In this study, we investigate a theory that the perturbed thermodynamic profile caused
by MCV will promote the spin-up of incipient vortex through the generation of
bottom-heavy mass flux profile. We initialize VVM with a very weak, 3 m s-1 Rankine vortex that
represents an idealized MCV embedded into a quiescent environment. Six sensitivity
experiments are carried out with the maximum wind speed situating at different heights
that creates different perturbed potential temperature profile. We find that the MCV with
maximum wind speed placed at 4500 m is the fastest to undergo cyclogenesis, the surface
vortex case and the vortex with height of maximum wind speed at 7500 m barely develops
after 144 hours of integration. We can divide the sensitivity experiments to developing
sets and non-developing sets accordingly.
The direct response to different structure of MCV is the degree of thermodynamic
stabilization. To quantify thermodynamic stabilization, we define stability index as the
difference between the averaged moist entropy at 1000-3000 m and 7000-9000 m. We
find that the stability index is lower (more stable) in DS prior genesis. With more
stabilization, the saturation fraction around the center of the vortex will increase more
efficiently and the convection in such environment, have two distinct features: (1) The
better organization of convection around the vortex center, which can concentrate heating
and moisturizing near the center of incipient vortex. (2) A higher probability of generating vertical mass flux profiles that is more “bottom heavy”, increasing local vorticity through
stretching.
Both of these features have a positive contribution to the intensification of the incipient
vortex, hence faster time to cyclogenesis could be achieved. We are aware that in recent
cloud resolving simulations (Kilroy et al., 2017, 2018), a pre-existing MCV is not
required to spin-up incipient vortex. In this study, we do not argue that the formation of
MCV is a necessary trigger for cyclogenesis. Rather, the favorable thermodynamic
environment collaterally created by the MCV can act as a catalyst to speed up the process,
preventing the weak vortex to dissipate when entering unfavorable environment before it
even had a chance to develop. On the other hand, the stabilization could also be achieved
by other means such as microphysical processes and large scale warming and cooling
with elevated background vorticity and does not exclusively require a MCV, which could
be addressed in future studies.
The key findings of this study are as follow:
⚫ In addition to the mere existence of a MCV, the vertical position of the MCV also
plays a role in the time required for the incipient vortex to develop into a
tropical-depression strength system. When the MCV is placed at different level, it alters
the initial environmental stabilization hence the evolution of stabilization and
column moisture. In our current initial conditions, MCV placed at 4500 m seems
to be the “sweet spot” for the vortex to develop.
⚫ Stabilized environment are more likely to generate bottom heavy mass flux profile
found in two-dimensional cloud model (Raymond & Sessions, 2007) also applies
to three-dimensional environments.
⚫ The aggregation of deep convection in environments with high saturation fraction
that is evident in previous experiments conducted in high resolution models are
also occurs in our experiment. Due to the deep convection characterized in our
experiment is highly rotational, the process is analogous to the upscale growth of
VHT involved in cyclogenesis described by Montgomery et al. (2006). We find
that the aggregation takes place at a scale between 104 – 105 km3, which is 10–
100 times larger than their results.
Finally, we propose a simple conjecture in the cyclogenesis framework that combines
theories from previous researches and knowledge obtained from this study, shown in the
flowchart below.
Figure 29. Flow chart of an updated cyclogenesis framework combining the VHT paradigm and the role of stabilization caused by MCV.
Some questions remain of interest and can be discussed in future studies:
◼ As showed in 3.1, there seems to be a preference of position which large
convection are activated. According to previous studies, convection seems to
prefer to activate on the down-shear left of storm relative winds. Although
strong shear is known to be detrimental to tropical cyclone development due to
vortex tilting and ventilation of low entropy air, weakly-sheared environment
is also showed to be favorable for convection to aggregate (Tsai & Wu, 2017).
It would be interesting to conduct the experiment in a weak-moderately sheared
environment to inspect the net contribution of shear to genesis between these
competing effects.
◼ Will different initial soundings, such as soundings from Western Pacific Ocean,
or a perturbed initial water vapor mixing ratio, affect the outcome of the
experiment?
◼ Positive radial gradient of column integrated moist entropy flux divergence, or
Gross Moist Stability (GMS) is observed to reach maximum shortly prior
genesis (Tang, 2017). It would be interesting to compare the evolution of GMS
in these experiments.
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