Refining paleoaltimetry reconstructions of the Sierra Nevada, California, using air parcel trajectories

Stable isotope paleoaltimetry studies often implicitly assume that atmospheric flow interactions with topography can be simply modeled as a Rayleigh distillation process in which air parcels consistently ascend topographic barriers. We present a modern (A.D. 1979-2010) air parcel trajectory analysis using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model that shows that this fundamental assumption is often violated in the Sierra Nevada region of the western United States. Observed trajectory patterns and scaling calculations indicate that windward orographic blocking frequently occurs when trajectories encounter high elevations (>2.5 km) in the central and southern Sierra Nevada. As a result, trajectories reaching the Sierran lee commonly travel around, rather than over, the highest range elevations. Redirection effects are particularly pronounced at leeward sites distal (>150 km) to the Sierran crest, but are also evident in trajectory patterns in the northern Sierra Nevada. This trajectory analysis improves the interpretability of regional meteoric water and proxy isotopic records and has particular relevance to stable isotope-based reconstructions of Sierran paleoelevations. Specifically, stable isotope methods alone provide only limited insight into the elevation history of the Sierra Nevada and are likely insufficient to resolve proposed late Cenozoic elevation gains on the order of 1-2 km that may have raised the Sierra to its modern elevations.