Simulations of Idealized Flow Over Complex Terrain

Many geological studies of the surface elevation history of the Sierra Nevada and other mountain ranges ignore basic atmospheric dynamics in basing their assumptions on two-dimensional models of flow over topography. This is not an appropriate assumption when applying the technique to realistic three-dimensional terrain. California's Sierra Nevadas, for example, can be idealized as a single ridge with varying elevations: to the south the ridge reaches an elevation of >4 km and to the north an elevation of <3 km. We suggest that due to the three-dimensionality of the Sierra Nevadas the technique of determining paleoaltimetery from isotope-based proxy records is not applicable and that the complexity of the terrain accounts for the discrepancies between the geologic and isotope-based proxy records. Through our sensitivity analysis we constrain the low-level flow deflection around an idealized mountain range and assess how lateral variations in elevation influence the flow deflection. Using idealized three-dimensional WRFV3.5.1 simulations of smoothed complex terrain we test the sensitivity of air masses to flow deflection around an idealized ridge with laterally varying elevation. The simulations are run in an idealized configuration on a domain of 450 points in the x direction and 200 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. Rayleigh damping is applied to the upper 15 km of the domain and constant horizontal and vertical diffusion is used in the model. The topography consists of a single idealized ridge 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. Preliminary results based on simulations using an idealized modern Sierran topography indicate that flow is not two-dimensional as is assumed in interpretations of isotope-based proxies. Instead streamlines are deflected around the highest topography and over the lower parts of the range. Although our results suggest that the isotope-based paleoaltimetry technique is not applicable to the Sierra Nevada the technique might still apply to other mountain ranges which exhibit more two-dimensional flow. For future work we aim to determine which mountain range this method can be used for, for instance, the Southern Alps in New Zealand.