Orographic Influence on a Drifting Cyclone

In this study, a primitive equation numerical model is adopted to investigate the orographic influence on a drifting cyclone over an idealized topography similar to that of Taiwan. For a cyclone propagating from the east and impinging on the central portion of the mountain, a northerly surface jet tends to form upstream of the mountain between the primary cyclone and the mountain due to blocking and channeling effects. Two pressure ridges and one trough are also produced. When the cyclone approaches the mountain, the low-level vorticity and low pressure centers decelerate and turn southward upstream of the mountain due to orographic blocking. At the same time, the upstream low-level vorticity is blocked by the mountain. The abrupt increase of surface vorticity and the contraction of cyclone scale on the lee side are explained by the generation of new potential vorticity (PV) due to wave breaking associated with the severe downslope wind and hydraulic jump. The generation of this new PV is evidenced by the transition from the regime dominated by flow splitting to the regime dominated by wave breaking and the dominance of mixing and diffusion term in the vorticity and PV budgets. At this stage, the cyclone and low pressure centers appear to accelerate or jump over the mountain. At the same time, the surface low shifts to the south of the original westward track, which is primarily influenced by strong adiabatic warming associated with the downslope wind. The primary surface cyclone then resumes its original westward movement and symmetric circulation on the lee side once it moves away from the mountain. The deflection of the cyclone and low pressure centers at midlevels, such as σ = 3 km, are similar to those at the surface. Both vorticity and PV budgets are calculated to help understand the contributions from individual terms at different stages when a cyclone drifts over an idealized topography.