterflux isotopic distillation column. It was designed to represent a minimal set of processes yielding the steep gradients of deuterium content observed in atmospheric moisture profiles. It is formulated on a multi-cell grid and is built upon the assumption of adiabatic ascent, and features a bulk representation of sedimenting condensate.

Here, we increase the fidelity of the model by replacing the bulk microphysics with a probabilistic particle-based representation covering the complete warm-rain formation and precipitation pathway inherently coupled with ambient aerosol and moisture reservoirs. To this end, the open-source PySDM particle-based modeling framework is extended with a representation of water isotopic composition, including 2H and 18O dynamics that stem from both equilibrium and kinetic fractionation effects throughout diffusional growth and evaporation under super- and sub-saturated conditions, respectively.

We explore the differences in simulated ambient vapor deuterium content profiles between the bulk and super-particle formulations of the model. We further discuss the potential for inclusion of isotopic fractionation effects in particle-based models covering other isotopologues as well as the ice-phase of water.