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

represents the meridional anomaly and arrows show the composite vectors in each figure. Full field composites of 850-hPa wind are also available in Fig. S4 . There are widespread northerly 850-hPa winds on day −2 in both SON and DJF ( Figs. 6a,d ) resulting in northerly moisture flux ( Figs. 7a,d ). As expected based on the relative magnitudes of the low-level lee troughs between the spring and summer ( Figs. 4b and 5b ), the actual ( Fig. S4 ) and anomalous ( Figs. 6a,d ) northerly wind into the La

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

simulation and line along which the west-to-east-oriented vertical cross sections of terrain are shown in (c) (red dashed line), and (c) west-to-east-oriented vertical cross sections of terrain height taken through the peak of the terrain for each simulation [km; see red dashed line in (b)]. Radiation and surface fluxes of heat, moisture, and momentum were included to allow for the development of realistic slope flows and natural DCI without prescribing a warm/cold “bubble” in proximity to the heated

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Jeremiah O. Piersante, Russ. S. Schumacher, and Kristen L. Rasmussen

to the frequent occurrence of mesoscale convective systems (MCSs; Velasco and Fritsch 1987 ; Laing and Fritsch 1997 ; Houze 2004 ; Zipser et al. 2006 ; Houze et al. 2015 ). Generally, convection initiation for MCSs in both continents involves the same key ingredients: 1) a poleward flux of low-level moisture from the Gulf of Mexico or the Amazon via the low-level jet (LLJ), 2) a capping inversion that prolongs the release of instability via westerly mid- to upper-level dry air subsidence

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Hernán Bechis, Paola Salio, and Juan José Ruiz

mechanisms have been proposed to explain the strength of the low-level moisture gradient leading to the formation of drylines. Frontogenesis caused by shearing deformation and confluence acting over the moisture fields is one of them ( Anthes et al. 1982 ; Ziegler et al. 1995 ). Drylines can also appear as a consequence of inhomogeneities in land cover type or soil conditions. For example, strong differences in soil moisture can lead to horizontal gradients in surface moisture fluxes which can directly

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

( Morrison 2017 ); thus, initial updraft width could be one factor governing CI. Numerical representation of updraft size and vertical mass flux is sensitive to the model grid resolution (e.g., Bryan et al. 2003 ; Varble et al. 2014 ; Varble et al. 2020 ; Hirt et al. 2020 ), as well as other physical parameterizations, limiting what can be ascertained about updraft-environment interactions using convection-allowing mesoscale models. A more complete understanding of CI requires synchronized

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

the warm air advection at ~30°S at the lower level. To understand the role of lower-level moisture advection during this event we plot a Hovmöller diagram of meridional wind ( V wind; Fig. 8a ) and moisture flux due to meridional wind ( Vq ; Fig. 8b ). It is clear that prior to the event ~0000 UTC 27 November, there is a strengthening of northerly SALLJ (deep purple shading) at ~850 mb carrying moisture southward. We see a reversal of lower-level wind direction after the event ~0000 UTC 28

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