Three-Dimensional Idealized Simulations of Barrier Jets along the Southeast Coast of Alaska

Abstract

Three-dimensional idealized simulations using the fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5) down to 6-km grid spacing were performed in order to understand how different ambient conditions (wind speed and direction, stability, and inland cold pool) and terrain characteristics impact barrier jets along the southeastern Alaskan coast. The broad inland terrain of western North America is important in Alaskan jet development, since it rotates the impinging flow cyclonically (more coast parallel) well upstream of the coast, thus favoring more low-level flow blocking while also adding momentum and width to the barrier jet. Near the steep coastal terrain, the largest wind speed enhancement factor (1.9?2.0) in the terrain-parallel direction relative to the ambient onshore-directed wind speed occurs at relatively low Froude numbers (Fr ? 0.3?0.4). These low Froude numbers are associated with (10?15 m s?1) ambient wind speeds and wind directions orientated 30°?45° from terrain-parallel. For simulations with an inland cold pool and nearly coast-parallel flow, strong gap outflows develop through the coastal mountain gaps, shifting the largest wind speed enhancement to Fr < 0.2. The widest barrier jets occur with ambient winds oriented nearly terrain-parallel with strong static stability. The gap outflows shift the position of the jet maximum farther offshore from the coast and increase the jet width. The height of the jet maxima is typically located at the top of the shallow gap outflow (?500 m MSL), but without strong gap outflows, the jet heights are located at the top of the boundary layer, which is higher (lower) for large (small) frictionally induced vertical wind shear and weak (strong) static stability.