Extended Abstract (800K)9.2
Mesoscale eddie formation and shock features associated with a coastally trapped disturbance
Stephen D. Burk, NRL, Monterey, CA; and W. T. Thompson
Synoptic maps for 28-29 August 2002 indicate typical near-surface northerly flow along coastal California, while Goes-10 animated satellite imagery from this period show a stratus-capped, coastally trapped disturbance (CTD) propagating northward against the prevailing flow. A pronounced, linear, hydraulic-jump-like (‘shock’) feature develops in the cloud field south of Cape Mendocino (CM) and angles away from the coast. Over the next ~6 hours both the CTD and the oblique shock feature move northward and, as they approach the Cape, the leading edge of the CTD clouds roll-up into a visually striking cyclonic mesoscale eddy—with the shock feature being wrapped into the eddy. Although the CTD had been rapidly propagating northward, it rather abruptly stalls and fails to round CM. Further, a second cyclonic mesoscale eddy is observed to form SW of Point Arena. In this study we use the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) to forecast and analyze these dramatic mesoscale features. Using a triply-nested grid structure with a 3 km inner grid, the model forecasts many of the observed features of this case, including: (a) the cloud-filled, northward propagating CTD; (b) the development, linear structure, and orientation of the oblique shock feature; (c) the CTD rounding Point Arena, but failing to round CM; and (d) the formation of cyclonic mesoscale eddies near CM and Point Arena. With allowance for a several hour phase lag, time series of COAMPS winds and temperature are found to agree quite well with buoy observations. This model fidelity permits us to analyze the dynamics associated with the shock feature and formation of the eddies. We find that the interaction of the approaching surge with transcritical flow around CM generates the pronounced shock feature – with Froude number rapidly transitioning through unity across the oblique shock. COAMPS isentropic surfaces show a pronounced jump, with boundary layer depth abruptly increasing in the shock, while the wind speed undergoes a sharp decrease across the shock. Relative (and absolute) vorticity are enhanced in the shock by horizontal flow convergence and vertical vorticity stretching, while there is generation of potential vorticity by dissipation within the shock. Supercritical flow leeward of CM acts to dampen, and halt, the northward CTD propagation. The instability characteristics of the strong, narrow shear zone between the CTD and the northerly background flow are analyzed using a classical stability analysis. Perturbation growth rates and wavelengths of maximum growth rate from parallel shear flow stability theory are in very rough agreement with the observed and modeled eddy growth and spacing. These results may be improved by accounting for topographic forcing in the selection of the fastest growing mode. Finally, a sensitivity study was conducted in which the ocean roughness length was replaced with a typical land value. The horizontal shear between the southerly flow of the CTD and the northerly background flow is substantially diminished in this case, and only a weak, transient eddy develops SW of Point Arena. However, the enhanced surface roughness does not substantially alter the development of the shock feature.
Session 9, Coastal Orographic Effects
Friday, 8 August 2003, 1:30 PM-3:10 PM
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