A comparison of two coastal barrier jets along the southeast Alaskan coast during the SARJET field experiment

The Alaskan coastline bordering the Gulf of Alaska has steep coastal topography, as well as a variety of bays, straits, and sounds. The low-level flow frequently accelerates down the pressure gradient along the coast and out of coastal gaps to form complex three-dimensional coastal jets, which can be dangerous to the aviation and shipping industries. Because this region has little in situ observational data, a combination of spaceborne synthetic aperture radar (SAR), high-resolution PSU-NCAR MM5 simulations, and Wyoming King-Air aircraft data from September-October 2004 of SARJET were utilized in order to improve our understanding of these coastal wind events.

Field observations and simulations are presented from SARJET in order to contrast the structural evolution and dynamics of two distinctly different jets. The IOP 1 event (26 Sept 2004) represents a “classic” barrier jet to 35 m/s, in which slightly stable onshore flow persisted throughout the event. This event is compared with the “hybrid” jet event of IOP 7 (12 Oct 2004), which featured offshore gap flow and downslope coastal winds early in the event, and stable onshore flow ascending over the gap outflow later in the case. The Penn-State-NCAR MM5 was nested down to 1.33-km grid spacing using the GFS analyses for initial and boundary conditions, the Eta-PBL, and the Grell convective parameterization on the 36- and 12-km domains. The simulations for both IOPs were validated with King-Air aircraft data, and the MM5 realistically simulated the complex coastal flows. A backward-trajectory analysis illustrates that IOP 1 was similar to the standard conceptual model of a barrier jet, with on-shore directed flow and turning of the wind just upstream of the coast due to blocking. Meanwhile, during IOP 7 the cold low-level outflow through the Cross Sound gap rotated anticyclonically and merged with the ambient southerly winds offshore, while the flow above this gap outflow (~850 mb) had an onshore component, similar to a more classical barrier jet structure.

A momentum budget from the MM5 was completed for both IOPs in order to show the force balance differences between the IOPs. In order to further examine the gap outflow, the coastal gap was filled in the model. Without the local gap outflow, the jet intensity was reduced by about 10% (3 m/s), and the offshore scale of the jet was reduced by about 20-30 km. The gap outflow influence was further confirmed using three-dimensional idealized 4-km MM5 simulations with a southerly flow of 15 m/s. With an inland cold pool perturbation of 10 degC initialized from 0-1 km ASL and a linear decrease of the perturbation to zero by 2-km, the barrier jet width was 50% larger as compared to a control simulation without the cold pool initialized.