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M. Susan Lozier
,
Lawrence J. Pratt
,
Audrey M. Rogerson
, and
P. D. Miller

Abstract

In an effort to understand the extent to which Lagrangian pathways in the Gulf Stream indicate fluid exchange between the stream and its surroundings, trajectories of RAFOS floats are viewed in a frame of reference moving with the dominant zonal phase speed associated with the periodic flow. In such a frame, geometrical structures emerge that more clearly delineate the position of the parcel in relation to the jet core and its surroundings. The basic premise of this work is that the pathways of fluid parcels in the vicinity of stagnation points, as defined in the moving frame of reference, are susceptible to changes in their pathways, thereby facilitating fluid exchange between different regions of the flow field. Four representative RAFOS float trajectories are shown to exhibit the expected behavior in the vicinity of stagnation points. To further examine the mechanism of exchange in the vicinity of these geometrical features, concepts from dynamical systems theory are applied to a numerically simulated flow field. The entrainment and detrainment of parcels from the jet core are explained in the context of stable and unstable manifolds and their associated lobes. It is shown that the Lagrangian pathways from the numerical flow and the observational trajectories exhibit a similarity based on the kinematics of a meandering flow field. Overall, this study provides the first look at RAFOS float trajectories in a moving frame and provides insight as to how the temporal variability of a jet creates chaotic exchange.

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Kathleen A. Edwards
,
Audrey M. Rogerson
,
Clinton D. Winant
, and
David P. Rogers

Abstract

During summer, significant changes in marine atmospheric boundary layer (MABL) speed and depth occur over small spatial scales (<100 km) downstream from topographic features along the California coast. In June and July 1996, the Coastal Waves 96 project collected observations of such changes at capes with an instrumented aircraft. This paper presents observations from the 7 June flight, when the layer-averaged speed increased 9 m s−1 and depth decreased by 500 m over a 75-km downwind from Cape Mendocino, accompanied by enhanced surface fluxes and local cloud clearing. The acceleration and thinning are reproduced when the flow is modeled as a shallow transcritical layer of fluid impinging the bends of a coastal wall, leading to the interpretation that they are produced by an expansion fan. Model runs were produced with different coastlines and imposed pressure gradients, with the best match provided by a coastline in which the cape protruded into the flow and forced a response in the subcritical region upstream of the cape. A hydraulic jump was produced at a second bend, near where the aircraft's lidar observed the MABL height to increase. Light variable winds observed within Shelter Cove were replicated in model flows in which the flow separated from the coastline. Though highly idealized, the shallow-water model provided a satisfactory representation of the main features of the observed flow.

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Wendell A. Nuss
,
John ML Bane
,
William T. Thompson
,
Teddy Holt
,
Clive E. Dorman
,
F. Martin Ralph
,
Richard Rotunno
,
Joseph B. Klemp
,
William C. Skamarock
,
Roger M. Samelson
,
Audrey M. Rogerson
,
Chris Reason
, and
Peter Jackson

Coastally trapped wind reversals along the U.S. west coast, which are often accompanied by a northward surge of fog or stratus, are an important warm-season forecast problem due to their impact on coastal maritime activities and airport operations. Previous studies identified several possible dynamic mechanisms that could be responsible for producing these events, yet observational and modeling limitations at the time left these competing interpretations open for debate. In an effort to improve our physical understanding, and ultimately the prediction, of these events, the Office of Naval Research sponsored an Accelerated Research Initiative in Coastal Meteorology during the years 1993–98 to study these and other related coastal meteorological phenomena. This effort included two field programs to study coastally trapped disturbances as well as numerous modeling studies to explore key dynamic mechanisms. This paper describes the various efforts that occurred under this program to provide an advancement in our understanding of these disturbances. While not all issues have been solved, the synoptic and mesoscale aspects of these events are considerably better understood.

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