Aircraft Observations of Offshore-directed Flow near Wide Bay, Alaska

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  • 1 Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington
  • | 2 National Oceanic and Atmospheric Administration, Pacific Marine Environmental Laboratory, Seattle, Washington
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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.

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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