Orographic Effects on Track Deflection and Precipitation Associated with the Passage of an Idealized Tropical Cyclone

In this study, a serial of idealized numerical experiments are performed using the Advanced Research Weather Research and Forecast (ARW) model to help understand the orographic effects on the track deflection and structure of tropical cyclones (TCs). In these numerical experiments, TC is initialized by a bogus vortex in a conditionally unstable stratified airflow. For an idealized TC embedded in a 5 m s-1 easterly flow over a 1-km high mountain, the TC is deflected to the south upslope (eastern slope) and then to the north over the lee (western) slope while resuming its westward movement. The basic-flow and vortex Froude numbers are estimated to be U/Nh = 0.5 and Vmax/Nh = 5.5, respectively. Based on the findings of Lin et al. (2005 JAS – L05), this flow belongs to the moderately blocking regime. For a 2-km high mountain, the TC track is similar to that of 1-km high mountain, but the southward deflection is much more significant. The accumulated rainfall is much stronger, extends farther along the mountain range, and produces much more upslope rainfall, compared with that over the1-km high mountain. The estimated U/Nh and Vmax/Nh are 0.25 and 2.75 respectively, indicating that flow still belongs to the moderate blocking regime. Thus, the track is still continuous, consistent with L05's findings. Without latent heating, it is found that the dry vortex cannot survive the orographic blocking by the 2-km high mountain. Without both latent heating and planetary boundary layer forcing, the dry vortex is able to survive the orographic forcing, but its track becomes discontinuous in passing over a 2-km high mountain. Sensitivity experiments with various Froude numbers (U/Nh and Vmax/Nh) and lack of bogus vortex, PBL, and moisture effects will be conducted to help understand the dynamics of orographic effects on TC track deflection. In addition, vorticity budget analysis and potential vorticity budget analysis will be performed.