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The South American low‐level jet (SALLJ) is a narrow northerly wind speed maximum present just above the boundary layer. It is an important component of the tropical–extratropical heat and moisture exchange in South America and can favour deep moist convection in southeastern South America. The main objective of this study is to analyse the SALLJ characteristics at 21 upper‐air stations deployed between the Tropics and the subtropics from just east of the Andes to the eastern plains during the SALLJ Experiment (SALLJEX). The greatest wind speed occurs between 300 and 2,000 m AGL between 0300 and 1200 UTC, mainly in connection with greater northerly winds during an anticlockwise rotation of the wind from sunset to sunrise, thus suggesting the important role of the inertial oscillation in the wind's diurnal cycle. The spatial variation of the LLJ throughout the SALLJEX network shows a weakening of the maximum wind speed from stations near the Andes towards the Plains suggesting the presence of the LLJ core just east of the Andes around Santa Cruz de la Sierra's latitude (17°48′S). Weak, moderate, and strong SALLJ categories defined from a local maximum northerly wind speed threshold at each station are defined to analyse the relationship between the SALLJ intensity and the thermodynamic properties of the lower layers of the atmosphere. Strong SALLJs are frequently observed at night‐time, while weak SALLJs are likely to occur at any time of the day. Strong cases have deeper and less stable nocturnal boundary layers, which could be due to the SALLJ warm advection near the time of wind speed maximum (0600 UTC). Deeper convective boundary layers and higher low‐level temperatures observed at 1800 UTC prior to strong nocturnal SALLJs can potentially lead to larger amplitudes of inertial oscillation and contribute to generating stronger SALLJs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Six different satellite rainfall estimates are evaluated for a 24-hour accumulation period at 12 UTC with a 0.25 degree resolution. The rain gauge data are obtained from a dense inter-institutional station network for December 1, 2008 to November 30, 2010 over South America. The evaluated satellite rainfall products are the Tropical Rainfall Measuring Mission 3B42 V6, V7 and RT, the NOAA/Climate Prediction Center Morphing technique (CMORPH), Hydroestimator (HYDRO) and the Combined Scheme algorithm (CoSch). The validation and intercomparison of these products are focused on southern South America. The performance improves in the “blended” estimates by including microwave observations and surface observations in the adjustments, i.e., 3B42 V6, V7 and CoSch; however, large overestimations are detectable in CMORPH, principally for extreme values over plains areas. The estimates based on parameters associated with infrared images only (HYDRO) underestimate precipitation south of 20° S and tend to overestimate the warm precipitation to the north. The inclusion of observed precipitation data is convenient from monthly (3B42 V7 and V6) to daily scales (CoSch) and improves the estimates. The estimates that include microwave observations show a strong tendency to overestimate extreme values of precipitation over 70 mm. This effect is strongly evident in northern and central Argentina and southern Brazil. A deeper assessment is necessary, particularly over the Central Andes, where effects of topography principally associated with solid precipitation correspond to the persistence of majorly overestimated precipitation. [ABSTRACT FROM AUTHOR]

Thunderstorms in southeastern South America (SESA) stand out in satellite observations as being among the strongest on Earth in terms of satellite-based convective proxies, such as lightning flash rate per storm, the prevalence for extremely tall, wide convective cores and broad stratiform regions. Accurately quantifying when and where strong convection is initiated presents great interest in operational forecasting and convective system process studies due to the relationship between convective storms and severe weather phenomena. This paper generates a novel methodology to determine convective initiation (CI) signatures associated with extreme convective systems, including extreme events. Based on the well-established area-overlapping technique, an adaptive brightness temperature threshold for identification and backward tracking with infrared data is introduced in order to better identify areas of deep convection associated with and embedded within larger cloud clusters. This is particularly important over SESA because ground-based weather radar observations are currently limited to particular areas. Extreme rain precipitation features (ERPFs) from Tropical Rainfall Measurement Mission are examined to quantify the full satellite-observed life cycle of extreme convective events, although this technique allows examination of other intense convection proxies such as the identification of overshooting tops. CI annual and diurnal cycles are analyzed and distinctive behaviors are observed for different regions over SESA. It is found that near principal mountain barriers, a bimodal diurnal CI distribution is observed denoting the existence of multiple CI triggers, while convective initiation over flat terrain has a maximum frequency in the afternoon. [ABSTRACT FROM AUTHOR]

This paper describes the lessons learned from the implementation of a regional ensemble data assimilation and forecast system during the intensive observing period of the Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations (RELAMPAGO) field campaign (central Argentina, November–December 2018). This system is based on the coupling of the Weather Research and Forecasting (WRF) model and the Local Ensemble Transform Kalman Filter (LETKF). It combines multiple data sources both global and locally available like high-resolution surface networks, AMDAR data from local aircraft flights, soundings, AIRS retrievals, high-resolution GOES-16 wind estimates, and local radar data. Hourly analyses with grid spacing of 10 km are generated along with warm-start 36-h ensemble-forecasts, which are initialized from the rapid refresh analyses every three hours. A preliminary evaluation shows that a forecast error reduction is achieved due to the assimilated observations. However, cold-start forecasts initialized from the Global Forecasting System Analysis slightly outperform the ones initialized from the regional assimilation system discussed in this paper. The system uses a multi-physics approach, focused on the use of different cumulus and planetary boundary layer schemes allowing us to conduct an evaluation of different model configurations over central Argentina. We found that the best combinations for forecasting surface variables differ from the best ones for forecasting precipitation, and that differences among the schemes tend to dominate the forecast ensemble spread for variables like precipitation. Lessons learned from this experimental system are part of the legacy of the RELAMPAGO field campaign for the development of advanced operational data assimilation systems in South America. • A LETKF-WRF system was run successfully in real-time to support RELAMPAGO operations. • A reduction in forecast error was shown due to data assimilation cycles. • 60-member RRR analyses and forecasts are available for the research community. [ABSTRACT FROM AUTHOR]