Three-dimensional idealized simulations of barrier jets along the southeast coast of Alaska

The low-level flow from the Pacific Ocean interacts with the steep coastal terrain of Alaska to create strong (> 25 m s-1) terrain-parallel winds known as barrier jets. Given the complex orography of southern Alaska, with its inland plateau, steep coastal terrain, and many prominent coastal gaps, it needs to be determined how barrier jets evolve for different ambient conditions for this region. A set of three-dimensional idealized dry barotropic simulations on an f-plane (58 oN) was constructed over the Gulf of Alaska using the Penn State/NCAR MM5 version 3.7 (Grell et al. 1995). The MM5 was configured with three domains with horizontal grid spacings of 54-, 18-, and 6-km (Fig.1a) and were run using a one-way nest communication. For the hybrid jet runs, an inland cold pool was initialized over the land grid points over the Alaska-Yukon Territory. The cold pool had temperature perturbation magnitudes of -5, -10, and -15oC. The ambient wind speeds were incremented every 5 m s-1 from 10 to 25 m s-1 and the wind direction every 20 degrees from 160o to 220o, which corresponds to ~25o to 85o from coast-parallel. The static stability was incremented every 0.005 s-1 from N = 0.005 to 0.015 s-1.

The broad inland terrain of western North America produces a large mountain anticyclone, which rotates the onshore winds cyclonically 500-1000 km upstream of the coast. This adds momentum to the barrier jet and it reduces the cross-barrier component of the impinging flow, which favors more flow blocking. As a result, those simulations with a broad mountain create stronger and wider barrier jets along the southeast Alaskan coast than simulations without inland terrain.

The largest wind speed enhancement factors (1.9-2.0) in the terrain-parallel direction relative to the ambient onshore-directed wind speed occur for the classic (primarily onshore flow) and hybrid jets (includes offshore-directed gap flow) at low Froude numbers (Fr < 1), with a maximum at Fr ~0.3-0.4. These faster jet cases are associated with (10-15 m s-1) ambient wind speeds and wind directions orientated 30-45o from terrain-parallel. The widest barrier jets occur with ambient winds oriented nearly terrain-parallel with strong static stability. The gap outflows at the coast during hybrid jets shift the position of the jet maximum farther away from the coast than the classical jets, and the height of the hybrid jet maxima are typically lower than the classical simulations, since the hybrid jets are often located at the top of the shallow gap outflow.