Orographic Effects on Track Deflection and Structure Change of an Idealized Tropical Cyclone Passing over a Mesoscale Mountain Range

In this study, a number of numerical experiments are performed using the Advanced Research Weather Research and Forecast (ARW) model to help understand the fundamental orographic effects on the track deflection and structure of an idealized tropical cyclone (TC) passing over an idealized mountain range. 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 (U) 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 (control case), respectively, where Vmax is the maximum TC tangential velocity, N is the Brunt-Vaisala frequency of the environment, and h is the mountain height. Based on the findings of a previous study (Lin et al. 2005 - L05), this flow belongs to the moderately blocking regime except with stronger blocking associated with a much longer mountain range. With U/Nh = 0.25 and Vmax/Nh = 2.75, the TC track is still continuous but the southward deflection is much more pronounced, similar to the control case. This is consistent with L05's findings because both cases belong to moderate blocking regime. The accumulated rainfall is much stronger, extends farther along the mountain range, and produces much more rainfall at the upslope, compared to the control case. The fundamental mechanism of track deflection with moisture is attributed mainly to the vorticity stretching associated with the outer circulation of the TC vortex and latent heating, based on the vorticity budget analysis. With a shorter mountain range, the vorticity advection terms shows a potential deflection to the north. It is shown that the latent heating is able to help hold the integrity of the TC vortex thus makes the TC track more continuous, based on a sensitivity experiment with latent heating deactivated. It is also found that the inclusion of planetary boundary layer forcing is required to sustain the TC vortex. The orographic effects on structure change of the TC are examined by analyzing both horizontal and vertical cross sections when it passes over the mountain range. It is found that the vertical structure of the TC becomes asymmetric during and after the passage over the mountain and the downstream flank of the eyewall is significantly weakened by the mountain.