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

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Aiswarya Lakshmi K. K.
,
Swaroop Sahoo
,
Sounak Kumar Biswas
, and
V. Chandrasekar

Abstract

Weather radars with dual-polarization capabilities enable the study of various characteristics of hydrometeors, including their size, shape, and orientation. Radar polarimetric measurements, coupled with Doppler information, allow for analysis in the spectral domain. This analysis can be leveraged to reveal valuable insight into the microphysics and kinematics of hydrometeors in precipitation systems. This paper uses spectral polarimetry to investigate precipitation microphysics and kinematics in storm environments observed during the Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) field experiment in Argentina. This study uses range–height indicator scan measurements from a C-band polarimetric Doppler weather radar deployed during the field campaign. In this work, the impact of storm dynamics on hydrometeors is studied, including the size sorting of hydrometeors due to vertical wind shear. In addition, particle microphysical processes because of aggregation and growth of ice crystals in anvil clouds, as well as graupel formation resulting from the riming of ice crystals and dendrites, are also analyzed here. The presence of different particle size distributions because of the mixing of hydrometeors in a sheared environment and resulting size sorting has been reported using spectral differential reflectivity (sZdr) slope. Spectral reflectivity sZ h and sZdr have also been used to understand the signature of ice crystal aggregation in an anvil cloud. The regions of pristine ice crystals are identified from vertical profiles of spectral polarimetric variables in anvil cloud because of sZ h < 0 dB and sZdr values around 2 dB. It is also found that the growth process of these ice crystals causes a skewed bimodal sZ h spectrum due to the presence of both pristine ice crystals and dry snow. Next, graupel formation due to riming has been studied, and it is found that the riming process produces sZ h values of about 10 dB and corresponding sZdr values of 1 dB. This positive sZdr indicates the presence of needle/columnar secondary ice particles formed by ice multiplication processes in the riming zones. Last, the temporal evolution of a storm is investigated by analyzing changes in hydrometeor types with time and their influence on the spectral polarimetric variables.

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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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