Stable Water Isotopes as Tracers in Global Precipitation

Abstract

Stable water isotopes (H2O, H18O, and HDO) are incorporated into the microphysics schemes of two different atmospheric models. This thesis describes the use of these molecules as tracers in precipitation budgets to assess the processes controlling the isotopic signatures of precipitation in the tropics and orographic snow in the mid-latitudes. The idealized simulations of seasonal precipitation budgets in the tropics determine that increased vapor convergence during intense precipitation is most important for setting the isotopic composition of the convective precipitation. The isotopic signal of the converged vapor is more important than the local evaporation and smaller scale post-condensational processes. Flow over a 2D-mountain and realistic simulations of orographic clouds show that the isotopic signature of precipitation is more sensitive to changes in mountain height and initial temperature profiles than to the cloud droplet number concentration. Riming of cloud liquid and vapor deposition onto ice are the largest source terms for orographic precipitation, and have distinct isotopic signatures that are altitude-dependent. When riming is the larger source term, precipitation tends to be more enriched than when vapor deposition dominates.