The structure and dynamics of barrier jets along the southeast Alaskan coast

Abstract

Coastal barrier jets along the complex orography of southeastern Alaska were investigated using high resolution observations and model simulations. Barrier jet events were sampled with the Wyoming King-Air research aircraft during the Southeastern Alaskan Regional Jet (SARJET) field experiment in 2004. These observations, combined with simulations of select cases by the Penn State-NCAR Mesoscale Model (MM5), were used to better understand barrier jet structure and dynamics. A suite of idealized simulations were used to put the case studies in perspective with a larger set of atmospheric conditions, while also evaluating previous theoretical and observational results.

Two SARJET case studies were investigated along the tall and steep Fairweather Mountains near Juneau, Alaska. The first case (24 September 2004) was a classical barrier jet forced primarily by onshore flow and upslope adiabatic cooling, with maximum winds >30 m s^-1 at the coast between 600-800 m ASL and an offshore extent of ∼60 km. In contrast, the hybrid jet (12 October 2004) was influenced by an offshore-directed gap flow at the coast, which produced a warm anomaly over the coast associated with downslope flow and a wind maximum (∼30 m s^-1) that was displaced 30-40 km offshore at 500 m ASL. A sensitivity experiment in which the coastal mountain gap was filled led to a ∼40% reduction in the jet width, and the position of the jet maximum shifted ∼40 km to the coast, but the overall jet intensity remained approximately the same.

The generality of these SARJET results was tested by generating a set of three-dimensional idealized MM5 simulations by varying wind speeds, wind directions, and static stabilities for the classical jet simulations, while incrementing the magnitude of the inland cold pool (instead of static stability) for hybrid jet simulations. The broad inland terrain was shown to impact the upstream winds by rotating them cyclonically to become more terrain-parallel within 500-1000 km of the coast. This reduced cross-barrier component acted to reduce the local Froude number of the impinging flow, thus enhancing the potential for flow blocking. Thus, the enhancement of the large-scale mountain anticyclone by the inland terrain acts to “precondition” the impinging flow for barrier jet development.

The largest simulated wind speed enhancements (∼1.9-2.0) for the classic and hybrid jets occurred for low Froude numbers ( Fr), with a maximum at Fr ∼0.3-0.4. Low ambient wind speeds (10–15 m s^-1) and southerly (170-180°) wind directions (∼30-45° from coast-parallel) were also ingredients for the largest wind speed enhancements. The widest barrier jets were found in simulations with ambient winds oriented nearly terrain-parallel (∼160°) with strong static stability (N > 0.01 s^-1). Hybrid barrier jets were slightly wider than the classical, with the gap outflows acting to shift the position of the jet maximum further away from the coast. During periods of maximal gap outflow (hrs 6-18), the height of the jet maximums were typically lower than the classical simulations, since the hybrid jet maximum was located at the top of the shallow gap outflow. The jet height was most correlated with total wind speed, U_total, and negatively correlated with static stability, N, suggesting that the height of the jet maximum approximately scales as U _total/N, which is proportional to the vertical wavelength of a mountain wave.

Finally, a detailed assessment of the usefulness of the previous linear theory and scale analysis on barrier jets was performed. The high Fr relationship (L = Nh_m/ f) performed better than the low Fr relationship (L = U_n/f) in determining the offshore extent of the barrier jet. The implementation of the dividing streamline concept of Sheppard’s model for determining the proper blocking height (h_d) resulted in a modified form (L = Nh_d/ f), which improved the predictive skill. For the determination of maximum wind speeds, the high Fr relationship (ΔV = Nh_m) was found to be better correlated with the measured values than the low Fr relationship ( ΔV = U_n) throughout the full range of Fr. Two-dimensional linear theory performed poorly for Fr < 0.5. Modifications were made to these previous relationships to better account for the three dimensional winds, which helped to improve the estimated wind speed enhancements.