Convective Bursts Research Articles

Convective Bursts Episode of the Rapidly Intensified Typhoon Mujigae (2015)

Convective burst (CB) characteristics at distinct stages of a rapidly intensified Typhoon Mujigae (2015), are investigated based on a 72-h simulation. The spatial features show that almost all CB elements develop in the eyewall. The number of CBs in the inner-core region within a 100 km radius——which account for a large proportion of the total CBs, with a sharp increase about 6 h before the onset of rapid intensification (RI)——provides some indication of the RI of the typhoon. The CBs during pre-RI and RI are examined from dynamic and thermodynamic viewpoints. The combination of lower-level convergent inflow and upper-level divergent outflow pushes a relay-race-like transmission of convective activity, favorable for the development of deep convection. A double warm-core structure is induced by the centripetal outflow sinking and warming associated with CBs, which directly accelerates RI by a sudden decrease in hydrostatic pressure. By utilizing the convection activity degree (CAD) index derived from the local total energy anomaly, a correlation formula between CBs and CAD is deduced. Furthermore, an intense CAD (ICAD) signal threshold (with a value equal to 100) to predict CBs is obtained. It is verified that this ICAD threshold is effective for estimating the occurrence of a CB episode and predicting RI of a typhoon. Therefore, this threshold may be a valuable tool for identifying CB episodes and forecasting rapidly intensified typhoons.

Diurnal variability of convection over northwest Indian subcontinent observed by the Doppler weather radar data

The diurnal cycle of convection over a sub-tropical semi-arid inland station—Delhi—has been analyzed in this study based on three different rainfall episodes. Two of these cases represent convection in association with low precipitable water content ( 60 mm) and low vertical wind shear (< 2 m/s). It has been noted that for all the three cases, convection was initiated during the morning hours in the form of single cells, which evolved into multi-cellular convection zones later on those days. The most common mesoscale organization of the clouds in all the three cases had been in the form of convective lines which moved along the mean steering flow in the lower troposphere. However, for case 1, squall line formation and movement were observed during a period of 6 h, which was aided by the high unidirectional shear in the lower-to-middle troposphere, that was absent in other cases. These squall lines were associated with severe surface winds. The convection zones were found to be short lived with less stratiform outflow for case 1, more stratiform outflow for case 2 and longest lifetimes and most stratiform outflow for case 3. This study also indicates that the primary peak of convection and associated rainfall over the region, irrespective of the season, is in the afternoon hours between 1730 Indian Standard Time (IST) and 2030 IST, and lags the diurnal temperature maximum (around 1430 IST) by 3–5 h. When there is sufficient moisture in the atmosphere and convection persists throughout the entire diurnal cycle, a second peak in convection and associated rainfall appears over the region in the early morning hours (between 0230 and 0530 IST). This night time-early morning peak has a greater fraction of stratiform clouds at the beginning of a rainfall episode. As the moisture build up in the atmosphere on day 2 and later of a long-lived episode, new convection was initiated in the night time with increase in the night time rainfall intensity. This implies that pre-monsoon convection over Delhi in the presence of low moisture is primarily unimodal, characterized by short bursts of intense convection with narrow and short-lived cells. Monsoon convection on the other hand, is essentially bimodal, with the early morning peak, often pre-dominating over the afternoon peak and characterized by longer lived cells which are less intense than cells of the pre-monsoon weather systems.

High‐Resolution Modeling of ENSO‐Induced Precipitation in the Tropical Andes: Implications for Proxy Interpretation

AbstractSediment records from Lake Pallcacocha, Ecuador, have been interpreted as proxies of El Niño–Southern Oscillation (ENSO) variability, owing to increased precipitation in the area during El Niño events. However, the lake's watershed receives precipitation from processes arising from both the eastern and western Andes, where ENSO has different impacts; this has led to ambiguity in observed regional ENSO signals and has consequently challenged the suitability of the lake's records as ENSO proxies. Here, a mesoscale weather prediction model is used to investigate the regional circulation dynamics and precipitation response during different ENSO events, namely, Eastern Pacific (EP), Central Pacific (CP), coastal El Niño (COA), and La Niña (LN). The region receives more accumulated precipitation during COA and LN compared to EP and CP events. However, during EP and COA events, the region is prone to extreme precipitation associated with convective bursts originating from the Pacific. During CP and LN, moisture originates from the Atlantic and may reach the area as broader‐scale less‐intense precipitation. Statistical analysis of modeled precipitation reveals consistency between the number of threshold‐exceeding precipitation events in the high Andean elevations and the number of events identified in the late Holocene Pallcacocha record. These results illustrate the importance of considering ENSO flavors when interpreting paleoclimate proxies, highlight the role of COA events in understanding eastern Pacific proxy records, and support the hypothesis that Holocene changes in the number of events recorded in the lake sediment may indicate a change in the relative frequency of ENSO flavors.

Effects of microphysical processes on the rapid intensification of Super-Typhoon Meranti

Typhoon Meranti (2016) underwent extreme rapid intensification (RI) over the western North Pacific (WNP), just before its landfall in Fujian Province, China. In this study, the extreme RI, track, and structures of the inner core and spiral cloud bands of Meranti were simulated, reasonably realistically, using WRF-ARW (the Advanced Research version of the Weather Research and Forecasting model). Firstly, the convective bursts (CBs) and associated microphysical processes were analyzed in detail to evaluate the probable mechanism responsible for the RI of Meranti. The results showed that Meranti experienced RI over the warm ocean with high ocean heat content and decreasing environmental vertical shear. The onset of RI was triggered by CBs, which penetrated the upper troposphere, leading to upper-tropospheric warming and the formation of an upper-level warm core. Diagnosis of the complete Q-vector showed that the CBs of the inner core were jointly triggered by the dynamic forcing of the frontogenesis and the non-adiabatic phase transition process. Further analysis of the heat budget of microphysical processes revealed that the condensation process (accounting for 76.3% of the total heating rate of the warming processes) played a leading role for CBs in the lower and middle troposphere, while the deposition process (accounting for 19% of the total heating rate of the warming processes) was a critical factor of the CBs in the upper level of the troposphere.

Inner-core lightning outbreaks and convective evolution in Super Typhoon Haiyan (2013)

Using lightning data from the World Wide Lightning Location Network, infrared satellite imagery, and microwave observations, this study investigates lightning outbreaks and convective evolution in the inner core (0–100 km) of Super Typhoon Haiyan (2013), the strongest storm on record to make landfall in the northwest Pacific. This storm was characterized by intense lightning activity with half of the strokes occurring in the inner core. Three major inner-core lightning outbreaks and convective bursts (CBs) were observed during rapid intensification (RI), maximum intensity (MI), and weakening stages. These outbreaks coincided with favorable large-scale environmental conditions for TC development with higher sea surface temperature (29–30 °C), higher relative humidity (75–80%), and weaker deep-layer vertical wind shear (3–8 m s−1), compared to the climatological averages for the month of November in the northwest Pacific.The RI lightning outbreak occurred primarily in the downshear quadrants and CBs were located inside the radius of maximum wind (RMW). The MI lightning outbreak occurred just after the eyewall replacement cycle, inducing marked depression of brightness temperature at 91-GHz. The lightning outbreak during Haiyan's weakening stage preferred the upshear–left quadrant outside the RMW. In contrast, relative lack of cloud-to-ground lightning in the rainbands was observed during all three main outbreaks. The radial and azimuthal distributions of lightning outbreak within the inner core provided indicative information on the relationships between convective structure and intensity changes of Haiyan.

Rapid intensification of tropical cyclones in the context of the solar wind-magnetosphere-ionosphere-atmosphere coupling

Rapid intensification of tropical storms is examined in the context of solar wind coupling to the magnetosphere-ionosphere-atmosphere system. Tropical cyclone “best tracks” in the southern and northern hemispheres are used in the superposed epoch analysis of time series of solar wind parameters. The results indicate that rapid intensification of tropical storms tends to follow arrivals of high-speed streams from coronal holes or interplanetary coronal mass ejections, which can trigger geomagnetic storms. The ensuing auroral and polar cap activity including ionospheric currents and ionospheric convection generates atmospheric gravity waves that propagate from the high-latitude lower thermosphere both upward and downward. If ducted in the lower atmosphere, they can reach tropical troposphere. Despite significantly reduced wave amplitudes, but subject to amplification upon reflection in the upper troposphere, these gravity waves can trigger/release moist instabilities to initiate convective bursts, with the latent heat release leading to intensification of storms. Convective bursts have been linked to rapid intensification of tropical cyclones. Cases of tropical cyclone intensification closely correlated with the solar wind structure are found to be preceded by atmospheric gravity waves generated by the solar wind magnetosphere-ionosphere-atmosphere coupling process. The gravity waves are observed in the ionosphere as traveling ionospheric disturbances. Their propagation in the lower atmosphere is examined by ray tracing in a model atmosphere to show that they can reach tropical cyclones. It is suggested that the interaction of aurorally-generated gravity waves with the tropical cyclone vortex and the inner primary eyewall could play a role in the intensification process. Assuming that quasi-periodic convective bursts lead to vortex waves, a two-dimensional barotropic approximation is used to obtain asymptotic solutions representing propagation of vortex waves and their absorption by the mean flow in the critical layer.

Evolution of Thermodynamic Structures During Rapid Growth and Decay of Extremely Severe Cyclonic Storm Chapala (2015)

ABSTRACT The structure and evolution of inner-core convective bursts and their differences associated with rapid intensification (RI) and rapid decay (RD) of tropical cyclone CHAPALA are examined. The inception of RI was associated with substantial increase of convective heating and its vertical extent in the inner core. Increase in diabatic heating was of the order of 12-21 oC, particularly in the middle and upper troposphere. Latent heat release produced a diabatically generated potential vorticity (PV) in vertical column. The immediate cause of RI was a significant increase of moisture flux from surface to 500 hPa. This was accomplished primarily by updrafts of the order of 6-12 Pa s− 1, representing the strong vertical motion distribution inside the warm core convective zone. The episode of deep convective bursts transpired during the period of RI. The evolving flow became highly symmetric and dominated by deep convective axisymmetric vortex structures. The RD coincided with the significant weakening in updraft of moisture flux consequently decrease of diabatic heating in the middle and upper troposphere and dissipation of upper and lower PV.

Mesoscale Circulations and Organized Convection in African Easterly Waves

Abstract Global convection-permitting model simulations and remote sensing observations are used to investigate the interaction between organized convection, both moist and dry, and the atmospheric circulation in the case of an African easterly wave (AEW). The wave disturbance is associated with a quadrupole structure of divergence, with two convergence centers slightly ahead of the trough. Moisture transport from southeast of the trough to the area in front and lower midtropospheric moisture convergence precondition and organize convection. The main inflow into the squall-line cluster is from behind. The moisture-abundant inflow collides at the low level with monsoon air with high moist static energy and establishes a frontal line of updrafts at the leading edge of the propagating mesoscale convective system. A mantle of moisture surrounds the convective core. A potential vorticity budget analysis reveals that convective latent heating is driving the evolution of the wave but not in a quasi-steady way. The wave propagation includes a succession of convective bursts and subsequent dynamic adjustment processes. Dry convection associated with the Saharan air layer (SAL) and SAL intrusions into the wave trough together with vorticity advection can play a role in intensifying AEWs dynamically as they move from the West African coast across the Atlantic Ocean. Our analysis demonstrates that the synoptic-scale wave and convection are interlinked through mesoscale circulations on a continuum of scales. This implies that the relation between organized convection and the atmospheric circulation is intrinsically dynamic, which poses a particular challenge to subgrid convection parameterizations in numerical models.

The Role of Polygonal Eyewalls in Rapid Intensification of Typhoon Megi (2010)

Abstract High-resolution numerical experiments for Typhoon Megi (2010) in the western North Pacific are conducted using the Advanced Research version of the Weather Research and Forecasting (WRF) Model to understand the mechanisms of rapid intensification (RI). With a dynamically initialized vortex, sensitivity experiments are carried out focusing on the planetary boundary layer (PBL) and the following microphysics schemes: WRF single-moment 6-class (WSM6) and WRF double-moment 6-class (WDM6) microphysics with Yonsei University, Mellor–Yamada–Janjić (MYJ), and Mellor–Yamada–Nakanishi–Niino (MYNN) 2.5-level (MN2.5) and 3.0-level (MN3) PBL schemes. The largest differences are found between WSM6-MN3 and WDM6-MN3, and we therefore examine RI mechanisms based on the results of these experiments. Prior to RI, WDM6-MN3 shows a drier environment and stronger downdrafts in the lower troposphere than in WSM6-MN3. As a result, during the RI period, WSM6-MN3 (WDM6-MN3) significantly intensifies with the minimum sea level pressure decreasing by 51 (29) hPa and the maximum surface wind increasing by 28 (12) m s−1 in 24 h. In both experiments, the maximum values of surface heat fluxes, potential vorticity (PV), radial absolute angular momentum advection, inertial stability, supergradient wind, and convective bursts inside the radius of maximum winds are frequently observed at each vertex of polygonal eyewalls in the lower troposphere. In particular, WSM6-MN3 exhibits more convective cells inside the inner-core region, a more persistent and thicker polygonal eyewall in the lower troposphere, and a more robust vertical structure of hydrometeors and vertical velocity than WDM6-MN3. This study suggests that within the inner-core region, polygonal eyewalls like WSM6-MN3 provide favorable conditions for RI.

A Numerical Study of Convective‐Scale Structures in the Outer Cores of Sheared Tropical Cyclones: 1. Updraft Traits in Different Vertical Wind Shear Magnitudes

AbstractThis study investigates the outer‐core convective‐scale updraft features of numerically simulated tropical cyclones under vertical wind shear of different magnitudes. Updrafts in the outer core were weighted in favor of downshear formation, and the increase in the shear value led to more short‐lived updrafts. The tops of detected updrafts tended to cluster at either 4‐ to 6‐km or 12‐ to 14‐km height. Downshear‐left updrafts mostly moved radially inward, and updrafts on the left side of the shear vector moved more rapidly azimuthally. Longer‐lived updrafts gained greater vertical mass fluxes during their life cycles, with those in strongly sheared tropical cyclones having larger vertical mass fluxes for the same updraft life span. The height of stronger vertical velocity in convective bursts decreased as shear increased, whereas larger vertical velocity within nonconvective‐burst cells held between 3 and 4 km. The values of several environmental parameters (such as the supercell composite parameter and the energy‐helicity index) associated with updrafts in the downshear‐left quadrant were larger than those in the downshear‐right quadrant for the weakly sheared tropical cyclone, whereas they became much larger in the downshear‐right quadrant for highly sheared vortices. There were no relationships between the parameters and updraft mean mass fluxes, so the use of these parameters for predicting the likelihood of updraft strength in tropical cyclones requires caution.

The Influence of CO2Forcing on North American Monsoon Moisture Surges

Widespread multiday convective bursts in the southwestern United States during the North American monsoon are often triggered by Gulf of California moisture surges (GoC surges). However, how GoC surges, and the amount and intensity of associated precipitation, will change in response to CO2-induced warming remains little known, not least because the most widely available climate models do not currently resolve the relevant mesoscale dynamics because of their coarse resolution (100 km or more). In this study, a 50-km-resolution global coupled model is used to address this question. It is found that the mean number of GoC surge events remains unchanged under CO2doubling, but intermediate-to-high intensity surge-related precipitation tends to become less frequent, thus reducing the mean summertime rainfall. Low-level moisture fluxes associated with GoC surges as well as their convergence over land to the east of the GoC intensify, but the increases in low-level moisture are not matched by the larger increments in the near-surface saturation specific humidity because of amplified land warming. This results in a more unsaturated low-level atmospheric environment that disfavors moist convection. These thermodynamic changes are accompanied by dynamic changes that are also detrimental to convective activity, with the midlevel monsoonal ridge projected to expand and move to the west of its present-day climatological maximum. Despite the overall reduction in precipitation, the frequency of very intense, localized daily surge-related precipitation in Arizona and surrounding areas is projected to increase with increased precipitable water.

Air-sea interactions during rapid intensification of typhoon Fengshen (2008)

Typhoon (TY) Fengshen (2008) rapidly intensified from 35 to 70 kt (about 18–36 m/s) in 24 h between 0000 UTC 19 June and 0000 UTC 20 June 2008 east of the Philippines. Based on multi-platform observations, the oceanic and atmospheric processes responsible for its rapid intensification (RI) is investigated in this study. We find that the anomalously high tropical cyclone heat potential (110–200 kJ/cm2) (TCHP) exerts a great influence on RI of Fengshen compared with other atmospheric environmental factors. In 24 h of RI, TY Fengshen moves over the abnormally warm ocean where the maximum isotherm depth of 26 °C is 200 m which is 80 m deeper than that under the climatological condition. The thick and warm mixed layer restrains the sea surface cooling by limiting entrainment of the cold thermocline water, and thus enhances the ocean to atmosphere heat transfer. In areas of abnormally high TCHP, the occurrence of convective bursts (CBs) results in extra latent heat release (LHR). During the RI phase, abundant LHR causes the warm anomaly of the middle and upper troposphere around the eye, and thereby results in RI of Fengshen. In addition, we also find that the role of high TCHP in CBs and the following RI may be different between asymmetric and axisymmetric processes.

Erratum to: Relationship between convective bursts and the rapid intensification of Typhoon Mujigae (2015)

Erratum to: Relationship between convective bursts and the rapid intensification of Typhoon Mujigae (2015)

Relationship between convective bursts and the rapid intensification of Typhoon Mujigae (2015)

Typhoon Mujigae was the strongest typhoon in 2015, and underwent an extreme rapid intensification (RI). The RI of Mujigae was reasonably reproduced by a 72‐hr control simulation. By enhancing two updrafts in the eye wall, connective bursts (CBs) were found to have played an important role in the formation of the two warming cores in the middle and upper troposphere, respectively, which triggered and maintained the RI of Typhoon Mujigae. Meanwhile, CBs induced the upper‐tropospheric potential vorticity anomaly and caused the downward transfer of momentum from the upper to the middle troposphere which was also conducive to the RI of the typhoon. Sensitivity tests confirmed our conclusion.

The Relationship between Spatial Variations in the Structure of Convective Bursts and Tropical Cyclone Intensification as Determined by Airborne Doppler Radar

Abstract The relationship between radial and azimuthal variations in the composite characteristics of convective bursts (CBs), that is, regions of the most intense upward motion in tropical cyclones (TCs), and TC intensity change is examined using NOAA P-3 tail Doppler radar. Aircraft passes collected over a 13-yr period are examined in a coordinate system rotated relative to the deep-layer vertical wind shear vector and normalized by the low-level radius of maximum winds (RMW). The characteristics of CBs are investigated to determine how the radial and azimuthal variations of their structures are related to hurricane intensity change. In general, CBs have elevated reflectivity just below the updraft axis, enhanced tangential wind below and radially outward of the updraft, enhanced vorticity near the updraft, and divergent radial flow at the top of the updraft. When examining CB structure by shear-relative quadrant, the downshear-right (upshear left) region has updrafts at the lowest (highest) altitudes and weakest (strongest) magnitudes. When further stratifying by intensity change, the greatest differences are seen upshear. Intensifying storms have updrafts on the upshear side at a higher altitude and stronger magnitude than steady-state storms. This distribution provides a greater projection of diabatic heating onto the azimuthal mean, resulting in a more efficient vortex spinup. For variations based on radial location, CBs located inside the RMW show stronger updrafts at a higher altitude for intensifying storms. Stronger and deeper updrafts inside the RMW can spin up the vortex through greater angular momentum convergence and a more efficient vortex response to the diabatic heating.

Relation of Convective Bursts to Changes in the Intensity of Typhoon Lionrock (2016) during the Decay Phase Simulated by an Atmosphere-Wave-Ocean Coupled Model

Relation of Convective Bursts to Changes in the Intensity of Typhoon Lionrock (2016) during the Decay Phase Simulated by an Atmosphere-Wave-Ocean Coupled Model

Relationship between Tropical Cyclone Intensification and Cloud-Top Outflow Revealed by Upper-Tropospheric Atmospheric Motion Vectors

AbstractThe temporally dense geostationary satellite observations made possible by recent technological advances enable atmospheric motion vectors (AMVs) to be derived that are suitable for capturing atmospheric flows even of mesoscale phenomena, for which in situ data are scarce. Tropical cyclone (TC) outflows around the cloud top, reflecting TC secondary circulation, were computed by using AMVs derived from successive Multifunctional Transport Satellite (MTSAT) imagery, and the relationship between TC intensification rate (defined as the change of the best-track maximum sustained wind in the previous 24 h) and the outflow was investigated for 44 TCs occurring during 2011–14. During the TC intensification phase, temporal changes in the outflow were generally synchronous with changes in the cloud-top temperature of TC inner-core convective clouds detected by MTSAT infrared band. It was noteworthy that the intensification rates of 66% of the TCs peaked 0–36 h after outflow maximization and that the intensification rate for TCs with a maximum rate of &gt;15 m s−1 day−1 peaked after the outflow maximum. Furthermore, TCs with a large intensification rate and latent-heat release around the midlevel tended to have a large outflow during the intensification phase. A comparison of TCs with and without convective bursts (CBs) revealed that the correlation between outflow and the TC intensification rate was higher for TCs accompanied by CBs than for those without CBs, implying that a rapid deepening of inner-core convection is important for intensification of a TC’s secondary circulation. The outflow tended to be most correlated with the TC intensification rate 0–6 h earlier.

Analyzing Simulated Convective Bursts in Two Atlantic Hurricanes. Part II: Intensity Change due to Bursts

AbstractThis paper investigates convective burst (CB) evolution in Weather Research and Forecasting (WRF) Model simulations of two tropical cyclones (TCs), focusing on the relationship between CBs and TC intensity change. Analysis of intensity change in the simulations shows that there are more CBs inside the radius of maximum winds (RMW) during times when the TCs are about to intensify, while weakening/steady times are associated with more CBs outside the RMW, consistent with past observational and theoretical studies. The vertical mass flux distributions show greater vertical mass flux at upper levels both from weaker updrafts and CBs for intensifying cases. The TC simulations are further dissected by past intensity change, and times of sustained intensification have more CBs than times when the TC has been weakening but then intensifies. This result suggests that CB development may not always be predictive of intensification, but rather may occur as a result of ongoing intensification and contribute to ongoing intensification. Abrupt short-term intensification is found to be associated with an even higher density of CBs inside the RMW than is slower intensification. Lag correlations between CBs and intensity reveal a broad peak, with the CBs leading pressure falls by 0–3 h. These relationships are further confirmed by analysis of individual simulation periods, although the relationship can vary depending on environmental conditions and the previous evolution of the TC. These results show that increased convection due to both weak updrafts and CBs inside the RMW is favorable for sustained TC intensification and show many details of the typical short-term response of the TC core to CBs.

Analyzing Simulated Convective Bursts in Two Atlantic Hurricanes. Part I: Burst Formation and Development

AbstractUnderstanding the structure and evolution of the tropical cyclone (TC) inner core remains an elusive challenge in tropical meteorology, especially the role of transient asymmetric features such as localized strong updrafts known as convective bursts (CBs). This study investigates the formation of CBs and their role in TC structure and evolution using high-resolution simulations of two Atlantic hurricanes (Dean in 2007 and Bill in 2009) with the Weather Research and Forecasting (WRF) Model.Several different aspects of the dynamics and thermodynamics of the TC inner-core region are investigated with respect to their influence on TC convective burst development. Composites with CBs show stronger radial inflow in the lowest 2 km, and stronger radial outflow from the eye to the eyewall around z = 2–4 km, than composites without CBs. Asymmetric vorticity associated with eyewall mesovortices appears to be a major factor in some of the radial flow anomalies that lead to CB development. The anomalous outflow from these mesovortices, along with outflow from supergradient parcels above the boundary layer, favors low-level convergence and also appears to mix high-θe air from the eye into the eyewall. Analyses of individual CBs and parcel trajectories show that parcels are pulled into the eye and briefly mix with the eye air. The parcels then rapidly move outward into the eyewall, and quickly ascend in CBs, in some cases with vertical velocities of over 20 m s−1. These results support the importance of horizontal asymmetries in forcing extreme asymmetric vertical velocity in tropical cyclones.

On the Processes Leading to the Rapid Intensification of Typhoon Megi (2010)

Abstract The processes leading to the rapid intensification (RI) of Typhoon Megi (2010) are explored with a convection-permitting full-physics model and a sensitivity experiment using a different microphysical scheme. It is found that the temporary active convection, gradually strengthened primary circulation, and a warm core developing at midlevels tend to serve as precursors to RI. The potential vorticity (PV) budget and Sawyer–Eliassen model are utilized to examine the causes and effects of those precursors. Results show that the secondary circulation, triggered by the latent heat associated with active convection, acts to strengthen the mid- to upper-level primary circulation by transporting the larger momentum toward the upper layers. The increased inertial stability at mid- to upper levels not only increases the heating efficiency but also prevents the warm-core structure from being disrupted by the ventilation effect. The warming above 5 km effectively lowers the surface pressure. It is identified that the strong secondary circulation helps to accomplish the midlevel warming within the eye. The results based on potential temperature (θ) budget suggest that the mean subsidence associated with detrainment of active convection is the major process contributing to the formation of a midlevel warm core. On the possible causes triggering the inner-core active convection, it is suggested that the gradually increased vortex-scale surface enthalpy flux has a leading role in the development of vigorous convection. The results also highlight the potentially dominant role of weak to moderate convection in the onset of RI, while the convective bursts play a supporting role. Based on the aforementioned analyses, a schematic diagram is shown to describe the plausible path leading to RI.