Environmental and Orographic Effects on Track Deflection of Idealized Tropical Cyclones

In this study, a series of systematic, idealized numerical experiments is performed to help understand the effects of orography, beta, shear, and physical parameterizations, including planetary boundary layer (PBL), cumulus (CP) and microphysics (MP) parameterization schemes, on the track deflection of tropical cyclones (TCs). In these numerical simulations, the TC is spun up by an initial bogus vortex, which is in gradient wind balance, in a conditionally unstable stratified fluid flow. In an easterly flow with PBL and moisture but no orography, we found the bogus TC moves northwestward, as found in previous studies and explained by the beta effect. However, unlike that found in previous studies in a quiescent fluid, no significant northwestward movement is produced on an f-plane in either a quiescent fluid or a uniform flow. With a backward (westward) shear in either a quiescent fluid or easterly flow, the TC moves toward right (northward), as also found in earlier studies. With a 15 km resolution, we find that simulated TC tracks are not sensitive to MP schemes, although this needs to be further confirmed by simulations with finer horizontal resolution. When CP is activated, the change in track deflection is insignificant. It is also found that it is necessary to activate the PBL parameterization; otherwise no TC would be spun up due to lack of convergence associated with the vortex in the boundary layer.

For an idealized TC embedded in a 5 m/s easterly flow over a 1 km high mountain, which gives a Froude number (Fr) of about 0.5, the TC is deflected to the south upstream and then recover its westward movement on the lee side of the mountain, similar to that in strong blocking regime as found in Lin et al. (2005 JAS – L05). For a 2 km high mountain (Fr~0.25), the southward deflection is much larger due to extremely strong blocking. In fact, the TC is deflected slightly to the north first when it is far upstream from the mountain and then deflected to the south when it is near the mountain. Unlike a dry vortex, the TC near surface is destroyed by the orographic blocking when it crosses the mountain range, and is unable to reorganize on the lee side. The accumulated rainfall is much stronger, extends farther along the mountain range, and produces much more upslope rainfall, compared with that over 1 km high mountain. Unlike hypothesized in earlier studies, cumulus heating does not force the cyclonic circulation around the mountain. Vorticity budget analysis and potential vorticity budget analysis will be performed to help understand the dynamics of orographic effects on TC track deflection.