Sensitivity of coastally trapped disturbance dynamics to barrier height and topographic variability in a numerical model

K.J. Tory
P.L. Jackson
C.J.C. Reason


Submitted to Mon. Wea. Rev., in revised form March 21, 2001.

Abstract

The sensitivity of a coastally trapped disturbance (CTD) to topographic height is examined using simulations of the 15-18 May 1985 CTD. These simulations include three with uniform topography, in which the North American west coast mountains are represented by a three piece uniform ramp at the coast leading to a constant plateau inland, and three with realistic topography. In each trio of uniform and realistic topography simulations, there is a control case in which the terrain height closely approximates reality, and two variations in which the topography is multiplied everywhere by a Topographic Multiplication Factor (TMF) of 1.5 and 0.5 to assess the sensitivity of the simulation to the barrier height. Average propagation speeds increased (decreased) by 15--20 % with the increased (decreased) TMF.

It was found that the position of the CTD leading edge generally followed close behind a leading pressure trough minimum, which propagated northward along the coast in a manner similar to a Topographically Trapped Rossby Wave (TTRW). The propagation of both the CTD and TTRW was diurnally modulated, with slowing during the day. The diurnal effects were stronger on the CTD propagation which led to a CTD lag after the heating period followed by an acceleration back towards the position of the trough minimum. Further variability in the CTD propagation was present in the more realistic topography simulations caused by pressure variations ahead of the CTD related to alongshore differences in Marine Boundary Layer (MBL) structure.

The average propagation speed of the leading coastal trough was proportional to barrier height and not barrier slope which is consistent with TTRW theory applied to the model barrier structure. Due to the dominant influence of the coastal trough on CTD propagation this led to an average CTD propagation speed proportional to TMF.