Sensitivity of typhoon track to asymmetric latent heating/rainfall induced by Taiwan topography: A numerical study of Typhoon Fanapi (2010)

Abstract

Typhoon Fanapi (2010) traveled westward across the Central Mountain Range of Taiwan on 19 September and its rainfall shifted from a symmetric to an asymmetric pattern with convection mostly to the south and southeast. Meanwhile, the storm slowed down from 22 to 14 km h−1 for 12 h upon leaving Taiwan, and led to heavy rainfall (>800 mm) and serious flooding over the low‐lying southwestern plains. Through simulation and sensitivity tests using the Cloud‐Resolving Storm Simulator at 3 km grid size, this study shows that the sudden and temporary speed reduction was caused by the asymmetric latent heating (LH), not the environmental flow. Specifically, over a 9 h period, the model storm moved westward at 16 km h−1 in the control run, but increasingly faster and more toward the northwest when the moisture (and thus the LH effect and its asymmetry) is gradually reduced. Steering flow analysis and estimation using model results suggest an eastward motion vector of about 8 km h−1, consistent with the observation, is produced by the asymmetric LH effect, when the effects from the vertical wind shear and beta‐drift are both taken into account. This result is further supported by the diagnosis on storm motion based on potential vorticity tendency. Although important, such feedback to typhoon track from rainfall asymmetry that is induced by the blocking effect of topography have not been reported or studied.