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

Abstract

Subtropical South America (SSA) east of the Andes Mountains is a global hotspot for mesoscale convective systems (MCSs). Wide convective cores (WCCs) are typically embedded within mature MCSs, contribute over 40% of SSA’s warm-season rainfall, and are often associated with severe weather. Prior analysis of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data identified WCCs in SSA and associated synoptic conditions during austral summer. As WCCs also occur during the austral spring, this study uses the 16-year TRMM PR dataset and ERA5 reanalysis to compare anomalies in environmental conditions between austral spring (SON) and summer (DJF) for the largest and smallest WCCs in SSA. During both seasons, large WCCs are associated with an anomalous mid-level trough that slowly crosses the Andes Mountains and a northerly South American low-level jet (SALLJ) over SSA, though the SON trough and SALLJ anomalies are stronger and located farther northeastward than in DJF. A synoptic pattern evolution resembling large WCC environments is illustrated through a multi-day case during the RELAMPAGO field campaign (10-13 November 2018). Unique high-temporal resolution soundings showed strong mid-level vertical wind shear associated with this event, induced by the juxtaposition of the northerly SALLJ and southerly near-surface flow. It is hypothesized that the Andes help create a quasi-stationary trough/ridge pattern such that favorable synoptic conditions for deep convection persist for multiple days. For the smallest WCCs, anomalously weaker synoptic-scale forcing was present compared to the largest events, especially for DJF, pointing to future work exploring MCS formation under weaker synoptic conditions.

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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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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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Sujan Pal, Francina Dominguez, María Eugenia Dillon, Javier Alvarez, Carlos Marcelo Garcia, Stephen W. Nesbitt, and David Gochis

hydrologic modeling approach is also useful in flood-prone watersheds, to evaluate the flood predictability and possible hydrologic response in a changing climate ( Moore and Clarke 1981 ; Clark et al. 2008 ; Fenicia et al. 2011 ). For this study, we use the WRF-Hydro modeling system as an uncoupled (i.e., driven by independent meteorological forcing), distributed hydrologic model over the basin to assess its capability in a flash flood hindcast and forecasting framework. WRF-Hydro is currently the

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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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