Onshore flow and coastally trapped disturbances: an idealised simulation

Onshore flow and coastally trapped disturbances: an idealised simulation

During summer months, winds along the West Coast of the United States are climatologically northerly, driven by the north-eastern Pacific subtropical high pressure and the thermal low inland. This northerly flow is often interrupted by short periods of southerly winds, some of which are associated with coastally trapped disturbances (CTDs). The climatological study by Mass and Bond (1996) indicated that a number of CTDs are associated with a significant onshore flow to the south of the region where a CTD was observed. One of such events occurred in May 1985, during which a non-uniform onshore flow was observed.

This study uses the Regional Atmospheric Modeling System (RAMS) to conduct a sequence of idealised simulations in which a non-uniform onshore flow is imposed. The onshore wind is specified by a Gaussian distribution along the N-S direction with the maximum speed of 5 ms^-1, and there is no initial vertical variation in wind speed except in the boundary layer. The initial depth of the marine boundary layer is 400 m and a capping inversion aloft has strength of 10 ^oK in potential temperature. The Froude number corresponding to the boundary layer is 0.43. This value is less than unity and the flow in the boundary layer will largely be blocked by the coastal mountain. Associated with this blocking is a CTD mainly within the marine boundary layer. The disturbance is characterised by a raised marine boundary layer, trapped southerly winds, and a high-pressure ridge along the coast. Trapped southerly winds appear within and below the raised marine layer and the alongshore scale increases with time. Two modes of southerly winds are resolved by the model: a standing mode and a moving mode. The moving mode progresses northward and its phase speed is independent to the inversion layer thickness and to the width and strength of initial onshore flow. The propagation speed is in good agreement with that of a linear Kelvin wave. The structure of disturbance resembles that of a Kelvin wave in an early stage of the evolution. The forces are in quasi-geostrophic balance in the across-shore direction, while in the alongshore direction local acceleration is also important. The ratio of kinetic to potential energy of the disturbance also possesses the feature of a Kelvin wave. We conclude that the response to the non-uniform onshore flow forcing is characterised by a Kelvin wave during early stage. The CTD is gradually damped as it propagates northwards.