Tropical Cyclone (TC) is an intense atmospheric vortex with a warm core and low pressure structure, and generates over the tropical or subtropical warm ocean. The problem of TC genesis has been paid great attention by scientists since the 1950s, but due to the lack of the observation data over sea, this problem has become the most difficult and challenging topic in the researches of TC.
Cumulus convections are considered to be the most basic element in the TC generation process. The formation of TC in the Northwest Pacific is often associated with the mesoscale convective system (MCS) or mesoscale convective complex (MCC). Meanwhile, in the stratiform cloud precipitation area of MCS or MCC, a mesoscale convective vortex (MCV) can be produced, and the MCV is considered to be the embryo of TC. However, how the cumulus convections and the embryo organize, develop and finally form a synoptic-scale vortex has always been difficult to answer.
For a long time, the effect of cumulus convection was explained by the CISK (conditional instability of the second kind) mechanism and WISHE (wind-induced surface heat exchange) mechanism. Since the beginning of the 21st century, researchers have gradually realized that these two mechanisms can explain the process of TC intensification, but neither of them is a reasonable explanation for the process of TC genesis. Because they all implicitly assume that a finite amplitude cyclonic circulation system with intense surface vorticity already exists. Therefore, the cumulus convection organization process before the establishment of the surface cyclonic circulation is still a difficult problem that needs to be investigated deeply.
TC Durian generated over the South China Sea on June 28, 2001. "Based on the multi-scale interaction mechanism, we described a series of stories about the genesis of TC Durian. In the first story, we mainly told about the activities of the monsoon surge associated with the cross-equatorial jet and the low level jet from Somalia, and the upscaling process of the vorticity zone accompanying the band-shaped convective clouds, and the role of large scale conditions on dominating the time and location of TC genesis. The occurrence of MCC is the result of the organization of large scale monsoon trough. Among the types of TC genesis in which the monsoon trough acts as a large-scale disturbance in the lower troposphere in the Western Pacific, it is inevitable that MCC is prone to occur as a precursor." Dr. Zhang Wenlong said.
"In the second story, we focused on the role of the mesoscale system, the middle-level MCV in the genesis of TC durian. The first is the mesoscale organization role of the MCV, which makes the vortical hot towers (VHTs) in the area tend to gradually concentrate in the central area and interact easily between each other, and promotes the intense VHTs to be axisymmetrically distributed in the embryonic area. The second is its storage role, which can reserve and maintain the heat, water vapor, and vorticity carried by the died VHTs, making the MCV area to be more favorable to the TC genesis, and ultimately become the embryo of Durian." Dr. Zhang Wenlong said.
In the tropical large scale disturbance systems, why do some convective cloud clusters eventually develop into TCs, while others fail to develop into TCs? Now Dr. Zhang Wenlong and his collaborators have written a new story about the genesis of TC Durian, looking for new explanations from the vertical coupling (VC) characteristics of TC and the role of the vertical connection of small-scale VHTs.
The new story is titled: "Vertical Coupling Characteristics and Mechanism of Tropical Cyclone Durian (2001)", published in the 3rd issue of "Science China: Earth Science" in 2021, by Dr. Zhang Wenlong from Beijing Institute of Urban Meteorology, professor Cui Xiaopeng from the Institute of Atmospheric Physics, Chinese Academy of Sciences, and professor Dong Jianxi from the National Marine Environmental Forecasting Center, China, and the corresponding author is Dr. Zhang Wenlong. The study adopts the numerical simulation method and compares the related results with that of the famous senior meteorologist Montgomery. It is revealed that even if TC genesis occurs in a barotropic environment, the VC process still occurs between the trough (vortex) at the middle level and that at the lower level in the TC embryo area, and the VHTs play vertical connecting roles and are the actual practitioners of the VC.
"The VC process and its mechanism in TC genesis is an important but unanswered basic scientific problem hidden under the condition of weak vertical shear of horizontal wind, the key factor for TC genesis." Professor Cui Xiaopeng said, "The VC problem has not been given a targeted study. One of the possible reasons is that most part of the previous numerical simulation experiments of TC genesis are the ideal experiments with an artificially constructed axisymmetric vortex in the initial field, and a few are real cases simulations starting when a tropical depression circulation already exists. These numerical simulations all skip the VC stage that may bring difficulties to the simulations, and therefore the researchers may miss the opportunity to observe the VC characteristics of TC. "
Based on the simulation of TC genesis starting from the monsoon trough with unclosed circulation, Zhang Wenlong research group successfully observed the VC process of TC embryo in a barotropic environment. The study pointed out that through the VHTs' vertical connections, the middle- and lower-troposphere trough axes move towards each other and realize the VC. The VHTs promote the VC of the wind field through the stretching term and tilting term of vorticity budget, promote the VC of the temperature field through the release of latent heat, and promote the VC of the humidity field through the deep cloud towers. Due to the collective contributions of the VHTs, the embryo area develops into a warm, nearly saturated core with strong cyclonic vorticity. 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. "Only the TC embryo that has achieved VC may be the 'real' embryo that can further develop into a tropical storm with spiral cloud belts." Professor Dong Jianxi said.
This research has enriched and deepened the link of the small-scale VHTs in the multi-scale interaction mechanism of TC genesis, and is of great significance to the basic theoretical research of TC genesis. Meanwhile, more case studies are expected to verify and improve these understandings.
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This work was supported by the National Basic Research Program of China (Grant No. 2015CB452804) and the National Natural Science Foundation of China (Grant No. 41475051).
See the article: Zhang W, Cui X, Dong J. 2021. Characteristics and mechanism of vertical coupling in the genesis of tropical cyclone Durian (2001). Science China Earth Sciences, 64(3): 440-457, https://doi.org/10.1007/s11430-019-9681-x https://engine.scichina.com/publisher/scp/journal/SCES/64/3/10.1007/s11430-019-9681-x?slug=fulltext