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Nicholas A. Bond and S. Allen Macklin

Abstract

The effects of the upstream orography of the Alaska peninsula on the low-level flow in the coastal region are studied using observations from two NOAA P-3 research flights. The terrain in this region includes a low sill at Wide Day (approximately 300 m high and 80 km wide), which is flanked by moderate terrain to the southwest (∼900 m high) and higher terrain to the northeast (∼1500 m high). For the case of 26 February 1987, a large Froude number (Fr ∼ 1.6) characterized the incident flow. Boundary-layer wind speeds were approximately 30 m s−1 downstream of the gap at Wide Bay and the moderate terrain. The cross-terrain component of the wind above the boundary layer was 24 m s−1 upstream of the barrier and as lame as 45 m s−1 approximately 70 km downstream of the barrier. Wind speeds were significantly less above and downstream of this wind maximum, as with a hydraulic jump. A prominent trough in sea level pressure was observed in the lee of the higher terrain; the largest 100-m wind speeds (∼34 m s−1) observed were near this trough. For the case of 3 March 1987, the incident flow over the Alaska peninsula was weak, and the Froude number was small (Fr ∼ 0.4). In this situation, a low-level outflow (∼300 m high) with large wind speeds, cold air temperatures, and high sea level pressure was isolated to the region downstream from the gap at Wide Bay.

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S. Allen Macklin, Gary M. Lackmann, and Judith Gray

Abstract

The thermal contrast between cold air over continental Alaska and relatively warm marine air over the Gulf of Alaska causes frequent, low-level, offshore-directed winds over the south-central Alaskan coast during the cold season. Coastal mountains affect these winds by inhibiting low-level mixing of continental and marine air masses near the coast, by providing channels that focus and accelerate drainage winds, and by exciting mountain-lee waves.

Offshore-directed winds were observed twice with a research aircraft. The strongest winds were measured at the mouth of the Copper River and over and downwind of Resurrection Bay. The synoptic weather pattern and its orientation to local topographic features influenced wind magnitude. With the pressure gradient perpendicular to the coast, offshore-directed flow was light except at the Copper River and Resurrection Bay drainages. When the pressure gradient was aligned with the shore, regional surface winds were stronger and more uniform, although the Copper River and Resurrection Bay drainages wore still discernible.

Evaluation of local force balances showed the largest ageostrophy at coastal locations downwind of bays and river valleys. Seaward from the coast, ageostrophic accelerations and cross-isobaric wind components were smaller, indicating a transition toward geotriptic equilibrium. This spatial adjustment pattern is consistent with the hypothesis that equilibrium is achieved within a distance similar to the regional Rossby radius of deformation.

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S. Allen Macklin, Nicholas A. Bond, and Jeffrey P. Walker

Abstract

During February 1982 a NOAA research aircraft investigated a cold, low-level jet blowing from a gap between mountain ranges on line west side of Cook Inlet, Alaska. The jet blew 200 km southeastward across Cook Inlet between the Kenai Peninsula and the Kodiak archipelago, passing into the Gulf of Alaska where it merged with the large-scale marine wind field. Measurement commenced ∼35 km downstream of the seaward end of the gap. The jet's internal boundary layer accelerated by 5% and grew 20% in depth for ∼50 km; thereafter, wind speed and boundary-layer depth were nearly constant for the next 100 km. The strongest winds (>20 m s−1 at a height of 80 m) were observed on the south side of the jet's thermal axis and 90 km downstream from the coast. Budget analyses reveal that the down-gradient acceleration within the jet was principally opposed by surface friction, and the cold air advection was balanced by a strong upward-directed sensible heat flux from Cook Inlet and entrainment of warmer air from aloft.

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