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

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. Significance Statement Unprecedented balloon-borne measurements of the atmosphere (known as soundings) were collected in Argentina in 2018-19, during the RELAMPAGO-CACTI field projects. These measurements allowed us to characterize the conditions supporting some of the most intense thunderstorms on Earth, which are known to occur in this region. The ingredients needed for severe thunderstorms-moisture, instability, lift, and vertical wind shear-were present in several multiday episodes. The sounding data revealed that conditions were often favorable for rotating storms and large hail, but not for tornadoes. Measurements taken from mobile platforms also revealed detailed pictures of how storms influence their surroundings. This dataset will enable comparisons between storm environments in South America and other regions with intense thunderstorms that have not previously been possible.