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  • RELAMPAGO-CACTI: High Impact Weather in Subtropical South America x
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Timothy J. Lang, Eldo E. Ávila, Richard J. Blakeslee, Jeff Burchfield, Matthew Wingo, Phillip M. Bitzer, Lawrence D. Carey, Wiebke Deierling, Steven J. Goodman, Bruno Lisboa Medina, Gregory Melo, and Rodolfo G. Pereyra

), which is physically separate from the much larger Andes range located to its west ( Fig. 1 ). The SDC interacts with the warm and moist air from the South American low-level jet (SALLJ), mechanical subsidence in the lee of the Andes, and other meteorological features to provide orographic forcing of deep, intense convection that often back builds along the terrain ( Rasmussen and Houze 2011 , 2016 ; Rasmussen et al. 2014 ; Bruick et al. 2019 ). This creates a relatively geographically confined

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Jake P. Mulholland, Stephen W. Nesbitt, Robert J. Trapp, and John M. Peters

forcing owing to the stronger deep-layer wind shear in the higher-terrain supercell inflow may have also resulted in the stronger upward vertical velocities, especially in the low to midlevels (e.g., Weisman and Rotunno 2000 ; Peters et al. 2019b , 2020b ). Overall, these results suggest that terrain-induced variations to vertical wind profiles were mainly responsible for the differences noted in convective morphology as compared to terrain induced thermodynamic variations, in line with conclusions

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Robert J. Trapp, Karen A. Kosiba, James N. Marquis, Matthew R. Kumjian, Stephen W. Nesbitt, Joshua Wurman, Paola Salio, Maxwell A. Grover, Paul Robinson, and Deanna A. Hence

relationship owes to the critical dependence of the linear and nonlinear dynamics forcing of vertical accelerations on vertical shear. Note that because wide updrafts provide larger volumes for hail growth (e.g., Dennis and Kumjian 2017 ), an updraft-width enhancement by the terrain-enhanced vertical wind shear may have contributed to the large hail on 10 November 2018 despite the relatively short duration of updraft rotation in the IOP4 storm; future work will explore this possible connection. Wide

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Zachary S. Bruick, Kristen L. Rasmussen, Angela K. Rowe, and Lynn A. McMurdie

. 2009 ). In subtropical South America, correlations between rainfall and ENSO have been demonstrated, especially for the La Plata basin encompassing northeast Argentina, Paraguay, and southeastern Brazil. Rainfall tends to be maximized in this area during El Niño, leading to flooding within the basin ( Camilloni and Barros 2003 ; Cavalcanti et al. 2015 ). Synoptic forcing for enhanced rainfall in the La Plata basin may be provided by a stronger subtropical jet with increased cyclonic vorticity

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Jake P. Mulholland, Stephen W. Nesbitt, and Robert J. Trapp

(LLJ), and upper-level negative geostrophic potential vorticity (weak ambient inertial instability) all favored the most rapid transition of discrete convective cells into an MCS. Furthermore, Dial et al. (2010) found that for cases of convection initiation (CI) along a frontal or similar boundary, the potential for UCG increased when the cloud-layer wind and deep-layer vertical wind shear vectors were nearly parallel to the initiating boundary. Additionally, as the magnitude of low-level forcing

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Zachary S. Bruick, Kristen L. Rasmussen, and Daniel J. Cecil

, 2016 ) because of the impingement of the SALLJ on the topography. The orographic forcing helps to overcome any mechanical capping produced by subsiding upper-level air in the lee of the Andes. Additionally, the SDC and the plains immediately to their east were the focus of the RELAMPAGO and CACTI field campaigns. To understand the life cycle of intense convection, the TRMM PR data were separated into three categories, including deep, deep and wide, and wide convective cores (DCCs, DWCCs, and WCCs

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Hernán Bechis, Paola Salio, and Juan José Ruiz

moist tropical air mass to the north of the line and dry, warm air, which moves leeward of the Andes slopes in a zone of prevailing westerly flow. The regional circulation that leads to this airmass contrast is linked to the characteristics of the topography. North of 35°S the Andes block the low-level flow, forcing a mainly meridional displacement of air masses. In particular, the channeling of warm, moist air masses from low latitudes leads to the formation of the South American low-level jet

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Jake P. Mulholland, Stephen W. Nesbitt, Robert J. Trapp, Kristen L. Rasmussen, and Paola V. Salio

northwesterly flow pattern aloft across the tracking domain ( Fig. 12b ), whereas MCS events display a more westerly component across the tracking domain ( Fig. 12a ). MUN events also tend to have a slightly more amplified upper-level trough off the west coast of South America, potentially resulting in greater quasigeostrophic (QG) forcing for synoptic-scale ascent or supporting frontal intrusions in some events, favoring more widespread CI. The largest differences in the upper-level patterns exist between

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Matthew R. Kumjian, Rachel Gutierrez, Joshua S. Soderholm, Stephen W. Nesbitt, Paula Maldonado, Lorena Medina Luna, James Marquis, Kevin A. Bowley, Milagros Alvarez Imaz, and Paola Salio

force, as it had penetrated 2–3 cm into the ground when she found it. The photographs from shortly after it was retrieved ( Figs. 8a,b ) show a rather round stone with scalloped lobes (e.g., Knight and Knight 1970 ) covering much of the surface, and no large icicle lobes. This implies no preferred orientation direction in its final growth layer, presumably owing to random tumbling during its descent. She recounted that many of the stones had similar roundish shapes, with clear outsides and milky

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