Numerical prediction of tropical cyclogenesis. Part II: Identification of large‐scale physical processes under the monsoon shear line synoptic pattern

Abstract

This is the second part of a two-part study to investigate the numerical prediction of tropical cyclogenesis. Part I presents a comprehensive statistical assessment of the performance of the high-resolution European Centre for Medium-Range Weather Forecasts deterministic forecasts in predicting tropical cyclone (TC) genesis over the western North Pacific between 2007 and 2018 using The International Grand Global Ensemble data. Among these forecasts, the ones that predict TC genesis occurring in the monsoon shear line synoptic pattern have the highest 5-day predicted skill. In this article (Part II), the large-scale dynamic and thermodynamic variables associated with this type of genesis are examined to identify the physical processes likely responsible for TC genesis. Moist convection appears to be one of the first important factors. Deep convection at the initial time (Day −5) is concentrated near the inner core of the pre-genesis vortex. In the lower troposphere, the northeasterly flow to the north and southwesterly flow to the south of the pre-genesis vortex begin to strengthen at Day −4 and Day −3 respectively, and then the maximum wind centres move close to the genesis position (GP) from Day −3 to Day 0. Strong low-level radial wind and the associated absolute angular momentum (AAM) import develop on Day −4, which effectively forces a transverse circulation and spins up a surface vortex near the GP. The vertical gradients of latent heat release and the decrease in mean-sea-level pressure by Day −3 further support the import of low-level AAM and the development of the low-level vortex. At the upper levels, an anticyclone moves close to the GP, reducing the vertical wind shear at Day −2, fully establishing the secondary circulation system. The positive feedback between the environmental flow and the internal thermal forcing conditions from Day −2 to the genesis time are also important to genesis.