ALBERT

Description: The Southeastern Alaskan Regional Jets experiment investigated the structures and physical processes of barrier jets along the coastal Fairweather Mountains near Juneau, Alaska, from 24 September to 21 October 2004. This paper compares in situ aircraft data and high-resolution simulations from the first intensive observation period (IOP1), which featured a nearly terrain-parallel barrier jet (classical jet) with another coastal jet event (IOP7) that was influenced by offshore-directed gap flows at the coast (hybrid jet). IOP1 featured southerly onshore flow preceding a landfalling trough, which was blocked by the coastal terrain and accelerated down the pressure gradient to produce a 5–10 m s−1 wind enhancement in the alongshore direction in the lowest 1 km MSL. In contrast, IOP7 featured higher surface pressure and colder low-level temperatures to the east (inland) of the study area than did IOP1, which resulted in offshore-directed coastal gap flow exiting Cross Sound below ∼500 m that turned anticyclonically and merged with the ambient flow. Unlike the classical jet (IOP1), IOP7 had a surface warm anomaly adjacent to the steep coastal terrain, while a cold anomaly existed farther offshore within the gap outflow. Above the shallow gap flow (〉500 m MSL), there were more classical barrier jet characteristics. High-resolution fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) simulations were performed to compare the structures and underlying dynamics between the two cases. Model trajectories show that coastal winds for IOP1 originated offshore, while much of the coastal flow in IOP7 had gap flow origins near the surface and offshore origins above the gap outflow. A model momentum budget suggests that the vertical mixing of southerly momentum from aloft forced the gap outflow in IOP7 to turn anticyclonically more rapidly than an inertial circle. A simulation of IOP7 with the Cross Sound gap removed (filled in) produced a coastal jet with similar maximum wind speeds to the control but resulted in a reduction in the width of the coastal jet by about 40%.

Description: The Southeastern Alaskan Regional Jets experiment investigated the structures and physical processes of barrier jets along the coastal Fairweather Mountains near Juneau, Alaska, from 24 September to 21 October 2004. This paper compares in situ aircraft data and high-resolution simulations from the first intensive observation period (IOP1), which featured a nearly terrain-parallel barrier jet (classical jet) with another coastal jet event (IOP7) that was influenced by offshore-directed gap flows at the coast (hybrid jet). IOP1 featured southerly onshore flow preceding a landfalling trough, which was blocked by the coastal terrain and accelerated down the pressure gradient to produce a 5–10 m s−1 wind enhancement in the alongshore direction in the lowest 1 km MSL. In contrast, IOP7 featured higher surface pressure and colder low-level temperatures to the east (inland) of the study area than did IOP1, which resulted in offshore-directed coastal gap flow exiting Cross Sound below ∼500 m that turned anticyclonically and merged with the ambient flow. Unlike the classical jet (IOP1), IOP7 had a surface warm anomaly adjacent to the steep coastal terrain, while a cold anomaly existed farther offshore within the gap outflow. Above the shallow gap flow (〉500 m MSL), there were more classical barrier jet characteristics. High-resolution fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) simulations were performed to compare the structures and underlying dynamics between the two cases. Model trajectories show that coastal winds for IOP1 originated offshore, while much of the coastal flow in IOP7 had gap flow origins near the surface and offshore origins above the gap outflow. A model momentum budget suggests that the vertical mixing of southerly momentum from aloft forced the gap outflow in IOP7 to turn anticyclonically more rapidly than an inertial circle. A simulation of IOP7 with the Cross Sound gap removed (filled in) produced a coastal jet with similar maximum wind speeds to the control but resulted in a reduction in the width of the coastal jet by about 40%.

Description: 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.

Description: A technique for initializing realistic idealized extratropical cyclones for short-term (0–72 h) numerical simulations is described. The approach modifies select methods from two previous studies to provide more control over the initial cyclone structure. Additional features added to the technique include 1) deformation functions to initialize more realistic low-level fronts, tropopause structure, and enhanced jet maximum at upper levels; 2) a barotropic shear function to help develop different cyclone and frontal geometries; and 3) damping functions to create an isolated baroclinic wave in the horizontal; therefore, the initialized cyclone is not influenced by the domain boundaries for relatively short simulations. Since this procedure allows for control of the initialized cyclone structures, it may be useful for studies of frontal and cyclone interaction with topography and mesoscale predictability. The initialization system produces a variety of basic states and synoptic disturbances, ranging from weak to explosively developing cyclones. Examples are shown to provide some insight on how to adjust selected parameters. The output is compatible with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model and the Weather Research and Forecasting model. This note describes the procedure as well as presents an example of a landfalling cyclone along the U.S. west coast with and without terrain.