Simulation of cloud microphysical effects on water isotope fractionation in a frontal system

Abstract:

The stable water isotopic composition changes due to fractionation during phase changes. This information is useful for understanding the water cycle, such as the water vapor source, transport and cloud microphysical processes. In conventional atmospheric models, the isotope exchange between liquid and gas phase is usually assumed to be in an equilibrium state, which is not sufficient to describe the highly kinetic phase transformation processes in clouds. In this study, a two-moment microphysical scheme incorporated into the NCAR Weather Research and Forecasting (WRF) model is modified to simulate the isotope fractionations. Experimentally determined stable water isotope thermal equilibrium data are converted into isotope saturation vapor pressure, which is then put into the two-stream Maxwellian kinetic equation to calculate the fractionation during vapor condensation/evaporation or deposition/sublimation. Isotope mass transfer between liquid- and ice-phase hydrometeors during freezing/melting are also considered explicitly. The simulation results were compared with rainwater isotope measurements and showed fairly good agreement. Sensitivity tests were also conducted to quantify the contribution of rainwater isotopic due to water vapor source and transport, condensation environment conditions, and cloud microphysical processes. The results show that isotopic water vapor source dominates the stable isotope concentration in rainwater but the cloud microphysical processes including the ice-phase processes are also quite important. The results also showed that the two-stream Maxwellian kinetic method would cause significantly more deuterium to be transported into higher altitudes during convection than the thermal equilibrium method.