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Zhixiao Zhang, Adam Varble, Zhe Feng, Joseph Hardin, and Edward Zipser

1. Introduction A deep convective cloud cluster meets the criterion for a mesoscale convective system (MCS) when its contiguous precipitation area reaches a scale of 100 km or more in one direction (e.g., Houze 1993 , 2004 ). An MCS often contains both convective towers and stratiform regions ( Houze 1993 ) and can develop mesoscale circulations during its mature stage (e.g., Houze 1993 , 2004 ; Chen and Frank 1993 ) that differentiate it from isolated deep convection. MCSs significantly

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

environments supporting it with adequate spatial and temporal resolution, as well as an incomplete understanding of environment–cloud interactions supporting growing congestus (e.g., Crook 1996 ; Weckwerth and Parsons 2006 ; Houston and Niyogi 2007 ; Lock and Houston 2014 ; Rousseau-Rizzi et al. 2017 ; Weckwerth et al. 2019 ). For CI to occur, the atmosphere requires three fundamental ingredients: static instability, moisture, and a triggering mechanism (e.g., surface airmass boundaries, orographic

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

subtropical South America (SSA) are deeper and more frequent than those east of the Rocky Mountains in North America ( Zipser et al. 2006 ; Houze et al. 2015 ). Specifically, the cloud shields associated with SSA mesoscale convective systems (MCSs) are approximately 60% larger than those occurring in the continental United States (CONUS; Velasco and Fritsch 1987 ) and their precipitation areas are larger and longer lived ( Durkee et al. 2009 ; Durkee and Mote 2010 ), contributing up to ~95% of warm

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

November–18 December 2018; https://www.eol.ucar.edu/field_projects/relampago ) and the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign (1 October 2018–30 April 2019; https://www.arm.gov/research/campaigns/amf2018cacti ). This study will contribute to a better global understanding of hailstorms to help to improve forecasting and diagnosis of hailstorms in subtropical South America. 2. Method Because of the lack of a hail-report database in subtropical South America, satellite

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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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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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