CSDMS 2016 annual meeting poster LaurenWheeler
The influence of elevation on the isotopic composition of orographically enhanced precipitation
Joseph Galewsky, University of New Mexico Albuquerque New Mexico, United States.
Abstract:
Windward isotope proxies of precipitation preserved in the geologic record are commonly used to determine the uplift history of a mountain range. In a 2D model of orographic precipitation, the incoming air is lifted and cooled, and the heavier isotopes are preferentially condensed and rained out along the windward path. Windward isotope-based paleoaltimetry assumes that changes in elevation do not significantly alter the distribution of the precipitation and the isotopic composition of that precipitation. Studies have shown that increased elevation acts to shift orographic precipitation upstream of the mountain range and that the isotopic composition of that precipitation diverges from Rayleigh condensation models. Pure orographic precipitation is rare though; more common is orographically enhanced precipitation. Using the Weather Research and Forecasting (WRF) model V3.5.1 we test how changes in elevation and topographic configuration can affect the distribution of orographically enhanced precipitation and isotopic composition on the windward face of mountain ranges. We use the Baroclinic Wave model in WRF, an idealized model that establishes a storm system that dominates weather in the mid-latitudes. The model includes two modifications: the addition of topography to the model setup and an isotope physics calculation included into the full microphysics scheme. The isotope physics are incorporated in the pre-existing Kessler microphysics scheme within WRF, which is altered to include a Perfect Precipitation model (PPM). Precipitation is generated in the PPM when a gridpoint reaches saturation, the water vapor is condensed and falls out as precipitation. Isotope tracers for the initial water vapor mixing ratio and δ18O are added to the PPM and advected and modified within the full microphysics scheme. The isotopic fractionation takes place upon condensation according to temperature-dependent equilibrium factors.
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