Tropical cyclogenesis, the process of a tropical cloud cluster transforming into a
quasi-stable, long-lasting mesoscale vortex, is a multiscale and finite amplitude process
(Emanuel, 1989). During the last few decades, different theories were proposed to
explain the processes involved in tropical cyclogenesis, which can be divided into two
main school of thoughts. “Top-down” development proposes that low-level cyclone
develops by the down-transport and merger of vorticity that source from mesoscale
convective vortices (MCV, Bister & Emanuel, 1997). The “bottom-up” development
theory (Hendricks et al., 2004; Montgomery et al., 2006) proposed that rotating deep
convection, which they called vortical hot towers (VHT), are main coherent structure
embedded in the MCV. The existence of MCV creates vertical wind shear, hence
horizontal vorticity components. These VHT forms by tilting of horizontal vorticity
through updrafts, and are able to spin up the vortex through multiple diabatic merger
events. The merger of VHTs have two major contributions to vortex spin up: (1) the
contribution in stretching term in vertical vorticity equation through intense helical
motions and (2) diabatic merger collectively heating up the lower troposphere near the
center of the incipient vortex, creating a secondary circulation due to radial gradient of
diabatic heating.
The collective effects of diabatic heating from the deep convection near the regions of
high inertial stability induces a toroidal secondary circulation, which could be explained
from the RHS in the balanced Sawyer-Eliassen Equation on a cylindrical coordinate
A, B and C represents static stability, baroclinity and inertial stability respectively. g is
gravitational acceleration and 𝜃0 is a reference value of potential temperature. 𝑄 is the
diabatic heating rate and 𝜓 is the mass stream-function on (r,z) plane. The amplification
in this secondary circulation induces increased low-level inflow that precedes wind
induced surface heat exchange (WISHE, Rotunno & Emanuel, 1987) to spin up the
incipient vortex.
Although the exact framework leading to genesis remains uncertain, column
saturation is commonly observed to precede tropical cyclogenesis. According to many
studies (Dunkerton et al., 2009; Nolan, 2007; Wang, 2018), it is evident that the region
near the center of incipient disturbance will reach and remain near column saturation
prior to the cyclogenesis. The sustained moisturization will “lock” the development of
convection in the inner region close to the vortex center, preventing gravity waves from
dissipating energy to the quiescent environment and continue to moisturize the
environment through diabatic heating.
Recent advances in high resolution cloud modeling opened an opportunity to marry
these two school of thoughts. Raymond and Sessions (2007) uses a two-dimensional
cloud model to study the evolution of convection in different magnitudes of stabilization
of thermodynamic profile through adding different magnitudes of potential temperature
perturbation at 3000m and 10000m into a background environment under
radiative-convective equilibrium (RCE). Their result revealed that a more stable potential
temperature profile will lead to a convective vertical mass flux that is concentrated in the
lower-middle troposphere, which is “bottom-heavy”, as opposed to the “top-heavy”
convective vertical mass flux profile in a environment under RCE. This allows vorticity
convergence in lower troposphere and ventilation of high entropy air into the
atmospheric column through mass convergence and detraining the lower entropy air in
the mid-troposphere. In addition to numerical studies, observational studies
(Gjorgjievska & Raymond, 2014; Raymond et al., 2011) using data from THORPEX
Pacific Asian Regional Campaign/Tropical Cyclone Structure Experiments
(TPARC/TCS-08) also showed an evolution of convection from “top-heavy” to
“bottom-heavy” prior the genesis of tropical storm Nuri, and showed that the “bottom-heaviness”
of convection in the environment is related to the stabilization of environment.
Additionally, they found that the stabilization of environment is inversely related to the
saturation fraction, and proportional to the vorticity at mid-levels.
Based on these results, Raymond et al. (2014) proposed a framework to explain how
the existence of a mid-level vortex can provide a favorable environment for the
intensification of a surface vortex. First they discussed that the early misconception that
“direct down (up) transport” of mid-level vorticity or any rotating systems on different
scale appearing in both “top-down” and “bottom-up” theories to amplify the surface
vortex is not possible, postulated by Kelvin’s Circulation Theorem. Alternatively, the
role of MCVs is the supply of a stabilized saturated entropy (s*) environment due to
thermal wind balance, and the stabilized environment can promote “bottom-heavy”
convection, inducing sustained feedback of environment moisturizing, which creates a
positive feedback through stretching to increase low-level vorticity. This conclusion is
intriguing due to the attempt to marry the “top-down” meso-𝛽 environment triggering a
“bottom-up” upscale cascade of vorticity by convective scale aggregations.
This study aims to supplement the framework of Raymond and Sessions (2007).
First, we want to test if their hypothesis from two-dimensional configuration and
horizontally homogeneous potential temperature perturbation is also valid in a
three-dimensional environment. We use a different approach, implanting idealized,
axisymmetric vortices in a three-dimensional cloud resolving model and let the
thermodynamic environment adjusts to the vortex to achieve the stabilization analogous
to their initial condition. Second, since heating/cooling of the troposphere can produce
different MCV structures, we want to test if different vertical profiles of initial MCV
influence the evolution of the low-level vortex. Chapter 2 describes model and the
experiment setup. Chapter 3 analyzes the evolution of environment in these experiments.
Chapter 4 presents the evolution of convection under different environments.
Concluding remarks and future work are discussed in Chapter 5.