Abstract

An idealized, fully nonlinear atmospheric model was used to evaluate the effects of stratified atmospheric flows on orographic precipitation isotopic ratios, with the aim of improving the foundation for interpreting proxy records used for paleoelevation studies. Orographic precipitation isotopic ratios can be understood in terms of two nondimensional parameters: the flow parameter Nh/U, where N is the moist Brunt-Väisälä frequency, a measure of atmospheric stratification, h is the large-scale relief, and U is the upstream horizontal wind speed perpendicular to the orogen; and the horizontal aspect ratio β, the ratio of the along-strike length of the orogen to its across-strike width. Existing Rayleigh condensation models for paleoaltimetry studies are only strictly applicable when Nh/U << 1. Most mountain ranges of geological interest are characterized by Nh/U > 0 much of the time. High values of both Nh/U and β favor precipitation upstream of the terrain, leading to isotopic ratios that are more depleted than predicted by the Rayleigh model. High Nh/U and low β favor low-level deflection of winds around the terrain, which substantially reduces lifting and condensation, leading to precipitation isotopic ratios in that are less depleted than predicted by the simple Rayleigh model. Changes in climate or the horizontal terrain aspect ratio can change precipitation isotopic ratios at least as much as changes in surface elevation, and surface uplift need not always affect precipitation isotopic ratios, suggesting that proxy records must be interpreted within a broad context of climate variability and landscape evolution.

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