An Acad Bras Cienc (2021) 93(3): e20190157 DOI 10.1590/0001-3765202120190157 Anais da Academia Brasileira de Ciências | Annals of the Brazilian Academy of Sciences Printed ISSN 0001-3765 I Online ISSN 1678-2690 www.scielo.br/aabc | www.fb.com/aabcjournal GEOSCIENCES Comparing explosive cyclogenesis Running title: COMPARING cases of different intensities EXPLOSIVE CYCLONES BY INTENSITY occurred in Southern Atlantic VILSON D. DE AVILA, ANDRÉ B. NUNES & RITA DE CÁSSIA M. ALVES Academy Section: GEOSCIENCES Abstract: Six events of explosive cyclogenesis occurred in the south Atlantic were compared using reanalysis data and satellite water vapor imagery. Cases of different e20190157 intensities (weak, moderate and strong) occurred during 2014 summer season and 2012 winter season were studied. Despite the similarities the tropopause anomaly was more prominent and vertical movements were stronger in the strong cyclogenesis cases. 93 The tropopause anomalies behind the cold front and ahead of the warm front appear (3) only in the mature stage of the weak and moderate cases while in the strong case it 93(3) is already evident and more intense behind the cold front since the beginning of the DOI cycle. In all the cases confl uence of the jet streams took place at higher levels forming 10.1590/0001-3765202120190157 a jet streak with difl uence occurring downstream and the cyclone beginning in the exit region. The trajectories of the cyclones were in the southeast direction but longer and more meridional in the strong cases. The results indicated the baroclinicity of the region as the main mechanism for the development of these cyclones as well as the amplitude of the upper level jet stream perturbation. Furthermore, all the explosive cyclones developed following the Shapiro & Keyser cyclone conceptual model. Key words: explosive cyclone, tropopause anomaly, Shapiro-Keyser, seclusion. INTRODUCTION the case of the Itaipu Dam, one of the world’s biggest hydroelectric power plants. The southern region of South America is very The weather and climate of the southern influenced by the passage of many types of region of South America are strongly infl uenced meteorological transient systems such as fronts, by the occurrence of extratropical cyclones since cyclones, squall lines, mesoscale convective this kind of meteorological transient system and systems and even tornados. their associated fronts are the most frequent Generally, in the absence of climatic phenomena reaching the region (Reboita et al. anomalies the south region of Brazil, Paraguay 2010) and maintaining the regime of abundant and Uruguay have a practically homogeneous and well distributed precipitation in space and precipitation regime with territorially well time. distributed precipitations along the year with The first cyclone conceptual model was elevated annual totals (Reboita et al. 2010) designed by Bjerknes (1919), improved by which favors the agricultural activities, the Bjerknes & Solberg (1922), followed by works vegetable extractivism, the livestock farming like Sutcliffe (1947), Petterssen (1956), Palmen and the hydroelectric energy production as in & Newton (1969), among many others, with the An Acad Bras Cienc (2021) 93(3) VILSON D. DE AVILA, ANDRÉ B. NUNES & RITA DE CÁSSIA M. ALVES COMPARING EXPLOSIVE CYCLONES BY INTENSITY publication of an additional conceptual model Thereafter, other studies found the existence proposed by Shapiro & Keyser (1990). of more cyclogenetic regions in South America. Till 1990 there were a few studies about Hoskins & Hodges (2005) and also Reboita et al. cyclogenesis in South America (e.g., Taljaard (2005) found a third cyclogenetic region in the 1967, Necco 1982, Satyamurty et al. 1990). They east coast to the south and southeast of Brazil found discrepant results because of the different with higher frequency of events in the summer. periods in time and size as well as the data used. Allen et al. (2010) found another cyclogenetic In order to understand the actual behavior of region in the south extreme of South America. cyclogenesis Gan & Rao (1991) computed the Among the extratropical cyclones there is frequency of surface cyclogenesis over South a special kind which develops in a very rapid America during the period of 1979-1988. From the and intense way with respect to the deepening 1091 cases observed in that period they found of the central atmospheric surface pressure. that the frequency of cyclogenesis is higher in This kind of cyclone is frequently referred as the winter in accord to Necco (1982) with the Bomb Cyclone (Bluestein 1993), commonly maximum frequency in May and the minimum in Explosive Cyclone and also East Coast Cyclone. December. In addition to the seasonal variation They generate severe weather causing serious of cyclogenesis they find interannual variation risk in regions near the coast because they are with major frequency in El Niño years and also potentially destructive and difficult to predict spatial variation with emphasis to two preferred due to the very fast deepening of the central regions for the occurrence of cyclogenesis: surface pressure, to the lack of meteorological One in the region of Uruguay where the major observation data over the ocean as well as frequency was in the winter influenced by the knowledge of the dynamic and thermodynamic Andes Cordillera and the baroclinic instability, factors that determine their occurrence. and the other near the San Mathias gulf where Originally the explosive cyclogenesis is the frequency was higher in the summer characterized by an intense and rapid deepening associated to the baroclinic instability of the in the surface pressure originally defined as a westerly flow. pressure drop of at least 1 hPa/h during a 24 After 1990 automatic schemes for detection hours period in the latitude of 60°. The criterion and tracking of cyclones and anticyclones were for its recognition corrected for any latitude is developed (Murray & Simmonds 1991) and many the value of the Normalized Deepening Rate works utilized reanalysis data with different (NDR) of the central surface pressure (Sanders spatial and temporal resolutions (Sinclair 1995, & Gyakum 1980) given in Bergeron units (B) as: Mendes et al. 2010). As an example, Sinclair sinφ NDR= ∆ p/ 24 hours (1) (1995) utilized reanalysis data from European sin 60° Centre for Medium-Range Weather Forecasts (ECMWF) while Simmonds & Keay (2000) utilized In this equation ϕ is the mean latitude of the reanalysis data from National Centers for explosive cycle and is the surface pressure drop Environmental Prediction / National Center for during that cycle. When the NDR reaches at least Atmospheric Research (NCEP/NCAR) and both 1 B the cyclogenesis is considered explosive. obtained results that agreed and confirmed According to Sanders & Gyakum (1980) the those results obtained by Gan & Rao (1991). explosive cyclones produce great amounts of precipitation, strong winds, high waves, and poor An Acad Bras Cienc (2021) 93(3) e20190157 2 | 23 VILSON D. DE AVILA, ANDRÉ B. NUNES & RITA DE CÁSSIA M. ALVES COMPARING EXPLOSIVE CYCLONES BY INTENSITY visibility. They are included among the most Pinto & Rocha (2011) used the Lorenz Energy severe events that affect the coastal regions Cycle to analyze the cyclogenesis dynamic where the demographic density is high and also aspects over the east coast cyclogenetic region in the open sea. of South America founding that the cyclogenesis Various authors (e.g., Hoskins et al. 1985, may have various different features there. In Uccellini et al. 1985, Wang & Rogers 2001, Avila a pioneering way Black & Pezza (2013) have et al. 2016, Nunes & Avila 2017) have indicated analyzed the Lorenz Energy Cycle associated with the association between the variations in the explosive cyclogenesis for the four regions with tropopause height and the occurrence of surface major explosive activity of the globe and found a cyclogenesis. It was observed that the occurrence robust signature in that energy cycle associated of tropopause local lowering frequently referred to anomalous energy conversion that are not as “tropopause folding” (Reed 1955) was present in normal cyclogenesis processes. The remarkable in cases of explosive cyclogenesis. latter believe that their results open a new way In some articles as Santurette & Georgiev (2005) to explore the behavior of explosive storms it was shown that the surface of 1.5 Potential based on the great scale environment and may Vorticity Unit (PVU) is representative of the so provide its prediction. called “dynamic tropopause” although some Various studies on explosive cyclogenesis authors consider 1.0 PVU and others even 2.0 were made for South America focusing mainly PVU. Bechis et al. (2018) suggested that the Ertel climatological aspects (e.g., Mendes et al. Potential Vorticity (EPV) threshold corresponding 2010, Bitencourt et al. 2013), the role of surface to the dynamic tropopause (indicated by the sensible and latent heat fluxes (Piva et al. 2011) World Meteorological Organization as -1.6 PVU and some others concerning certain dynamic for Southern Hemisphere) should take into or synoptic features (e.g., Iwabe & Rocha 2009, account the seasonality and the topography Avila et al. 2016). for its determination. Thus it is common in Bittencourt et al. (2013) presented a Meteorology to identify the tropopause height climatologic study for the explosive cyclones by the Ertel Potential Vorticity (Hoskins et al. in the South America cyclogenetic region based 1985, Schubert et al. 2004) with value of 1.5 on NCEP reanalysis data and tracking automatic PVU (where 1 PVU = 1 x 10-6 K kg-1 m2 s-1, having scheme of Murray & Simmonds (1991). They found positive sign in the Northern Hemisphere and that explosive cyclones are more frequent in the minus sign in the Southern Hemisphere). winter being a rare phenomenon although the The satellite water vapor imagery is useful South America cyclogenetic area has the major for identifying the dry air intrusions associated frequency of such cyclones in the southern with the tropopause folding as dark narrow hemisphere. Their results agree with those of bands (Santurette & Georgiev 2005).