Explosive Cyclones Research Articles

A high‐resolution climatological study of explosive cyclones in the Mediterranean region: Frequency, intensity and synoptic drivers

AbstractIn the complex context of climate change and in a heavily populated area like the Mediterranean region, understanding and studying explosive cyclones (ECs) is crucial because of their potential to cause extreme weather events, including strong winds, heavy rainfall, and significant disruptions to human activities and infrastructure, as happened in 2018 with the Vaia storm. The genesis and persistence of ECs, characterized by a rapid drop in central mean sea level pressure exceeding 12 hPa in 12 hours, are influenced by various physical processes and while water vapour convergence in the mid‐low troposphere is crucial, mid‐upper tropospheric cyclonic vorticity advection and upper tropospheric divergence also play significant roles. In this study, we used ERA5 hourly data of mean sea level pressure from 1979 to 2020 to analyse EC occurrences in the Mediterranean. The identification of EC events followed the 12‐hr deepening‐rate criterion introduced by Zhang et al. (2017), as shorter deepening rates definitions proved unreliable. A total of 80 ECs were identified and classified based on their spatial occurrence and magnitude, 34 of which were concentrated in the northwest of the basin, likely due to orographic deformation caused by the Alps on baroclinic waves originating from the Atlantic. The study also identified recurring synoptic patterns associated with the persistence of ECs over the sea, including the presence of potential vorticity (PV) streamers and mid‐tropospheric dry‐air intrusions. Empirical Orthogonal Function (EOF) analysis highlighted the Scandinavian pattern as the primary driver, with blocking conditions over western Russia and Scandinavia, and composite analyses revealed the intrusion of PV to various atmospheric levels, extending up to 500 hPa. In summary, this study offers valuable insights into the complex mechanisms governing EC formation in the Mediterranean Basin, shedding light on the synoptic patterns, atmospheric dynamics, and potential impacts on regional weather systems.

Explosive Cyclone Impact on the Power Distribution Grid in Rio Grande do Sul, Brazil

Southern Brazil is a region strongly influenced by the occurrence of extratropical cyclones. Some of them go through a rapid and intense deepening and are known as explosive cyclones. These cyclones are associated with severe weather conditions such as heavy rainfall, strong winds, and lightning, leading to various natural disasters and causing socioeconomic losses. This study investigated the interaction between atmospheric and oceanic conditions that contributed to the rapid intensification of the cyclone that occurred near the coast of South Brazil from 29 June to 3 July 2020, causing significant havoc. Hourly atmospheric and oceanic data from the ERA5 reanalysis were employed in this analysis. The results showed that warm air and moisture transportation were key contributors to these phenomena. In addition, the interaction between the jet stream and the cyclone’s movement played a crucial role in cyclone formation and intensification. Positive sea surface temperature anomalies also fueled the cyclone’s intensification. These anomalies increased the surface heat fluxes, making the atmosphere more unstable and promoting a strong upward motion. Due to the strong winds and the heavy rainfall, the explosive cyclone caused substantial impacts on the power services, resulting in widespread power outages, damaged infrastructure, and interruptions in energy distribution. This work describes in detail the cyclone development and intensification and aims at the understanding of these storms, which is crucial for minimizing their aftermaths, especially on energy distribution.

Explosive cyclones occurred between 2010 and 2020 in the South Atlantic under the perspective of two detection schemes.

Under two detection schemes, this study analyzes one of the most destructive weather systems - the explosive cyclones - in the South Atlantic, from 2010 to 2020. Then, two methods are presented to study these systems: the Observational Method (OBSM) and the Automated Method (AUTM). The first uses visual analysis of the mean sea level pressure (mslp) fields and functions to identify the local minimums using the Grid Analysis and Display (GrADS) software. The second utilizes a function from OpenGrADS called mfhilo. It shows the local minimum in the grid using laplacian, magnitude, and percentile. Two shell algorithms for data manipulation are used for the AUTM: one to trace the cyclones' trajectories according to a previously defined fixed area and the other to separate them into explosives. The OBSM methodology showed 271 cases averaging 25 yearly and revealed important characteristics regarding the intensities. According to AUTM's methodology, from the 2705 ordinary cyclone cases identified, 299 are explosives. There is a clear seasonality pattern in the systems' distribution along South America, similar to OBSM, but more highlighted. In summer, they concentrate at high latitudes, while in winter and spring, they are assembled near southern Brazilian and Uruguayan coasts.

Diagnostic Study of an Extreme Explosive Cyclone over the Kuroshio/Kuroshio Extension Region

Diagnostic Study of an Extreme Explosive Cyclone over the Kuroshio/Kuroshio Extension Region

Variations in Key Factors at Different Explosive Development Stages of an Extreme Explosive Cyclone over the Japan Sea

Explosive cyclones (ECs) occur frequently over the Japan Sea. The most rapidly intensifying EC over the Japan Sea during the 44-year period 1979–2022, in the cold season (October–April), was examined to reveal the variations in the key factors at different explosive development stages. The EC deepened at a maximum deepening rate of 3.07 bergerons and explosive development lasted for 15 h. At the initial moment of explosive development, the EC had distinctive low-level baroclinicity, the low-level water vapor convergence was weak, and mid-level cyclonic vorticity advection was far away from the EC’s center. At the moment at which the EC reached the maximum deepening rate, the low-level water vapor convergence and mid-level cyclonic vorticity advection increased distinctly and approached the EC’s center. A diagnostic analysis using the Zwack–Okossi equation showed that the main contributor to the initial explosive development was warm-air advection. Through the evolutionary process of the explosive development, the non-key factors of the cyclonic vorticity advection and diabatic heating at the initial explosive development stage increased quickly and became key factors contributing to the maximum explosive development. The key factors contributing to the explosive development varied with the stage of explosive development. The cross-section and vertical profile of each term suggested that the cyclonic vorticity advection was enhanced in the upper troposphere and diabatic heating increased in the middle troposphere.

The Generalized Application of a New Surface Pressure Tendency Equation in Synoptic Weather Systems

AbstractThe surface pressure tendency equation (SPTE) is an important theoretical tool in understanding the development of weather systems, with various forms being proposed over the past 100 years. In this study, a new version of the SPTE, which eliminates the explicit or implicit pressure tendency term on the right‐hand side, is applied to three typical synoptic weather systems in terms of tropical cyclones, explosive cyclones anticyclones, and two mesoscale systems in terms of squall lines and supercells based on numerical simulations. Results reveal promising performance of the new equation in understanding the surface pressure tendency of three synoptic systems, showing advantages in uncovering their dominant physical processes as well as inherent discrepancies. For the tropical cyclone, the latent heating is largely responsible for the surface pressure fall, whereas the surface pressure rise of the anticyclone is mainly fueled by the upper‐level advection of potential temperature. The leading factors for the explosive cyclone intensification demonstrate a transition from mid‐level diabatic heating at the early stage to upper‐level potential temperature advection related to stratospheric air intrusion during the later intensification period. For the squall line and supercell, however, local breakdown of hydrostatic balance can be as large as ∼10%, and consequently, caution should be taken when the equation is used in mesoscale systems, especially for the supercell. These results suggest that the new SPTE can be well applied in synoptic weather systems, and can provide new insights into their intensity evolutions and contributions of diverse physical processes.

Toward Eliminating the Decades-Old "Too Zonal and Too Equatorward" Storm-Track Bias in Climate Models.

Generations of climate models exhibit biases in their representation of North Atlantic storm tracks, which tend to be too far near the equator and too zonal. Additionally, models have difficulties simulating explosive cyclone growth. These biases are one of the reasons for the uncertainties in projections of future climate over Europe, and the underlying causes have yet to be determined. All three biases are shown to be related, and diabatic processes are pointed to as a likely cause. To demonstrate this, two hemispherically symmetric storm tracks forming downstream of an idealized sea surface temperature (SST) front on an aquaplanet are examined using the seamless ICOsahedral Non-hydrostatic weather and climate model (ICON) and its grid refinement capabilities. The analyzed perpetual boreal winter has a global grid spacing of 20km, two bi-directionally interacting grid nests over the Northern Hemisphere that refine the grid to 10-km spacing over much of the stormtrack and further to 5-km spacing near the SST front. In contrast, no grid refinement is performed for the Southern Hemisphere. Feature-based cyclone tracking shows that the poleward propagation in the NH is enhanced, so the high-resolution storm track is less equatorward and less zonal; explosive deepening rates are more frequent and precipitation rates are amplified. The implication is that resolving diabatic processes on the storm scale improves all three intersecting biases in the representation of storm tracks. While new challenges arise at cloud resolving scales, much improvement for the representation of storm tracks will be gained once climate models resolve the meso-γ scale.

Warm conveyor belts in present-day and future climate simulations – Part 2: Role of potential vorticity production for cyclone intensification

Abstract. Warm conveyor belts (WCBs) are strongly ascending, cloud- and precipitation-forming airstreams in extratropical cyclones. The intense cloud-diabatic processes produce low-level cyclonic potential vorticity (PV) along the ascending airstreams, which often contribute to the intensification of the associated cyclone. This study investigates how climate change affects the cyclones' WCB strength and the importance of WCB-related diabatic PV production for cyclone intensification, based on present-day (1990–1999) and future (2091–2100) climate simulations of the Community Earth System Model Large Ensemble (CESM-LE). In each period, a large number of cyclones and their associated WCB trajectories have been identified in both hemispheres during the winter season. WCB trajectories are identified as strongly ascending air parcels that rise at least 600 hPa in 48 h. Compared to ERA-Interim reanalyses, the present-day climate simulations are able to capture the cyclone structure and the associated WCBs reasonably well, which gives confidence in future projections with CESM-LE. However, the amplitude of the diabatically produced low-level PV anomaly in the cyclone centre is underestimated in the climate simulations, most likely because of reduced vertical resolution compared to ERA-Interim. The comparison of the simulations for the two climates reveals an increase in the WCB strength and the cyclone intensification rate in the Southern Hemisphere (SH) in the future climate. The WCB strength also increases in the Northern Hemisphere (NH) but to a smaller degree, and the cyclone intensification rate is not projected to change considerably. Hence, in the two hemispheres cyclone intensification responds differently to an increase in WCB strength. Cyclone deepening correlates positively with the intensity of the associated WCB, with a Spearman correlation coefficient of 0.68 (0.66) in the NH in the present-day (future) simulations and a coefficient of 0.51 (0.55) in the SH. The number of explosive cyclones with strong WCBs, referred to as C1 cyclones, is projected to increase in both hemispheres, while the number of explosive cyclones with weak WCBs (C3 cyclones) is projected to decrease. A composite analysis reveals that in the future climate C1 cyclones will be associated with even stronger WCBs, more WCB-related diabatic PV production, the formation of a more intense PV tower, and an increase in precipitation. They will become warmer, moister, and slightly more intense. The findings indicate that (i) latent heating associated with WCBs (as identified with our method) will increase, (ii) WCB-related PV production will be even more important for explosive cyclone intensification than in the present-day climate, and (iii) the interplay between dry and moist dynamics is crucial to understand how climate change affects cyclone intensification.

Historic and Future Perspectives of Storm and Cyclone

In weather sciences, the two specific terms “storm” and “cyclone” frequently appear in literature and usually refer to the violent nature of a number of weather systems characterized by central low pressure, strong winds, large precipitation amounts in the form of rain, freezing rain, or snow, as well as thunder and lightning. But what is the connection between these two specific terms? In this paper, the historic evolutions of the terms “storm” and “cyclone” are reviewed from the perspective of weather science. The earliest recorded storms in world history are also briefly introduced. Then, the origin of the term “meteorological bomb”, which is the nickname of the “explosive cyclone” is introduced. Later, the various definitions of explosive cyclones given by several researchers are discussed. Also, the climatological features of explosive cyclones, as well as the future trends of explosive cyclones under global climate change, are discussed.

A comparative study on initial developments between explosive and nonexplosive cyclones off the East Asian coast in winter

Explosive cyclones (ECs) pose serious challenges for weather forecasting and significant threats to human life and property. In searching for the key points that make a cyclone go through explosive deepening off the East Asian coast, we present a comparative analysis of ECs and nonexplosive cyclones (or ordinary cyclones; OCs) using 10 years of ERA5 reanalysis data with high temporal and spatial resolutions. Their differences in synoptic backgrounds are shown, and mechanisms of the initial developments are compared quantitatively from the perspective of potential vorticity (PV). Among the identified 135 cyclones, 72 went through explosive growth and 37.5/36.1/20.8/5.6% of these ECs are ranked as weak/medium/strong/super ECs. ECs feature stronger low-level baroclinicity and higher PV than OCs. The decomposition of the local PV tendency shows the dominant role of the PV advection (with a correlation coefficient of 0.8). During the initial development, ECs have an average meridional temperature contrast 4 K larger than OCs within 20 latitudes at the low troposphere in the upstream, due to a stronger cold advection. The upstream colder air increases the horizontal temperature gradient and thus produces steeper isentropic surfaces inclining to the west. Since the PV intrusion is mainly along the isentropic surfaces, the increase in their slope significantly enhances the downward transport of PV from upper air. The importance of the horizontal gradient of potential temperature is further proved by numerical experiments with the Weather Research and Forecast (WRF) model on typical winter ECs. In sensitivity experiments, the low troposphere meridional temperature contrast decreasing by the average difference between ECs and OCs significantly decreases PV and stops the cyclones from explosive deepening. Despite the importance of diabatic processes in the deepening of mid-latitude cyclones emphasized by many studies, this study shows that the PV intrusion dominated by cold air mass is the key cause of winter explosive cyclogenesis in this region.

Kinetic Energy and Vorticity Perspectives of the Rapid Development of an Explosive Extratropical Cyclone Over the Northwest Pacific Ocean in February 2018

Explosive extratropical cyclones (EECs) have long been a research focus for the meteorological society as they often cause serious economic losses and casualties. However, after a long period of research, there still remain some knowledge gaps about their rapid development. In this article, we conducted the first study by using both vorticity and kinetic energy (KE) budgets simultaneously on a typical EEC, which was the strongest EEC that affected the coastal areas of China in the last 3 years, to further the understanding of the mechanisms governing its rapid enhancement in rotation and wind speed. The vorticity budget shows that the lower-level convergence-related vertical stretching and the vertical transport of vorticity acted as the most and second most favorable factors for the increase in the cyclone's cyclonic vorticity, respectively, which were different from those findings based on the Zwack–Okossi vorticity budget. In contrast, the horizontal transport of vorticity and tilting mainly decelerated the EEC's development. Energetics features governing the rapid wind enhancement of the EEC were shown for the first time. It is found that the work on the rotational wind by the pressure gradient force and the net import transport of KE by the rotational wind contributed the largest and second largest to the cyclone's increase in wind speed. In contrast, the upward transport of KE and the cyclone's displacement mainly acted in an opposite way. Analysis based on the Green's theorem and rotational wind shows that the enhancement of the EEC's rotation and wind speed were linked to each other solidly.

Structure of the Potential Vorticity of an Explosive Cyclone over the Eastern Asian Region in Late November 2013

Structure of the Potential Vorticity of an Explosive Cyclone over the Eastern Asian Region in Late November 2013

Rapid Increase of Explosive Cyclone Activity over the Midwinter North Pacific in the Late 1980s

Abstract Long-term changes in the activity of explosively developing “bomb” cyclones over the wintertime North Pacific are investigated by using a particular version of a global atmospheric reanalysis dataset into which only conventional observations have been assimilated. Bomb cyclones in January are found to increase rapidly around 1987 in the midlatitude central North Pacific. Some of the increased bomb cyclones formed over the East China Sea and then moved along the southern coast of Japan before developing explosively in the central North Pacific. The enhanced cyclone activity is found to be concomitant with rapid warming and moistening over the subtropical western Pacific and the South and East China Seas under the weakened monsoonal northerlies, leading to the enhancement of the lower-tropospheric Eady growth rate and equivalent potential temperature gradient, setting a condition favorable for cyclone formation in the area upstream of the North Pacific storm track. Along the storm track, poleward moisture transport in the warm sector of a cyclone and associated precipitation along the warm and cold fronts tended to increase and thereby enhance its explosive development. After the transition around 1987, a bomb cyclone has become more likely to develop without a strong upper-level cyclonic vortex propagating from Eurasia than in the earlier period. The increased bomb cyclone activity in January is found to contribute to the diminished midwinter minimum of the North Pacific storm track activity after the mid-1980s.

Increase in the number of explosive low‒level cyclones around King George Island in the last three decades.

This paper documents an increase in the number of observed explosive cyclones (EC) at King George Island, South Shetland Islands, Antarctica, over the 1989‒2020 period. In ECs at 60o latitudes the surface atmospheric pressure drops ≥24 hPA in 24 hours. The annual EC frequency time series shows a significant positive trend of ~2.7 cyclones/decade, with a break in 2003 and average numbers of 7.3 and 11.8 events before and after that break, respectively. The increase follows closely earlier documented global sea surface temperature (SST) anomaly trends for the 1981‒2018 period, partially attributed to global warming and to the Pacific Decadal Oscillation (PDO). Connections between EC frequency and SST might occur through variations in SST in the southeastern Pacific and southwestern Atlantic, with anomalous cold conditions favoring an increase in ECs. We also found close relations between the number of ECs with simultaneous occurrences of PDO and Atlantic multidecadal oscillation in opposite phases, so that after 2003 they were in the cold and warm phases, respectively, and vice-versa before 2003. Both low-frequency modes seem to modulate the number of ECs. As per the authors knowledge these results have not been discussed before and may help climate modeling studies and weather forecasts.

South Atlantic explosive cyclones in 2014-2015: study employing NCEP2 and MERRA-2 reanalyses

An analysis of explosive cyclone cases was produced by comparing the reanalysis of MERRA-2 (high spatial resolution) and NCEP2 (low spatial resolution) to South Atlantic in the 2014-2015 period. A total of 51 cases were found, of which 49 were detected by the first reanalysis and 33 by the second (2 cases identified by NCEP2 were not identified by MERRA-2). Spring was the dominant season in the formation of the cases in both reanalyses. It was observed that most systems are formed preferentially eastward of a preexisting trough at higher levels, while others are formed under an almost zonal upper airstream. This difference is more evident in the NCEP2. It was also diagnosed that the MERRA-2 shows more clearly the diffluence in the 250 hPa flow. The analysis of the composite fields revealed a negative horizontal tilt of the trough in 500 hPa, influenced by intense convection as the system develops. Besides, it pointed to a more pronounced jet stream in intense explosive cyclones and more prominent diffluence in non-intense cases. Since the NCEP2 reanalysis detected fewer cases (and only 2 intense) than MERRA-2, it was considered that the former is less suited to the analysis of this type of event.

Analysis of the Most Intense Explosive Cyclone that Occurred Between 2010 and 2020 in the South Atlantic

The coastal region of South America is known for the high frequency of extratropical cyclones. From 2010 to 2020, there was an exceptional case regarding intensity, reaching 2.73 Bergeron, between January 02 and 03, 2019. To better understand the characteristics of this type of explosive cyclone, this work sought to investigate the synoptic conditions in this phenomenon. To this end, a visual inspection method of the sea level pressure charts was applied, allied with the functions available in the Grid Analysis and Display software. The cyclone began by transitioning from the continental low to the extratropical cyclone, associated with a trough at higher levels in a zone of weak temperature advection. As the system developed, there was a fracture in the upper air trough, acquiring negative horizontal inclination and the transition of the cyclone from the tropical to the polar side of the jet streak. Sea heat fluxes become relevant only 6 hours after cyclogenesis and enhance as the surface wind increases in the cold sector of the cyclone. In addition, a robust stratospheric ozone intrusion arose close to 700 hPa in the cyclone region, related to the dynamic tropopause folding.Keywords: Bombogenesis, Heat Fluxes, Dynamic Tropopause, ERA5, Synoptic Analysis. Análise do ciclone explosivo mais intenso ocorrido no período entre 2010 e 2020 no Atlântico SulR E S U M OA região da costa da América do Sul é conhecida pela alta frequência de ciclones extratropicais. Durante o período de 2010 a 2020, houve um caso excepcional com relação à intensidade, atingindo 2,73 Bergeron, entre 02 e 03 de janeiro de 2019. Para entender melhor as características associadas com este tipo de ciclone explosivo, este trabalho buscou investigar as condições sinóticas ocorridas neste fenômeno. Para tal, foi aplicado um método de inspeção visual das cartas de pressão ao nível médio do mar, aliado a funções disponíveis no software Grid Analysis and Display. O ciclone iniciou através da transição da baixa continental ao ciclone extratropical associado a um cavado em níveis superiores em uma zona de fraca advecção de temperatura. Conforme o sistema se desenvolveu, houve uma fratura no cavado em ar superior, adquirindo inclinação horizontal negativa e a transição do ciclone do lado tropical para o polar do jet streak. Os fluxos de calor do oceano se tornaram relevantes apenas 6 horas após a ciclogênese e se destacam com o aumento do vento à superfície no setor frio do ciclone. Além disso, observou-se uma forte intrusão de ozônio estratosférico até próximo de 700 hPa na região do ciclone, relacionado à dobra da tropopausa dinâmica.Palavras-chave: Bombogênese, Fluxos de Calor, Tropopausa Dinâmica, ERA5, Análise Sinótica.

Characteristics of an Explosive Cyclone over Northeast China Revealed by Satellite Water Vapor Imagery

Characteristics of an Explosive Cyclone over Northeast China Revealed by Satellite Water Vapor Imagery

Statistical Characteristics and Composite Environmental Conditions of Explosive Cyclones over the Japan Sea and Kuroshio/Kuroshio Extension

Statistical characteristics and composite synoptic-scale environmental conditions of explosive cyclones (ECs) over the Japan Sea and Kuroshio/Kuroshio Extension are examined and compared using ERA5 atmospheric reanalysis to give a better understanding of their differences. ECs over the Japan Sea frequently occur in late autumn and early winter and those over the Kuroshio/Kuroshio Extension mainly occur in winter and early spring. The maximum deepening rate, minimum central sea level pressure and explosive-developing lifetime of ECs over the Kuroshio/Kuroshio Extension are generally larger, lower and longer, respectively, than those over the Japan Sea. ECs over the Kuroshio/Kuroshio Extension formed over the East China Sea tend to develop more rapidly, and weak and moderate ECs generally begin to develop explosively over the sea to the east of the Japan Islands, while the strong and super ECs over the sea to the south of Japan Islands have longer explosive-developing tracks. Composite analysis shows that synoptic-scale environmental conditions favoring rapid EC development over these two regions are significantly different. ECs over the Japan Sea have stronger baroclinicity and cyclonic vorticity, but weaker water vapor convergence and upper-level jet stream than those over the Kuroshio/Kuroshio Extension. The key factor contributing to the baroclinicity is the cold air intrusion over the Japan Sea and the strong warm current heating over the Kuroshio/Kuroshio Extension. The potential vorticity shows anomalies in upper and low levels for both EC areas and extends further downwards over the Japan Sea.

Analysis of the Transition of an Explosive Cyclone to a Mediterranean Tropical-like Cyclone

This study investigates the dynamics of the development phases of a Mediterranean tropical-like cyclone (medicane) in the southern Ionian Sea, on 28 September 2018 that caused high impact phenomena in the central and eastern Mediterranean, focusing on the transition from explosive cyclone to medicane. The symmetry and the warm core structure of the system have been demonstrated via phase space diagrams determining three phases of the system development that are then supported on a dynamical basis. During the first phase of the system, baroclinic instability triggered the formation of the explosive cyclone, when strong upper-level PV anomalies at the dynamic tropopause level moved towards a pre-existed area of enhanced low-level baroclinicity over the coastal areas of Libya along with positive SST anomalies. The surface frontal structure was enhanced under the influence of the upper-level dynamic processes. During the second phase when the medicane formed, low-level diabatic processes determined the evolution of the surface cyclone, without any significant support from baroclinic processes in the upper troposphere. The distortion of the low-level baroclinicity and the frontal structure began after the initial weakening of the upper-level dynamics. During the third phase, the system remained barotropic, being affected by similar mechanisms as in the second phase but with lower intensity. The transition mechanism is not only the result of the seclusion of warm air in the cyclone core but, mainly, the continuation of an explosive cyclone or an intense cyclone when the occlusion began to form.

Physical Process Contributions to the Development of a Super Explosive Cyclone Over the Gulf Stream

Contributions of different physical processes to the development of a super explosive cyclone (SEC) migrating over the Gulf Stream with the maximum deepening rate of 3.45 Bergeron were investigated using the ERA5 atmospheric reanalysis from European Centre for Medium-Range Weather Forecasts (ECMWF). The evolution of the SEC resembled the Shapiro-Keyser model. The moisture transported to the bent-back front by easterlies from Gulf Stream favored precipitation and enhanced the latent heat release. The bent-back front and warm front were dominated by the water vapor convergence in the mid-low troposphere, the cyclonic-vorticity advection in the mid-upper troposphere and the divergence in the upper troposphere. These factors favored the rapid development of the SEC, but their contributions showed significant differences during the explosive-developing stage. The diagnostic results based on the Zwack-Okossi equation suggested that the early explosive development of the SEC was mainly forced by the diabatic heating in the mid-low troposphere. From the early explosive-developing moment to maximum-deepening-rate moment, the diabatic heating, warm-air advection and cyclonic-vorticity advection were all enhanced significantly, their combination forced the most explosive development, and the diabatic heating had the biggest contribution, followed by the warm-air advection and cyclonic-vorticity advection, which is different from the previous studies of ECs over the Northwestern Atlantic. The cross section of these factors suggested that during the rapid development, the cyclonic-vorticity advection was distributed and enhanced significantly in the mid-low troposphere, the warm-air advection was strengthened significantly in the mid-low and upper troposphere, and the diabatic heating was distributed in the middle troposphere.