An asymmetric tropical cyclone rainfall model in the Northern Vietnam coast

Rainfall associated with landfalling tropical cyclones (TCs) along the Northern Vietnam coast is examined to develop an asymmetric parametric TC-induced rainfall model starting from the axisymmetric Rain-Climatology and Persistence (R-CLIPER) model. We recalibrated the R-CLIPER model (original R-CLIPER denoted as NHC) against observed rainfall patterns of 14 landfalling TCs from 2001 to 2021 in the Northern Vietnam coast, while relaxing the model's underlying linear relationships. The recalibrated R-CLIPER (denoted as Fit-Ax), still axisymmetric, suggests that some parameters are better correlated with the normalized maximum wind speed using logarithmic and exponential relationships. Fit-Ax reduces the 12-hr total rainfall overall root-mean-square errors (RMSEs) and Bias magnitudes in the before- and after-landfall periods from NHC for the entire 500-km TC domain. We further redistribute the Fit-Ax rainfall intensity across the four quadrants with respect to the TC forward motion to account for the observed large asymmetry in quadrant rainfall (version denoted as Fit-As). The vertical wind shear (VWS) and landfall (before or after) are considered in this redistribution. Fit-As generally outperforms Fit-Ax and NHC in reproducing the observed rainfall distribution for the 14 TCs. At the quadrant level, both Fit-Ax and Fit-As show significant improvement in Bias over NHC. Fit-As is further better overall in RMSE and Skill when weighted by quadrant rainfall volume. In pattern matching, Fit-As produces the best grid-averaged Pearson correlation coefficients for 11 TCs. In addition, its equitable threat scores (ETSs) are best beyond the 20-mm rainfall threshold, with the maximum of 0.299 at the 90-mm rainfall threshold. Thus, our locally fitted asymmetric rainfall model demonstrates improved capability in reproducing the historical TC-induced rainfall along the Northern Vietnam coast.