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

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

Abstract

Hailstorms in subtropical South America are known to be some of the most frequent anywhere in the world, causing significant damage to the local agricultural economy every year. Convection in this region tends to be orographically forced, with moisture supplied from the Amazon rain forest by the South American low-level jet. Previous climatologies of hailstorms in this region have been limited to localized and sparse observational networks. Because of the lack of sufficient ground-based radar coverage, objective radar-derived hail climatologies have also not been produced for this region. As a result, this study uses a 16-yr dataset of TRMM Precipitation Radar and Microwave Imager observations to identify possible hailstorms remotely, using 37-GHz brightness temperature as a hail proxy. By combining satellite instruments and ERA-Interim reanalysis data, this study produces the first objective study of hailstorms in this region. Hailstorms in subtropical South America have an extended diurnal cycle, often occurring in the overnight hours. In addition, they tend to be multicellular in nature, rather than discrete. High-probability hailstorms (≥50% probability of containing hail) tend to be deeper by 1–2 km and horizontally larger by greater than 15 000 km2 than storms having a low probability of containing hail (<25% probability of containing hail). Hailstorms are supported synoptically by strong upper- and lower-level jets, anomalously warm and moist low levels, and enhanced instability. The findings of this study will support the forecasting of these severe storms and mitigation of their damage within this region.

Free access
Zachary S. Bruick
,
Kristen L. Rasmussen
,
Angela K. Rowe
, and
Lynn A. McMurdie

Abstract

El Niño–Southern Oscillation (ENSO) is known to have teleconnections to atmospheric circulations and weather patterns around the world. Previous studies have examined connections between ENSO and rainfall in tropical South America, but little work has been done connecting ENSO phases with convection in subtropical South America. The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) has provided novel observations of convection in this region, including that convection in the lee of the Andes Mountains is among the deepest and most intense in the world with frequent upscale growth into mesoscale convective systems. A 16-yr dataset from the TRMM PR is used to analyze deep and wide convection in combination with ERA-Interim reanalysis storm composites. Results from the study show that deep and wide convection occurs in all phases of ENSO, with only some modest variations in frequency between ENSO phases. However, the most statistically significant differences between ENSO phases occur in the three-dimensional storm structure. Deep and wide convection during El Niño tends to be taller and contain stronger convection, while La Niña storms contain stronger stratiform echoes. The synoptic and thermodynamic conditions supporting the deeper storms during El Niño is related to increased convective available potential energy, a strengthening of the South American low-level jet (SALLJ), and a stronger upper-level jet stream, often with the equatorward-entrance region of the jet stream directly over the convective storm locations. These enhanced synoptic and thermodynamic conditions provide insight into how the structure of some of the most intense convection on Earth varies with phases of ENSO.

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Jake P. Mulholland
,
Stephen W. Nesbitt
,
Robert J. Trapp
,
Kristen L. Rasmussen
, and
Paola V. Salio

Abstract

Satellite observations have revealed that some of the world’s most intense deep convective storms occur near the Sierras de Córdoba, Argentina, South America. A C-band, dual-polarization Doppler weather radar recently installed in the city of Córdoba in 2015 is now providing a high-resolution radar perspective of this intense convection. Radar data from two austral spring and summer seasons (2015–17) are used to document the convective life cycle, while reanalysis data are utilized to construct storm environments across this region. Most of the storms in the region are multicellular and initiate most frequently during the early afternoon and late evening hours near and just east of the Sierras de Córdoba. Annually, the peak occurrence of these storms is during the austral summer months of December, January, and February. These Córdoba radar-based statistics are shown to be comparable to statistics derived from Tropical Rainfall Measuring Mission Precipitation Radar data. While generally similar to storm environments in the United States, storm environments in central Argentina tend to be characterized by larger CAPE and weaker low-level vertical wind shear. One of the more intriguing results is the relatively fast transition from first storms to larger mesoscale convective systems, compared with locations in the central United States.

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