The Sensitivity of Orographic Precipitation to Flow Direction: An Idealized Modeling Approach

A method was developed for forcing a full-physics, full-terrain model with an idealized, balanced atmosphere determined by an input sounding. The initialization technique was applied to investigate the sensitivity of orographic precipitation in Pacific Northwest terrain to wind direction under both barotropic and sheared flow conditions approximating an atmospheric river. The model results agreed well with previous studies that considered typical conditions resulting in heavy precipitation, with the precipitation sensitivity to wind direction less than was estimated for similar deterministic cases. Further model forecasts of heavy precipitation cases were performed for comparison. To explore the causes of sensitivity in the Olympic Mountains of western Washington State, additional experiments were carried out using modified terrain fields with smoothed or idealized Olympic Mountains, or with nearby orography removed. Model simulations suggested that the sensitivity of Olympic Mountain precipitation to wind direction is more strongly modulated by the presence of surrounding orography than by the specific Olympic geometry.