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

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 stratuscapped, coastally trapped disturbance (CTD) propagating northward against the prevailing flow. A pronounced hydraulic-jump-like (‘shock’) feature develops in the cloud field south of Cape Mendocino (CM) and angles linearly away from the coast. Over the next ~6 hours both the CTD and the 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. The CTD rather abruptly stalls and fails to round CM. Further, a second cyclonic mesoscale eddy is observed to form SW of Point Arena. Using a mesoscale model, many of the observed features of this case are forecast, including: (a) the cloud-filled, northward propagating CTD; (b) the development, linear structure, and orientation of the oblique shock feature; (c) the CTD rounds Point Arena, but fails to round CM; and (d) the formation of cyclonic mesoscale eddies near CM and Point Arena. The model fidelity permits us to analyze the dynamics associated with the shock feature and eddy formation.