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Jeremiah O. Piersante, Kristen L. Rasmussen, Russ S. Schumacher, Angela K. Rowe, and Lynn A. McMurdie

-scale differences between MCSs across seasons, this study analyzes the anomalous synoptic environments associated with organized deep convection in both austral spring and summer in SSA. Based on previous studies in the CONUS ( Feng et al. 2019 ; Song et al. 2019 ), it is hypothesized that spring MCSs are associated with greater anomalous synoptic forcing. Because of this potential shift in synoptic forcing magnitude, we determine whether there is a relationship between anomalous synoptic forcing and MCS area

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T. Connor Nelson, James Marquis, Adam Varble, and Katja Friedrich

component, there is a similar mean upslope wind magnitude for all event types, differing by generally less than 1–2 m s −1 ( Fig. 9 ). Kirshbaum (2011) found, however, that that even somewhat small increases in the terrain-relative wind [ O (1) m s −1 ] could hamper the chances for orographic CI by disrupting or displacing the low-level convergence relative to the topographic thermal forcing. Contrary to his findings, our mean Null environments have the weakest overall terrain-perpendicular (upslope

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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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James N. Marquis, Adam C. Varble, Paul Robinson, T. Connor Nelson, and Katja Friedrich

horizontal flow convergence frequently aids convection initiation processes by forcing low-level air parcels upward, locally reducing CIN, deepening boundary layer moisture below cloud base, and providing a focal area for moist updrafts to detrain into the overlying free troposphere, reducing the negative entrainment effect ( Ziegler et al. 1997 ; Markowski and Richardson 2010 ; Moser and Lasher-Trapp 2017 ). Common mesoscale convergence features that trigger deep convection initiation (hereafter CI

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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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