The modulation of vortex growth by periodic convective activity

TL;DR

This work analyzes whether fluctuating convection components can accelerate tropical cyclone genesis by generating barotropic vorticity. Using an axisymmetric Boussinesq model with a bulk-plume convective scheme on the f-plane, the authors impose periodic, localized convection to mimic mesoscale systems and isolate convective momentum transfer (CMT). A center-domain theory and a two-mode vertical reduction show that CMT creates a phase lag between vertical velocity and vorticity, enabling cumulative production of a barotropic core wrapped by an anticyclonic shell, with growth enhanced by stronger forcing and moderated by entrainment. The results suggest diurnal and inertial-gravity-wave fluctuations could accelerate vortex development and highlight CMT as a crucial irreversible mechanism in vorticity production, while acknowledging simplifications and the need for 3D cloud-resolving confirmation.

Abstract

An important process in tropical cyclone formation is the development of a deep, warm core, which corresponds to the growth of a barotropic cyclone. Persistent convective activity is known to be crucial for the growth of barotropic vorticity. However, it remains unclear whether the fluctuating component of convective activity, such as that caused by the diurnal cycle and inertial-gravity waves, also accelerates the vortex development. To investigate this problem, numerical simulations are performed in an axisymmetric model with the Boussinesq approximation on the f-plane. Convection is parameterized with a bulk-plume mass-flux scheme. To represent a mesoscale convective system modulated by the diurnal cycle, periodic convective mass flux is imposed in a local region. The convection induces periodic diabatic heating and convective momentum transfer in the vertical direction (CMT). The CMT is an irreversible effect that breaks the quadrature phase relation between vertical velocity and vertical vorticity, producing a residual barotropic vorticity in each cycle. The barotropic vorticity consists of a barotropic cyclonic core and an anticyclonic shell. The cyclonic core is produced by the vertical advection and stretching of vertical vorticity. The anticyclonic shell is produced by the radial advection and tilting of radial vorticity. The analytical solution reproduces the formation and growth of the core-shell vorticity structure. This research reveals a potential acceleration effect of periodic convective activity on tropical cyclone genesis.

Figures (11)

• Figure 1: (a) The vertical profile of convective mass flux $M$ at the domain center, at the peak of the updraft phase (blue line) and at the peak of the downdraft phase (red line). (b) The fractional entrainment rate $\epsilon$ at the peak of the updraft phase (blue line) and at the peak of the downdraft phase (red line). The profiles in (a) only work for the domain center, and the profiles in (b) work uniformly in the domain.
• Figure 2: A snapshot of the reference axisymmetric simulation at $t=5$ days for the reference experiment. The first and second column shows the experiments without and with convective momentum transfer (CMT_off_6 and CMT_on_6). The first row shows the vertical velocity divided by the characteristic scale of vertical velocity $\Delta W \equiv M_*/\rho$, and the second row shows the vertical vorticity divided by $f$. Movies of all experiments can be downloaded from: https://box.nju.edu.cn/d/653d1189c69c40f8be58/.
• Figure 3: The Hovmöller diagram of (a) vertical velocity and (b) vertical vorticity at the domain center for the reference experiment (CMT_on_6). The horizontal axis is the time $t$ divided by the forcing period $T$, and the vertical axis is the height divided by the domain thickness $H$.
• Figure 4: A vertical mode decomposition analysis of the vertical vorticity at the domain center, using the reference experiment (CMT_on_6) and its counterpart (CMT_off_6) that turns off the CMT. (a) The barotropic vorticity $Z_0/f$. (b) The first baroclinic mode vorticity $Z_1/f$. (c) The second baroclinic mode vorticity $Z_2/f$.
• Figure 5: Results of the first two groups of experiments (CMT_off and CMT_on). (a) The barotropic vorticity at the domain center at $t=5$ days for experiments with different $M_*$. The red crosses show experimental results where CMT is turned off; the black crosses show the experiments where CMT is active. The black line shows the theoretical prediction when CMT is active. (b) is the same as (a) but for the phase lag of $Z_1$ to $w(z=H/2)$. The diagnosed phase lag has a "background level" that makes the lag slightly smaller than $\pi/2$. This background level is likely a technical issue in calculating autocorrelation due to the finite length of the data, and is hard to eliminate.