Dynamical influence of large valleys on the propagation of coastally trapped disturbances

Abstract

The influence on the propagation of coastally trapped disturbances (CTD) of large gaps in the bounding coastal mountains is studied via application of the Colorado State University Regional Atmospheric Modeling System (RAMS). By large gap it is meant that the width is of order 25–50% of the Rossby radius. In order to isolate the effects of the gaps, idealised simulations are used. Thus, RAMS is configured with a straight north-south coastline with a ramp-shaped coastal mountain range in which a gap is inserted and a gravity current-like CTD generated through cooling the low level atmosphere at the southern end of the domain. Radiative, land surface and cloud parameterisations are turned off to avoid sea breeze generation and other thermal effects and to focus on the dynamical effect of the gaps. Four simulations are conducted: no gap in the coastal mountain barrier, an insertion of a narrow (80 km) gap, a wide (160 km) gap, and an idealised island in the barrier. It is found that the gaps influence CTD propagation by effectively stalling the propagation at their mouth for a period and subsequently weakening the gravity current-like features (vertical extent, strength and abruptness of the wind and stratification changes at the leading edge). The idealised island has a less obvious impact since it is bounded by relatively narrow straits and does not allow communication with the model interior like the gaps do. Although lack of data and the nature of the idealised simulation prevents a detailed assessment of the simulations against real cases, qualitative comparisons with observed events in western North America and south-eastern Australia are made. Application of these results to air quality and forecasting issues is discussed.