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High-Resolution Observations and Numerical Simulations of Easterly Gap Flow through the Strait of Juan de Fuca on 9–10 December 1995

Brian A. ColleInstitute for Terrestrial and Planetary Atmospheres, State University of New York at Stony Brook, Stony Brook, New York

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Clifford F. MassDepartment of Atmospheric Sciences, University of Washington, Seattle, Washington

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Abstract

This paper examines the three-dimensional flow and dynamics of easterly gap flow through the Strait of Juan de Fuca on 9–10 December 1995 using high-resolution observations and model simulations. The observations were obtained during the fourth intensive observing period during the second phase of the Coastal Observation and Simulation with Topography field experiment. One of the most important data sources was the NOAA P-3 aircraft, which provided flight-level data, reflectivity, and Doppler winds as it flew down the Strait of Juan de Fuca. Below 900 m, the flow accelerated from 7 to 14 m s−1 at the entrance to the strait, with a significant cross-gap component and confluence over the eastern sections of the strait below 500 m. The easterly flow approaching the western exit of the strait accelerated from 15 to 20 m s−1. As the 900 m deep easterly flow exited the gap it decreased in depth and accelerated to ∼25 m s−1, creating a shallow swath of diffluent winds below 300 m. Later in the event when the gap flow became more shallow, the strongest low-level winds shifted eastward into the western section of the strait.

This event was simulated down to 1.33-km horizontal resolution using The Pennsylvania State University–NCAR Mesoscale Model version 5. Although many of the observed structures were realistically simulated, the model underestimated the sea level pressure difference down the strait by 30%, resulting in gap exit winds 2–5 m s−1 weaker than observed. The model also maintained the swath of low-level easterlies too far downwind of the gap. Sensitivity runs illustrate how the gap flow changes with different horizontal and vertical resolutions.

Trajectories and momentum diagnostics from the model indicate that this case did not represent the classic conceptual model of gap flow down a long channel. There was confluence within the gap as a result of northeasterly flow descending into the strait from southern Vancouver Island. This subsidence in the lee (south) of Vancouver Island created lee troughing, a significant cross-strait pressure gradient, and flow curvature over the gap at low levels. The gap outflow is shown to have structural similarities to classical expansion fan theory. However, unlike expansion fan theory, much of the low-level acceleration and subsidence at the gap exit was forced by downslope flow off the surrounding terrain, and the parcels associated with the gap outflow fan originated from many different locations.

Corresponding author address: Dr. B. A. Colle, Marine Science Research Center, State University of New York at Stony Brook, Stony Brook, NY 11794-5000.

Email: bcolle@notes.cc.sunysb.edu

Abstract

This paper examines the three-dimensional flow and dynamics of easterly gap flow through the Strait of Juan de Fuca on 9–10 December 1995 using high-resolution observations and model simulations. The observations were obtained during the fourth intensive observing period during the second phase of the Coastal Observation and Simulation with Topography field experiment. One of the most important data sources was the NOAA P-3 aircraft, which provided flight-level data, reflectivity, and Doppler winds as it flew down the Strait of Juan de Fuca. Below 900 m, the flow accelerated from 7 to 14 m s−1 at the entrance to the strait, with a significant cross-gap component and confluence over the eastern sections of the strait below 500 m. The easterly flow approaching the western exit of the strait accelerated from 15 to 20 m s−1. As the 900 m deep easterly flow exited the gap it decreased in depth and accelerated to ∼25 m s−1, creating a shallow swath of diffluent winds below 300 m. Later in the event when the gap flow became more shallow, the strongest low-level winds shifted eastward into the western section of the strait.

This event was simulated down to 1.33-km horizontal resolution using The Pennsylvania State University–NCAR Mesoscale Model version 5. Although many of the observed structures were realistically simulated, the model underestimated the sea level pressure difference down the strait by 30%, resulting in gap exit winds 2–5 m s−1 weaker than observed. The model also maintained the swath of low-level easterlies too far downwind of the gap. Sensitivity runs illustrate how the gap flow changes with different horizontal and vertical resolutions.

Trajectories and momentum diagnostics from the model indicate that this case did not represent the classic conceptual model of gap flow down a long channel. There was confluence within the gap as a result of northeasterly flow descending into the strait from southern Vancouver Island. This subsidence in the lee (south) of Vancouver Island created lee troughing, a significant cross-strait pressure gradient, and flow curvature over the gap at low levels. The gap outflow is shown to have structural similarities to classical expansion fan theory. However, unlike expansion fan theory, much of the low-level acceleration and subsidence at the gap exit was forced by downslope flow off the surrounding terrain, and the parcels associated with the gap outflow fan originated from many different locations.

Corresponding author address: Dr. B. A. Colle, Marine Science Research Center, State University of New York at Stony Brook, Stony Brook, NY 11794-5000.

Email: bcolle@notes.cc.sunysb.edu

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