A Dynamical Initialization Scheme for Tropical Cyclones under the Influence of Terrain

This study attempts to extend an earlier dynamical initialization (DI) scheme for tropical cyclones (TCs) to situations under the influence of terrain(s). When any terrain lower than 1 km exists between 150 and 450 km from the TC center, topographic variables are defined and a filtering algorithm is used to remove noise due to the presence of terrains before the vortex separation is conducted. When any terrain higher than 1 km exists between 150 and 450 km from the TC center or the TC center is within 150 km of land, a semi-idealized integration without the terrain is conducted to spin up an axisymmetric TC vortex before the inclusion of the terrain and the merging of the TC vortex with the large-scale analysis field. In addition, a procedure for the vortex size and intensity adjustments is introduced to reduce the initial errors before the forecast run. Two sets of hindcasts, one without (CTRL run) and one with the new DI scheme (DI run), are conducted for 9 TCs affected by terrains over the Northwest Pacific in 2015. Results show that the new DI scheme largely reduces the initial position and intensity errors. The 72-h position errors and the intensity errors up to 36 h forecasts are smaller in DI runs than in CTRL runs as well as smaller than those from the HWRF forecasts for the same TCs. The new DI scheme is also shown to produce the TC inner-core structure and rainbands more consistent with satellite and radar observations.