Uncertainty of Tropical Cyclone Genesis in Moderate VWS depending on Vertical Vortex Profiles

Moderate vertical wind shear (VWS) presents significant challenges in forecasting tropical cyclone (TC) behavior due to its highly variable outcomes. This variability is particularly evident in TC genesis (TCG), an area with limited understanding due to sparse observations and uncertain model simulations. A primary issue associated with moderate VWS is the vortex tilt, “displacement of low-level and mid-level vortex centers”, leading to asymmetric TC structures. This study investigates the impact of vertical vortex structure on TC genesis under moderate VWS using numerical simulations with the Weather Research and Forecasting Model (WRF). There is a variability in vertical profiles for pre-genesis vortices that serve as seeds for TCG. For example, African Easterly Waves (AEWs) have maximum vorticity in higher altitudes (near the mid-levels) whereas Western Pacific Easterly waves vortices are in lower altitudes (low-levels). Our experimental design compares mid-level (600 hPa) centered pre-genesis vortex and low-level (850 hPa) centered vortex. We analyze variations in genesis timing under moderate wind shear and reduced moisture content. Preliminary results show that mid-level vortex centers develop more slowly than low-level centers and exhibit greater variability. For both mid-level and low-level centered vortices, the timing of genesis depended on how many repetitive cycles of aligned precursor events the vortices had to reach genesis. The more failed attempts, the slower the onset of genesis. Key differences between low-level and mid-level centered vortices include: 1) the mid-level centered vortex shows greater downshear left convection prior to genesis 2) the mid-level centered vortex exhibits more evident mid-level centered vortex size expansion prior to genesis 3) the mid-level centered vortex exhibits greater tilt at genesis, which may require a larger circulation and vortex expansion for successful formation. This study aims to elucidate the mechanisms behind TCG depending on initial vortex profiles and shear interactions, thereby enhancing predictability and improving forecast models for tropical cyclone formation.