Gap winds and downslope winds

Strong surface winds can be generated by the interaction of the synoptic-scale flow with the topography through two different mechanisms, gap winds and downslope winds. Gap winds are produced when air is forced through a narrow break in a mountain barrier. Downslope winds may be generated when air flows across mountain ridges with steep lee slopes. Numerical simulations are presented that illustrate the relation and interaction between gap winds and downslope winds. The behavior of idealized flows encountering simplified topography is compared with a numerical case study of an extreme high wind event in Cook Straight, New Zealand on 19 October 1998. These simulations suggest that the maximum wind speeds produced by the gap-wind mechanism tend to be weaker than those produced by downslope winds, but that the areal extent of the high-wind region associated with gap winds may be significantly larger.