Orographic Influence on Basic Flow and Cyclone Circulation and Their Impacts on Track Deflection of an Idealized Tropical Cyclone

Abstract

A series of idealized numerical experiments and vorticity budget analyses is performed to examine several mechanisms proposed in previous studies to help understand the orographic influence on track deflection over a mesoscale mountain range. When an idealized tropical cyclone (TC) is embedded in a uniform, easterly flow and passes over a mountain with a moderate Froude number, it is deflected to the south upstream, moves over the mountain anticyclonically, and then resumes its westward movement. The vorticity budget analysis indicates that the TC movement can be predicted by the maximum vorticity tendency (VT). The orographic effects on the above TC track deflection are explained by the following: 1) Upstream of the mountain, the easterly basic flow is decelerated as a result of orographic blocking that causes the flow to become subgeostrophic, which advects the TC to the southwest, analogous to the advection of a point vortex embedded in a flow. The VT is primarily dominated by the horizontal vorticity advection. 2) The TC passes over the mountain anticyclonically, mainly steered by the orographically generated high pressure. This makes the TC move southwestward (northwestward) over the upslope (lee slope). The VT is mainly contributed by the horizontal vorticity advection with additional contributions from vorticity stretching and the residual term (which includes friction and subgrid turbulence mixing). 3) Over the lee slope and downstream of the mountain, the northwestward movement is enhanced by asymmetric diabatic heating, making the turning more abrupt. 4) Far downstream of the mountain, the VT is mainly contributed by the horizontal vorticity advection.

The influence of topography on midlatitude cyclones on Australia's east coast

“…This is consistent with the influence of topography on rainfall elsewhere in the world [ Tselioudis et al ., ; ter Maat et al ., ]. Globally, poor representation of topography contributes to biases in the global storm tracks [ Brayshaw et al ., ; Pithan et al ., ] and influences the deflection of tropical cyclone tracks [ Lin et al ., ], with east Asian orography in particular having a strong influence on both the Pacific and Atlantic storm tracks [ Saulière et al ., ]. However, idealized global studies have typically excluded the Australian landmass including east Australian topography from their model simulations, providing limited information about the impacts of topography on cyclones in the southern hemisphere [e.g., Brayshaw et al ., ; Saulière et al ., ].…”

Evolution, propagation and interactions with topography of hurricane-like vortices in a moist-convective rotating shallow-water model

“…This is why observational and theoretical studies of interactions of TC with such islands are recurrent in the literature, e.g. Chang, Yeh & Chen (1993), Yeh & Elsberry (1993a,b), Kuo et al (2001), Tang & Chan (2015), Wu, Li & Huang (2015), Tang & Chan (2016), Huang et al (2016) and Lin, Chen & Liu (2016). The observed TC tracks exhibit a variety of patterns with both northward and southward deflections.…”

Recent Progress in the Fundamental Understanding of Tropical Cyclone Motion

“…The interaction between terrain, basic flow and the TC is non-linear. Recent studies have made efforts in further understanding of the mechanisms leading to TC track deflection (Lin et al, 2016). The following mechanisms have been proposed to explain the upstream track deflection: 1 (Fig.…”

“…Other parameters such as the width and length of the mountain, and the radius of maximum wind (RMW) have relatively limited effects on the track of the TC. Lin et al (2016) used the vorticity tendency (VT) diagnostic to examine the southward deflection of TC. When the TC was upstream the mountain, the easterly basic flow became sub-geostrophic as a result of orographic blocking.…”

“…The TC decelerated and was deflected to the south, with the VT primarily dominated by the horizontal advection (see Fig. 1 of Lin et al (2016)). After that, the TC passed over the mountain clockwise, steered by the orographically generated high-pressure.…”