The vertical structure of linear coastal-trapped disturbances

The Vertical structure of coastal-trapped disturbances in several idealized models of a stably stratified lower atmosphere is examined. The vertical s tructure and phase speeds of the trapped modes depend on the resting strati fication and the height of the orographic step. The presence of a stable la yer above the boundary layer inversion increases the gravest-mode phase spe ed and supports the existence of higher vertical modes. Trapped wave soluti ons for the step orography are obtained for a lower atmosphere with constan t buoyancy frequency. The modes are primarily concentrated below the step b ut penetrate weakly into the stratified region above the step. The phase sp eed of the gravest trapped mode is greater than the gravest-mode Kelvin wav e speed based on the height of the step. Results from a linear two-layer mo del suggest that the observed vertical structure of isotherms at the leadin g edge of a 10-11 June 1994 event may arise during a transition from a dire ctly forced, barotropic, alongshore velocity response to a regime influence d by wave propagation, as the coastal-trapped vertical modes excited by the mesoscale pressure gradients begin to disperse at their respective phase s peeds. The results suggest also that the observed vertical structure of alo ngshore velocity, with largest velocities in the stable layer above the bou ndary layer, may arise from drag at the sea surface.