Using Air Parcel Trajectories to Clarify the Topographic Evolution of the Sierra Nevada

Stable isotope paleoaltimetry studies often implicitly assume that atmospheric flow interactions with topography can be simply modeled as a Rayleigh distillation process in which trajectories consistently ascend topographic barriers. We present a modern (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 US. Observed trajectory patterns and calculated Nh/U trajectory values indicate that orographic blocking frequently occurs on the windward (western) side of the Sierra Nevada when trajectories encounter high elevations (> 2.5 km) in the central and southern Sierra. As a result, trajectories reaching the Sierran lee commonly travel around, rather than over, the highest range elevations. These blocking and redirection effects are particularly pronounced at leeward sites that are distal (> 150 km) from the Sierran crest but are also evident in trajectory patterns for both windward and leeward locations 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 Sierra Nevada paleoelevations. Specifically, stable isotope methods alone provide only limited insight into the paleoelevation history of the Sierra Nevada and are likely insufficient to resolve Late Cenozoic elevation gains on the order of 1 – 1.5 km that have been proposed as part of the topographic evolution of the range.