Idealized WRF Simulations of Flow around the Sierra Nevada and Implications for Paleoaltimetry

Abstract:

Isotope-based evidence for the timing of the uplift of the Sierra Nevadas comes to conflicting conclusions with those from other lines of geologic evidence. The geologic evidence suggests that the Sierras uplifted >1 km 8 to 3.5 Ma, whereas isotope-based evidence suggests that there has been no significant uplift in the last 16 Ma. Recent studies have demonstrated that air trajectories in the Sierras are not consistent with the underlying two-dimensional assumptions used in the isotope-based approach and therefore the technique may not be applicable to the region. We suggest that due to the three-dimensionality of the Sierras, determining paleoaltimetery from lee-side isotope-based proxy records may not be applicable and that the complexity of the terrain accounts for the discrepancies between the geologic and isotope proxy records. Using idealized three-dimensional WRF v3.5.1 simulations of smoothed complex terrain we constrain and quantify the sensitivity of air masses to low-level flow deflection around a ridge with laterally varying elevation. The simulations are run in a domain of 560 points in the x direction and 250 points in the y direction with 4 km grid spacing and 121 unevenly spaced vertical points in a 30-km-high domain with open lateral boundaries with constant horizontal and vertical diffusion. Rayleigh damping is applied to the upper 15 km of the domain. The topography consists of a single idealized Sierran-type ridge with laterally varying elevation that encompasses both high and low values for the nondimensional number Nh/U, where N is the buoyancy frequency, h is the mountain height, and U is the horizontal wind speed. Our results indicate that flow is not two-dimensional as is assumed in interpretations of isotope-based proxies. Instead streamlines and trajectory paths are deflected around the highest topography and over the lower parts of the range. Although lee-side isotope-based paleoaltimetry sites may not be applicable to the Sierras, the technique might still apply to other mountain ranges which exhibit more two-dimensional flow or where the three-dimensional flows can be better constrained.