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1. Convective-Storm Environments in Subtropical South America from High-Frequency Soundings during RELAMPAGO-CACTI

   https://doi.org/10.1175/MWR-D-20-0293.1  

   Schumacher, Russ S. ; Hence, Deanna A. ; Nesbitt, Stephen W. ; Trapp, Robert J. ; Kosiba, Karen A. ; Wurman, Joshua ; Salio, Paola ; Rugna, Martin ; Varble, Adam C. ; Kelly, Nathan R. ( May 2021 , Monthly Weather Review)

   null (Ed.)

   Abstract During the Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations-Cloud, Aerosol, and Complex Terrain Interactions (RELAMPAGO-CACTI) field experiments in 2018–19, an unprecedented number of balloon-borne soundings were collected in Argentina. Radiosondes were launched from both fixed and mobile platforms, yielding 2712 soundings during the period 15 October 2018–30 April 2019. Approximately 20% of these soundings were collected by highly mobile platforms, strategically positioned for each intensive observing period, and launching approximately once per hour. The combination of fixed and mobile soundings capture both the overall conditions characterizing the RELAMPAGO-CACTI campaign, as well as the detailed evolution of environments supporting the initiation and upscale growth of deep convective storms, including some that produced hazardous hail and heavy rainfall. Episodes of frequent convection were characterized by sufficient quantities of moisture and instability for deep convection, along with deep-layer vertical wind shear supportive of organized or rotating storms. A total of 11 soundings showed most unstable convective available potential energy (MUCAPE) exceeding 6000 J kg −1 , comparable to the extreme instability observed in other parts of the world with intense deep convection. Parameters used to diagnose severe-storm potential showed that conditions were often favorable for supercells and severe hail, but not for tornadoes, primarily because of insufficient low-level wind shear. High-frequency soundings also revealed the structure and evolution of the boundary layer leading up to convection initiation, convectively generated cold pools, the South American low-level jet (SALLJ), and elevated nocturnal convection. This sounding dataset will enable improved understanding and prediction of convective storms and their surroundings in subtropical South America, as well as comparisons with other heavily studied regions such as the central United States that have not previously been possible. 

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2. Utilizing a Storm-Generating Hotspot to Study Convective Cloud Transitions: The CACTI Experiment

   https://doi.org/10.1175/BAMS-D-20-0030.1  

   Varble, Adam C. ; Nesbitt, Stephen W. ; Salio, Paola ; Hardin, Joseph C. ; Bharadwaj, Nitin ; Borque, Paloma ; DeMott, Paul J. ; Feng, Zhe ; Hill, Thomas C. ; Marquis, James N. ; et al ( April 2021 , Bulletin of the American Meteorological Society)

   null (Ed.)

   Abstract The Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign was designed to improve understanding of orographic cloud life cycles in relation to surrounding atmospheric thermodynamic, flow, and aerosol conditions. The deployment to the Sierras de Córdoba range in north-central Argentina was chosen because of very frequent cumulus congestus, deep convection initiation, and mesoscale convective organization uniquely observable from a fixed site. The C-band Scanning Atmospheric Radiation Measurement (ARM) Precipitation Radar was deployed for the first time with over 50 ARM Mobile Facility atmospheric state, surface, aerosol, radiation, cloud, and precipitation instruments between October 2018 and April 2019. An intensive observing period (IOP) coincident with the RELAMPAGO field campaign was held between 1 November and 15 December during which 22 flights were performed by the ARM Gulfstream-1 aircraft. A multitude of atmospheric processes and cloud conditions were observed over the 7-month campaign, including: numerous orographic cumulus and stratocumulus events; new particle formation and growth producing high aerosol concentrations; drizzle formation in fog and shallow liquid clouds; very low aerosol conditions following wet deposition in heavy rainfall; initiation of ice in congestus clouds across a range of temperatures; extreme deep convection reaching 21-km altitudes; and organization of intense, hail-containing supercells and mesoscale convective systems. These comprehensive datasets include many of the first ever collected in this region and provide new opportunities to study orographic cloud evolution and interactions with meteorological conditions, aerosols, surface conditions, and radiation in mountainous terrain. 

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3. Environmental Controls on Simulated Deep Moist Convection Initiation Occurring during RELAMPAGO-CACTI

   https://doi.org/10.1175/JAS-D-21-0226.1  

   Nelson, T. Connor ; Marquis, James ; Peters, John M. ; Friedrich, Katja ( July 2022 , Journal of the Atmospheric Sciences)

   This study synthesizes the results of 13 high-resolution simulations of deep convective updrafts forming over idealized terrain using environments observed during the RELAMPAGO and CACTI field projects. Using composite soundings from multiple observed cases, and variations upon them, we explore the sensitivity of updraft properties (e.g., size, buoyancy, and vertical pressure gradient forces) to influences of environmental relative humidity, wind shear, and mesoscale orographic forcing that support or suppress deep convection initiation (CI). Emphasis is placed on differentiating physical processes affecting the development of updrafts (e.g., entrainment-driven dilution of updrafts) in environments typifying observed successful and null (i.e., no CI despite affirmative operational forecasts) CI events. Thermally induced mesoscale orographic lift favors the production of deep updrafts originating from ∼1- to 2-km-wide boundary layer thermals. Simulations without terrain forcing required much larger (∼5-km-wide) thermals to yield precipitating convection. CI outcome was quite sensitive to environmental relative humidity; updrafts with increased buoyancy, depth, and intensity thrived in otherwise inhospitable environments by simply increasing the free-tropospheric relative humidity. This implicates the entrainment of free-tropospheric air into updrafts as a prominent governor of CI, consistent with previous studies. Sensitivity of CI to the environmental wind is manifested by 1) low-level flow affecting the strength and depth of mesoscale convergence along the terrain, and 2) clouds encountering updraft-suppressing pressure gradient forces while interacting with vertical wind shear in the free troposphere. Among the ensemble of thermals occurring in each simulation, the widest deep updrafts in each simulation were the most sensitive to environmental influences.

    

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4. Influences of the South American Low-Level Jet on the Convective Environment in Central Argentina Using a Convection-Permitting Simulation

   https://doi.org/10.1175/MWR-D-23-0122.1  

   Sasaki, Clayton R. S. ; Rowe, Angela K. ; McMurdie, Lynn A. ; Varble, Adam C. ; Zhang, Zhixiao ( February 2024 , Monthly Weather Review)

   This study documents the spatial and temporal distribution of the South American low-level jet (SALLJ) and quantifies its impact on the convective environment using a 6.5-month convection-permitting simulation during the Remote Sensing of Electrification, Lightning, And Mesoscale/Microscale Processes with Adaptive Ground Observations and Clouds, Aerosols, and Complex Terrain Interactions (RELAMPAGO-CACTI) campaigns. Overall, the simulation reproduces the observed SALLJ characteristics in central Argentina near the Sierras de Córdoba (SDC), a focal point for terrain-focused upscale growth. SALLJs most frequently occur in the summer with maxima to the northwest and east of the SDC and minima over the higher terrain. The shallower SALLJs (<1750 m) have a strong overnight skew, while the elevated jets are more equally spread throughout the day. SALLJ periods often have higher amounts of low-level moisture and instability compared to non-SALLJ periods, with these impacts increasing over time when the SALLJ is present and decreasing afterward. The SALLJ may enhance low-level wind shear magnitudes (particularly when accounting for the jet height); however, enhancement is somewhat limited due to the presence of speed shear in most situations. SALLJ periods are associated with low-level directional shear favorable for organized convection and an orientation of cloud-layer wind shear parallel to the terrain, which could favor upscale growth. A case study is shown in which the SALLJ influenced both the magnitude and direction of wind shear concurrent with convective upscale growth near the SDC. This study highlights the complex relationship between the SALLJ and its impacts during periods of widespread convection.

   Areas of enhanced low-level winds, or low-level jets, likely promote favorable conditions for upscale growth, the processes by which storms grow larger. Central Argentina is an ideal place to study the influence of low-level jets on upscale growth as storms often stay connected to the Sierras de Córdoba Mountain range, growing over a relatively small area. This study uses model data to describe the distribution and impact of the South American low-level jet on the storm environment. The South American low-level jet is frequently found near the Sierras de Córdoba, and moisture and convective instability increase when it is present. However, the jet’s impact on other conditions important for upscale growth, such as vertical wind shear, is not as straightforward.

    

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5. A Synoptic Evolution Comparison of the Smallest and Largest MCSs in Subtropical South America between Spring and Summer

   https://doi.org/10.1175/MWR-D-20-0208.1  

   Piersante, Jeremiah O. ; Rasmussen, Kristen L. ; Schumacher, Russ S. ; Rowe, Angela K. ; McMurdie, Lynn A. ( May 2021 , Monthly Weather Review)

   null (Ed.)

   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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