Mesoscale Eddy Formation and Shock Features Associated with a Coastally Trapped Disturbance

Stephen D. Burk Naval Research Laboratory, Monterey, California

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William T. Thompson Naval Research Laboratory, Monterey, California

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Abstract

On 28 August 2002, a visually striking sequence of events appeared in satellite imagery showing a coastally trapped disturbance (CTD) propagating northward along the coast of California against a northerly background flow. As a narrow tongue of coastal stratus indicative of the CTD propagated northward, a long, linear set of wave clouds developed ahead of the advancing CTD and angled away from the coast. The CTD and cloud lines moved northward over the next ∼6 h and, as they approached Cape Mendocino (CM), the leading edge of the CTD clouds rolled up into a cyclonic mesoscale eddy—with the wave clouds being wrapped into the eddy. The CTD abruptly stalled and failed to round CM. Further, a second cyclonic mesoscale eddy formed southwest of Point Arena (PA).

Although there has been extensive study of the propagation phase of CTDs, relatively little attention has been paid to the cessation of their propagation wherein mesoscale eddy development is not uncommon. Using the U.S. Navy's Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), run in an operational manner, numerous observed features of this case are forecast, including: (i) the cold, shallow, cloud-filled, northward-propagating CTD; (ii) the development, linear structure, orientation, and movement of an oblique hydraulic jump–like (“shock”) feature; (iii) a southerly wind shift associated with the CTD that precedes the advancing cloud tongue by several hours in both the observations and the model; (iv) the modeled CTD that rounds PA, but fails to round CM; and (v) the formation of modeled cyclonic mesoscale eddies near both CM and PA. North of PA, however, a phase error develops in which the modeled CTD propagation is too slow.

The model forecast cloud tongue behaves as a gravity current, and similar to earlier observational and modeling studies of CTDs, the model forecasts a bore propagating in the stratified atmosphere immediately above the marine boundary layer. Supercritical flow is forecast in the accelerating northerly flow rounding CM, and when the advancing bore interacts with this high Froude number region a pronounced oblique shock develops and the CTD stalls. Vorticity is enhanced along this shock due to vertical stretching and potential vorticity is generated within the shock. Additionally, juxtaposition of the CTD's southerly flow with the background northerly flow creates a vortex sheet–like shear zone along the offshore flank of the CTD, with the horizontal gradient of absolute vorticity changing signs, which is a necessary condition for classic barotropic instability.

Corresponding author address: Dr. Stephen D. Burk, Naval Research Laboratory, 7 Grace Hopper Ave., Monterey, CA 93943-5502. Email: burk@nrlmry.navy.mil

Abstract

On 28 August 2002, a visually striking sequence of events appeared in satellite imagery showing a coastally trapped disturbance (CTD) propagating northward along the coast of California against a northerly background flow. As a narrow tongue of coastal stratus indicative of the CTD propagated northward, a long, linear set of wave clouds developed ahead of the advancing CTD and angled away from the coast. The CTD and cloud lines moved northward over the next ∼6 h and, as they approached Cape Mendocino (CM), the leading edge of the CTD clouds rolled up into a cyclonic mesoscale eddy—with the wave clouds being wrapped into the eddy. The CTD abruptly stalled and failed to round CM. Further, a second cyclonic mesoscale eddy formed southwest of Point Arena (PA).

Although there has been extensive study of the propagation phase of CTDs, relatively little attention has been paid to the cessation of their propagation wherein mesoscale eddy development is not uncommon. Using the U.S. Navy's Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), run in an operational manner, numerous observed features of this case are forecast, including: (i) the cold, shallow, cloud-filled, northward-propagating CTD; (ii) the development, linear structure, orientation, and movement of an oblique hydraulic jump–like (“shock”) feature; (iii) a southerly wind shift associated with the CTD that precedes the advancing cloud tongue by several hours in both the observations and the model; (iv) the modeled CTD that rounds PA, but fails to round CM; and (v) the formation of modeled cyclonic mesoscale eddies near both CM and PA. North of PA, however, a phase error develops in which the modeled CTD propagation is too slow.

The model forecast cloud tongue behaves as a gravity current, and similar to earlier observational and modeling studies of CTDs, the model forecasts a bore propagating in the stratified atmosphere immediately above the marine boundary layer. Supercritical flow is forecast in the accelerating northerly flow rounding CM, and when the advancing bore interacts with this high Froude number region a pronounced oblique shock develops and the CTD stalls. Vorticity is enhanced along this shock due to vertical stretching and potential vorticity is generated within the shock. Additionally, juxtaposition of the CTD's southerly flow with the background northerly flow creates a vortex sheet–like shear zone along the offshore flank of the CTD, with the horizontal gradient of absolute vorticity changing signs, which is a necessary condition for classic barotropic instability.

Corresponding author address: Dr. Stephen D. Burk, Naval Research Laboratory, 7 Grace Hopper Ave., Monterey, CA 93943-5502. Email: burk@nrlmry.navy.mil

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