Characteristics and mechanism of vertical coupling in the genesis of tropical cyclone Durian (2001)

The vertical coupling (VC) process and mechanism during the genesis of a tropical cyclone (TC) implied by the weak vertical shear of horizontal wind, one of the key factors impacting TC genesis, constitute important but unanswered fundamental scientific problems. This paper carried out a targeted investigation of this problem through numerical simulation and theoretical analyses. The main conclusions are as follows. Even if TC genesis occurs in a barotropic environment, a VC process still occurs between the trough (vortex) at the middle level and that at the lower level in the TC embryo area. VC mainly occurs at the tropical disturbance (TDS) stage. Only after the VC is accomplished can the tropical depression (TD) organize further by itself and develop into the tropical storm (TS) stage or the stronger tropical typhoon (TY) stage through the WISHE (wind-induced surface heat exchange) mechanism. In the VC process, vortical hot towers (VHTs) play vertical connecting roles and are the actual practitioners of the VC. Through the VHTs’ vertical connections, the middle- and lower-troposphere trough axes move towards each other and realize the VC. VHTs can produce intensive cyclonic vorticity in the lower troposphere, which is mainly contributed by the stretching term. The tilting term can produce a single dipole or double dipole of vorticity, but the positive and negative vorticity pairs offset each other roughly. While the stretching term ensures that the cyclonic rotations of the wind field in the middle and lower levels tend to be consistent, the tilting term acts to uniformly distribute the horizontal wind in the vertical direction, and both terms facilitate the VC of the wind field. With the latent heat of condensation, VHTs heat the upper and middle troposphere so that the 352 K equivalent potential temperature contour penetrates vertically into the 925–300 hPa layer, realizing the VC of the temperature field. While forming cloud towers, VHTs make the ambient air become moist and nearly saturated so that the 95% relative humidity contour penetrates vertically into the 925–400 hPa layer, realizing the VC of the humidity field. Due to the collective contributions of the VHTs, the embryo area develops into a warm, nearly saturated core with strong cyclonic vorticity. The barotropic instability mechanism may also occur during TC genesis over the Northwest Pacific and provide rich large-scale environmental vorticity for TC genesis. The axisymmetric distribution of VHTs is an important sign of TC genesis. When a TC is about to form, there may be accompanying phenomena between the axisymmetric process of VHTs and vortex Rossby waves.