Analysis of Multi-Scale Circulation
and Non-Adiabatic Forcing Mechanisms Determining the Genesis and Intensity of Localized Torrential Rainfall
Beyond Local Factors
Muti-Scale Circulation & Non-Adiabatic Forcing : The Drivers of Extreme Rainfall
Local heavy rainfall is a major hazardous weather phenomenon
due to its high intensity & difficulty in prediction, drawing significant recent attention.
The variability in the occurrence and intensity of these downpours is complexly influenced by
long-term, large-scale atmospheric circulation patterns, such as the El Niño–Southern Oscillation (ENSO).
Therefore, explaining heavy rainfall over the Korean Peninsula requires analyzing the East Asian large-scale circulation, not just local weather factors. This circulation dictates the moisture channel & dynamic forcing over the peninsula.
Specifically, this analysis will deeply explore the multi-scale interaction mechanism by which
these elements, coupled with diabatic forcing, amplify the extreme intensity of heavy rainfall events.
ENSO's Definition & Teleconnection to Korean Precipiation Variability
The ENSO (El Niño–Southern Oscillation) is the most powerful natural variability phenomenon, resulting from the coupling of sea surface temperature (SST) and atmospheric circulation in the equatorial Pacific.
This recurring pattern involves the surface waters of the central and eastern tropical Pacific becoming either 1℃ to 3℃ warmer (El Niño) or colder (La Niña) than average.
The key mechanism by which ENSO influences rainfall variability in distant regions, including the Korean Peninsula, is through teleconnection.
Teleconnection: This refers to a climate anomaly where climate phenomena in one specific region are interconnected with weather conditions in other regions thousands of kilometers away. El Niño and La Niña are examples of such climate anomalies.
Fig 2 : [ Yong-sang Choi / Ewha Womans University, Climate Physics Chapter 6 Lecture Material ]
Changes in Walker Circulation
ENSO alters the distribution of tropical Pacific SST, which in turn changes
the center and intensity of the Walker Circulation.
Atmospheric Wave Induction
The change in the Walker Circulation shifts the center of convection and supplies significant latent heat
(thermal energy) to the upper atmosphere. This energy, combined with the Coriolis effect, generates
Rossby waves in the atmosphere.
Fig 3 : [ Yong-sang Choi / Ewha Womans University, Climate Physics Chapter 5 Lecture Material ]
El Niño
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The strong updraft in the Eastern Pacific increases
heat supply, causing Rossby waves to expand eastward.
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Consequently, the East Asian Westerly Jet (Jet Stream) bends southward more than usual.
This results in the North Pacific High shrinking or shifting eastward, leading to the weakening or southward movement of the Korean Peninsula's Jangma front.
La Niña
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Rossby wave activity tends to push the Westerly Jet Stream over East Asia northward more than usual.
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This northward shift of the jet stream creates an environment for the North Pacific High to expand.
The expanded High draws a warm and moist air mass
from the south into the Korean Peninsula, intensifying atmospheric instability.
In conclusion, ENSO does not merely involve ocean temperature changes;
rather, through changes in the Walker Circulation and the induction of Rossby waves,
it directly impacts the position of the upper-level jet stream and the expansion/contraction of the
North Pacific High, causing complex shifts in the Korean Peninsula's precipitation patterns.
The North Pacific High is the key synoptic-scale factor determining the summer precipitation pattern over the Korean Peninsula.
This massive oceanic anticyclone holds a hot and humid air mass, serving as a primary source providing the vast amount of moisture necessary for heavy rainfall.
The expansion and contraction of this High directly control
the position of the stationary front (Jangma front) that causes precipitation over the Peninsula.
Interaction with the North
Pacific High & Subtropical Jet
Role of the Low-Level Jet
The Low-Level Jet (LLJ), a strong southwesterly flow along the western edge of the High, is the main channel transporting moisture from the tropical and subtropical oceans to the Korean Peninsula. The amount of moisture transported determines the moisture budget, a critical element for the occurrence and intensity of heavy rain, and this transport capacity increases as the High expands strongly.
Influence of the Subtropical Jet & Jangma Front Induction
The moist environment in the lower atmosphere is closely linked to the Subtropical Jet (STJ), a powerful upper-tropospheric wind flow. The STJ regulates the northern boundary of the North Pacific High, and its positional variability directly impacts the High's expansion and contraction.
STJ shifts North
The High contracts, causing the Jangma front to shift south or weaken.
The High expands northward, causing the Jangma front to shift north or stall over the Peninsula.
STJ shifts South
Vertical Coupling of Westerly Jets
For localized heavy rainfall systems to form and persist,
the Upper-Level Westerly Jet and the Low-Level Jet must achieve an optimized vertical coupling.
This interaction simultaneously provides the moisture supply (fuel) and upward motion (pump) required by the convective system, maximizing the precipitation.
The Role of the Low-Level Jet (LLJ): The Moisture Transport Channel
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The LLJ is a strong wind belt with maximum speed at an altitude of approximately 1 to 3 km.
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It serves to intensively transport large amounts of moisture from tropical and subtropical regions toward the Korean Peninsula.
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When the LLJ flow meets and converges with mountain ranges or other air masses, it forces the air to rise, providing the vertical starting point for heavy rainfall.
Dynamic Forcing of the Upper-Level Jet (ULJ) : The Dynamic Pump
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The ULJ is a powerful flow occurring in the upper troposphere, at altitudes of 9 to 12 km.
It acts as the dynamic pump that triggers and sustains the heavy rainfall system.
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Upper-level divergence is induced in the entrance region where the jet stream accelerates.
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According to the law of mass conservation, this upper-level divergence initiates a secondary circulation that induces strong upward motion beneath it.
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This upward motion provides the initial trigger to lift the air, creating the essential environment for the condensation of the moisture supplied by the LLJ.
(Note: In the jet exit region where the flow decelerates, upper-level convergence and
lower-level subsidence are induced, usually leading to fair weather.)
Blocking & Prolonged Heavy Rain
Blocking is a phenomenon where the ULJ significantly wanders or stops moving, causing upper-level troughs or LLJ systems to stop for extended periods over the Korean Peninsula.
The stagnation of these systems ensures a continuous supply of warm and moist air to a specific region,
leading to the sustained regeneration of convective systems and the resulting extreme precipitation.
Therefore, the primary environment for heavy rainfall to exhibit unusual intensity and long-term persistence
is created by blocking events.
The Explosive Intensity of Torrential Rainfall
The core atmospheric dynamics driving the explosive intensity of torrential rainfall is the interplay between baroclinic instability and diabatic forcing.
Baroclinic instability acts as the dynamic trigger that initiates the upward motion, while diabatic forcing (latent heat release) serves as the thermodynamic engine that explosively amplifies this motion.
The Synergy of Baroclinic Instability & Diabatic Forcing
Dynamic Trigger :
Baroclinicity & Synoptic-Scale Forcing
Baroclinic Structure
A baroclinic structure is where isobars and isotherms cross each other, indicating a horizontal mixing of warm & cold air.
This state is inherently unstable, storing the potential energy needed to initiate
atmospheric circulation.
Fig 5 : [ Yong-sang Choi / Ewha Womans University, Climate Physics Chapter 5 Lecture Material ]
Initiation of Upward Motion :
Strong upward motion is triggered by the combination of an upper-level dynamic pump & maximum lower-level instability.
Upper-Level Dynamic Pump :
PVA occurs ahead of an upper-level trough, inducing divergence. According to the omega equation,
this forces upward motion in the mid-troposphere.
Lower-Level Instability :
Simultaneously, LLJ supplies a large volume of warm, moist air, maximizing the horizontal temperature gradient and drastically destabilizing the baroclinic structure.
The combined effect triggers intense initial upward motion, which is the starting point for the heavy rainfall system.
Positive Vorticity Advection (PVA) : The force that causes air to rise by transporting the air's rotation component (vorticity) to induce divergence in the upper atmosphere.
Omega Equation : A core formula for calculating the atmosphere's vertical motion (ω) in large-scale circulation. A negative value (ω < 0) signifies upward motion, while a positive value (ω > 0) signifies downward motion.
Thermodynamic Engine :
Diabatic Forcing & Extreme Amplification
Diabatic Forcing
Diabatic forcing refers to changes in atmospheric state and motion caused by non-adiabatic thermodynamic processes, primarily latent heat release from condensation.
This is the main energy source that rapidly intensifies the system.
Latent Heat Release :
When the abundant moisture that rose due to baroclinic instability condenses, a vast amount of latent heat is released into the surrounding air.
Buoyancy Increase :
This latent heat release rapidly raises the temperature of the surrounding air, lowering its density and explosively increasing buoyancy.
Positive Feedback Loop (self-amplification) :
The increased buoyancy accelerates the existing upward motion. The accelerated upward motion, in turn, draws up even more moisture, causing the release of more latent heat.
This positive feedback loop continuously strengthens the convective system to an extreme degree,
serving as the fundamental mechanism
that triggers short-term, destructive torrential rainfall.
Dominance at Peak Intensity :
During the peak intensity of heavy rainfall,
diabatic forcing dominates the upward motion, outweighing the initial dynamic forcing.
This means latent heat warming is the core engine that deepens convective instability and sustains the destructive intensity.
Conclusion
The genesis of torrential rainfall is a complex, multi-scale product where ENSO and the North Pacific High establish an environment of vertical coupling through the upper and lower-level jets, inducing a baroclinic structure.
However, the dominant force determining the extremity of its intensity is diabatic forcing.
Global warming is increasing the atmosphere's capacity to hold water vapor, thereby augmenting the potential amount of latent heat release.
This suggests that even if the dynamic environment induced by large-scale circulation patterns remains at average levels in the future, the resulting latent heat explosion during condensation will continue to increase the extreme intensity & frequency of localized torrential rainfall.