Tropical Cyclogenesis under the Influence of an Upper-Tropospheric Cold Low in Idealized Numerical Simulations

Previous studies have indicated that approximately 20% of tropical cyclones (TCs) form under the influence of an upper-level disturbance globally. This study investigates tropical cyclogenesis (TCG) within a lower-tropospheric wave pouch (WP) in the presence of an upper-tropospheric cold low (CL) using idealized simulations in a resting environment. The b effect induces distinctly different wind and humidity asymmetries in the CL and WP owing to their differing vertical and horizontal structures. The CL exhibits a wetter western sector and a drier eastern sector, with stronger winds in the eastern sector. In contrast, the eastern sector of the WP, where a mesoscale vortex emerges, is slightly wetter than its western sector. This results in the strongest dry-air intrusion when the WP is positioned northeast of the CL and the weakest dry-air intrusion when the WP is located southwest of the CL. Additionally, when the WP is positioned northeast of the CL, the vortex developing from their vorticity superposition competes with the WP-induced vortex. The anticyclone southeast of the CL, induced by the Rossby energy dispersion, disrupts vorticity aggregation and hinders TCG. In contrast, when the WP is located southwest of the CL, the superposition of the WP and CL vorticity accelerates TCG. These combined mechanisms make the southwest quadrant of the CL the most favorable region for TCG, while the northeast quadrant being the least favorable. The sensitivity of TCG to the CL depth and the CL-WP separation distance are also investigated through further experiments.