RELAMPAGO-CACTI: High Impact Weather in Subtropical South America

Description:

The special collection includes articles related to the major field experiments and associated research from two sister campaigns that studied the intense convective storms in Subtropical South America in 2018–2019: the Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations (RELAMPAGO) campaign, and the Clouds, Aerosols, and Complex Terrain Interactions (CACTI) campaign. These sister campaigns involved 200 scientists from 4 continents, and involved fixed ground, aircraft, and mobile observational assets, targeted satellite observations, and modeling components. These campaigns produced an unparalleled dataset involving many aspects of the intense convective storms, extreme hydrometeorological impacts, regional climate, and societal impacts of high-impact weather in this region. Aspects of studies including observing processes related to convection initiation, aerosol–cloud interactions over complex terrain, the production of giant hail in severe thunderstorms, and the rapid upscale growth and flooding from mesoscale convective systems, and the operational prediction and climate impacts of these storms. Project overview papers have been published in BAMS. The overview for RELAMPAGO is 10.1175/BAMS-D-20-0029.1, while the overview for CACTI is 10.1175/BAMS-D-20-0030.1.

Collection organizers:
Stephen Nesbitt, Department of Atmospheric Sciences, University of Illinois Urbana-Champaign
Adam Varble, Pacific Northwest National Laboratory
Paola Salio, University of Buenos Aires

RELAMPAGO-CACTI: High Impact Weather in Subtropical South America

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-yr TRMM PR and ERA5 datasets 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 midlevel 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 multiday case during the RELAMPAGO field campaign (10–13 November 2018). Unique high-temporal-resolution soundings showed strong midlevel 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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Russ S. Schumacher
,
Deanna A. Hence
,
Stephen W. Nesbitt
,
Robert J. Trapp
,
Karen A. Kosiba
,
Joshua Wurman
,
Paola Salio
,
Martin Rugna
,
Adam C. Varble
, and
Nathan R. Kelly

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

Abstract

Some of the most intense convective storms on Earth initiate near the Sierras de Córdoba mountain range in Argentina. The goal of the RELAMPAGO field campaign was to observe these intense convective storms and their associated impacts. The intense observation period (IOP) occurred during November–December 2018. The two goals of the hydrometeorological component of RELAMPAGO IOP were 1) to perform hydrological streamflow and meteorological observations in previously ungauged basins and 2) to build a hydrometeorological modeling system for hindcast and forecast applications. During the IOP, our team was able to construct the stage–discharge curves in three basins, as hydrological instrumentation and personnel were successfully deployed based on RELAMPAGO weather forecasts. We found that the flood response time in these river locations is typically between 5 and 6 h from the peak of the rain event. The satellite-observed rainfall product IMERG-Final showed a better representation of rain gauge–estimated precipitation, while IMERG-Early and IMERG-Late had significant positive bias. The modeling component focuses on the 48-h simulation of an extreme hydrometeorological event that occurred on 27 November 2018. Using the Weather Research and Forecasting (WRF) atmospheric model and its hydrologic component WRF-Hydro as an uncoupled hydrologic model, we developed a system for hindcast, deterministic forecast, and a 60-member ensemble forecast initialized with regional-scale atmospheric data assimilation. Critically, our results highlight that streamflow simulations using the ensemble forecasting with data assimilation provide realistic flash flood forecast in terms of timing and magnitude of the peak. Our findings from this work are being used by the water managers in the region.

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

Abstract

The Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) and Cloud, Aerosol, and Complex Terrain Interactions (CACTI) projects deployed a high-spatiotemporal-resolution radiosonde network to examine environments supporting deep convection in the complex terrain of central Argentina. This study aims to characterize atmospheric profiles most representative of the near-cloud environment (in time and space) to identify the mesoscale ingredients affecting storm initiation and growth. Spatiotemporal autocorrelation analysis of the soundings reveals that there is considerable environmental heterogeneity, with boundary layer thermodynamic and kinematic fields becoming statistically uncorrelated on scales of 1–2 h and 30 km. Using this as guidance, we examine a variety of environmental parameters derived from soundings collected within close proximity (30 km in space and 30 min in time) of 44 events over 9 days where the atmosphere either: 1) supported the initiation of sustained precipitating convection, 2) yielded weak and short-lived precipitating convection, or 3) produced no precipitating convection in disagreement with numerical forecasts from convection-allowing models (i.e., Null events). There are large statistical differences between the Null event environments and those supporting any convective precipitation. Null event profiles contained larger convective available potential energy, but had low free-tropospheric relative humidity, higher freezing levels, and evidence of limited horizontal convergence near the terrain at low levels that likely suppressed deep convective growth. We also present evidence from the radiosonde and satellite measurements that flow–terrain interactions may yield gravity wave activity that affects CI outcome.

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

Abstract

Orographic deep convection (DC) initiation and rapid evolution from supercells to mesoscale convective systems (MCSs) are common near the Sierras de Córdoba, Argentina, which was the focal point of the Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) field campaign. This study used an idealized numerical model with elongated north–south terrain similar to that of the Sierras de Córdoba to address how variations in terrain height affected the environment and convective morphology. Simulations used a thermodynamic profile from a RELAMPAGO event that featured both supercell and MCS storm modes. Results revealed that DC initiated earlier in simulations with higher terrain, owing both to stronger upslope flows and standing mountain waves. All simulations resulted in supercell formation, with higher-terrain supercells initiating closer to the terrain peak and moving slower off the terrain. Higher-terrain simulations displayed increases in both low-level and deep-layer wind shear along the eastern slopes of the terrain that were related to the enhanced upslope flows, supporting stronger and wider supercell updrafts/downdrafts and a wider swath of heavy rainfall. Deeper and stronger cold pools from these wider and stronger higher-terrain supercells led to surging outflow that reduced convective available potential energy accessible to deep convective updrafts, resulting in quicker supercell demise off the terrain. Lower-terrain supercells moved quickly off the terrain, merged with weaker convective cells, and resulted in a quasi-organized MCS. These results demonstrate that terrain-induced flow modification may lead to substantial local variations in convective morphology.

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Matthew R. Kumjian
,
Rachel Gutierrez
,
Joshua S. Soderholm
,
Stephen W. Nesbitt
,
Paula Maldonado
,
Lorena Medina Luna
,
James Marquis
,
Kevin A. Bowley
,
Milagros Alvarez Imaz
, and
Paola Salio

Abstract

On 8 February 2018, a supercell storm produced gargantuan (>15 cm or >6 in. in maximum dimension) hail as it moved over the heavily populated city of Villa Carlos Paz in Córdoba Province, Argentina. Observations of gargantuan hail are quite rare, but the large population density here yielded numerous witnesses and social media pictures and videos from this event that document multiple large hailstones. The storm was also sampled by the newly installed operational polarimetric C-band radar in Córdoba. During the RELAMPAGO campaign, the authors interviewed local residents about their accounts of the storm and uncovered additional social media video and photographs revealing extremely large hail at multiple locations in town. This article documents the case, including the meteorological conditions supporting the storm (with the aid of a high-resolution WRF simulation), the storm’s observed radar signatures, and three noteworthy hailstones observed by residents. These hailstones include a freezer-preserved 4.48-in. (11.38 cm) maximum dimension stone that was scanned with a 3D infrared laser scanner, a 7.1-in. (18 cm) maximum dimension stone, and a hailstone photogrammetrically estimated to be between 7.4 and 9.3 in. (18.8–23.7 cm) in maximum dimension, which is close to or exceeds the world record for maximum dimension. Such a well-observed case is an important step forward in understanding environments and storms that produce gargantuan hail, and ultimately how to anticipate and detect such extreme events.

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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

Abstract

During November 2018–April 2019, an 11-station very high frequency (VHF) Lightning Mapping Array (LMA) was deployed to Córdoba Province, Argentina. The purpose of the LMA was validation of the Geostationary Lightning Mapper (GLM), but the deployment was coordinated with two field campaigns. The LMA observed 2.9 million flashes (≥ five sources) during 163 days, and level-1 (VHF locations), level-2 (flashes classified), and level-3 (gridded products) datasets have been made public. The network’s performance allows scientifically useful analysis within 100 km when at least seven stations were active. Careful analysis beyond 100 km is also possible. The LMA dataset includes many examples of intense storms with extremely high flash rates (>1 s−1), electrical discharges in overshooting tops (OTs), as well as anomalously charged thunderstorms with low-altitude lightning. The modal flash altitude was 10 km, but many flashes occurred at very high altitude (15–20 km). There were also anomalous and stratiform flashes near 5–7 km in altitude. Most flashes were small (<50 km2 area). Comparisons with GLM on 14 and 20 December 2018 indicated that GLM most successfully detected larger flashes (i.e., more than 100 VHF sources), with detection efficiency (DE) up to 90%. However, GLM DE was reduced for flashes that were smaller or that occurred lower in the cloud (e.g., near 6-km altitude). GLM DE also was reduced during a period of OT electrical discharges. Overall, GLM DE was a strong function of thunderstorm evolution and the dominant characteristics of the lightning it produced.

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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

Abstract

On 10 November 2018, during the RELAMPAGO field campaign in Argentina, South America, a thunderstorm with supercell characteristics was observed by an array of mobile observing instruments, including three Doppler on Wheels radars. In contrast to the archetypal supercell described in the Glossary of Meteorology, the updraft rotation in this storm was rather short lived (~25 min), causing some initial doubt as to whether this indeed was a supercell. However, retrieved 3D winds from dual-Doppler radar scans were used to document a high spatial correspondence between midlevel vertical velocity and vertical vorticity in this storm, thus providing evidence to support the supercell categorization. Additional data collected within the RELAMPAGO domain revealed other storms with this behavior, which appears to be attributable in part to effects of the local terrain. Specifically, the IOP4 supercell and other short-duration supercell cases presented had storm motions that were nearly perpendicular to the long axis of the Sierras de Córdoba Mountains; a long-duration supercell case, on the other hand, had a storm motion nearly parallel to these mountains. Sounding observations as well as model simulations indicate that a mountain-perpendicular storm motion results in a relatively short storm residence time within the narrow zone of terrain-enhanced vertical wind shear. Such a motion and short residence time would limit the upward tilting, by the left-moving supercell updraft, of the storm-relative, antistreamwise horizontal vorticity associated with anabatic flow near complex terrain.

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Hernán Bechis
,
Paola Salio
, and
Juan José Ruiz

Abstract

Drylines have been identified as relevant synoptic-scale phenomena that frequently occur in several regions around the world. Despite previous works and the experience of local forecasters that recognizes the occurrence of drylines in Argentina and suggests its possible association with convection initiation, knowledge about the mechanisms leading to the genesis of these features is poor. This paper presents the first synoptic climatology of these drylines as well as a first approach to the understanding of the processes leading to their formation. The climatology is based on an automated algorithm for dryline identification applied to reanalysis data. We found that drylines are more frequent between the northern Patagonia plateau and the central Argentinean plains. A composite analysis is performed to analyze the processes leading to the formation of synoptic-scale drylines within this region. It was found that these drylines form in the confluence between a warm and moist air mass driven by a northwesterly flow and drier air flowing east over the northern Patagonia plateau. The dry air originates on top of the Pacific maritime boundary layer and experiences lee subsidence after crossing the Andes range creating an area of dry and warm air that is advected to the east by the westerly synoptic-scale flow, and transported downward during the day due to strong boundary layer turbulence. At the same time, surface heating over the plateau leads to substantial warming of the originally colder dry air behind the dryline, thus reversing the horizontal temperature gradient across the dryline.

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

Abstract

Satellite- and ground-based radar observations have shown that the northern half of Argentina, South America, is a region susceptible to rapid upscale growth of deep moist convection into larger organized mesoscale convective systems (MCSs). In particular, the complex terrain of the Sierras de Córdoba is hypothesized to be vital to this upscale-growth process. A canonical orographic supercell-to-MCS transition case study was analyzed to determine the influence that complex terrain had on processes governing upscale convective growth. High-resolution numerical modeling experiments were conducted in which the terrain height of the Sierras de Córdoba was systematically modified by raising or lowering the elevation of terrain above 1000 m. The alteration of the terrain lead to both direct and indirect effects on storm morphology. A direct effect included terrain blocking of cold pools, whereas indirect effects included terrain-induced variations in pertinent storm environmental parameters (e.g., vertical wind shear, convective available potential energy). When the terrain was raised, low-level and deep-layer vertical wind shear increased, mixed-layer convective available potential energy decreased, deep moist convection initiated earlier, and cold pools were blocked and generally became stronger and deeper. The reverse occurred when the terrain was lowered, resulting in a weaker supercell that did not grow upscale into an MCS. The control simulation supercell displayed the deepest cold pool and correspondingly fastest transition from supercell to MCS, potentially revealing that the unique terrain configuration of the Sierras de Córdoba was supportive of the observed rapid upscale convective growth of this orographic supercell.

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