Numerical simulations of coastally trapped disturbances

Coastally trapped disturbances ( CTDs) are trapped horizontally by the Coriolis force and vertically by a subsidence temperature inversion to propagate along coastal mountains. There are sudden temperature drops and wind reversals in CTD events. CTDs are initiated as gravity currents in a 3 dimensional mesoscale numerical model in this research by cooling the model lower atmosphere near the shore. The RAMS model is used in a sensitivity testing mode to examine the influences of the following factors on the evolution of CTDs: 1. different initial synoptic wind speeds 2. initial cooling amount and cooling area 3. topography that descends in the direction of propagation 4. different valleys that dissect coastal mountains 5. sea surface temperature 6. the presence of a near-shore island such as Vancouver Island The results show that: 1. The larger the initial northerly wind is, the smaller the offshore scale of a CTD is. 2. CTDs are very sensitive to initial cooling conditions by which they are created. The larger the cooling amount is, the stronger a CTD is. The effect of cooling area depends on initial cooling amount. The influence of different cooling areas can be the opposite with different initial cooling amount. 3. The propagation speed of CTDs is greatly reduced in the case of descending topography. 4. Valleys which dissect the coastal mountains greatly reduce the off-shore scales of CTDs. 5. When the sea surface temperature is close to the potential temperature of the lower atmosphere after cooling, a front comes into being in the lower atmosphere and the propagation speed of CTDs is enhanced. 6. Simulations show that near-shore islands may split a CTD with trapped disturbances propagating along both the island and mainland.