A Comparison of the Effects of an Upper-Level Anticyclone and a Lower-Level Cyclone on Tropical Cyclogenesis in Idealized Simulations

Abstract

This study examined the effects of an upper-level anticyclonic circulation and a lower-level cyclonic circulation on tropical cyclone (TC) genesis. We ran idealized simulations using the Advanced Research Weather Research and Forecasting (WRF-ARW) model. The simulation results show that the upper-level anticyclonic circulation makes a negative contribution to TC genesis, while the lower-level cyclonic circulation makes a positive contribution. The upper-level anticyclonic circulation results in slower TC genesis due to a substantial vertical zonal wind shear that shifts the upper-level vortex eastward from its initial position. This shift is unfavorable for the vortex's vertical alignment and warm core maintenance. This substantial vertical zonal wind shear is associated with the asymmetric vertical motion and associated diabatic heating, induced by the lower-level beta gyre. The upper-level anticyclonic circulation increases the westerly wind north of the vortex, resulting in a substantial vertical westerly wind shear. Thus, the initial upper-level anticyclonic circulation is unnecessary for TC genesis. The strong upper-level anticyclonic circulation, generally observed with a strong TC, should be considered a result of deep convection. The strong lower-level winds induce large surface heat fluxes and vorticity due to the superposition of the large-scale lower-level cyclonic circulation and vortex. These conditions lead to strengthened convection and diabatic heating and a quick build-up of positive vorticity, resulting in rapid TC genesis.