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.

中文翻译：

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SALLJEX期间南美低空急流的观测研究

南美低空急流 (SALLJ) 是出现在边界层上方的窄偏北风速最大值。它是南美洲热带-温带热湿交换的重要组成部分，有利于南美洲东南部的深层湿对流。本研究的主要目的是分析 SALLJ 实验 (SALLJEX) 期间部署在热带和亚热带之间从安第斯山脉以东到东部平原的 21 个高空站的 SALLJ 特征。最大风速出现在 300 和 2,000 m AGL 之间的 0300 和 1200 UTC 之间，主要与风从日落到日出逆时针旋转期间更大的北风有关，从而表明惯性振荡在风的昼夜循环中的重要作用. 整个 SALLJEX 网络中 LLJ 的空间变化表明，从安第斯山脉附近的站点到平原的最大风速减弱表明 LLJ 核心存在于安第斯山脉以东的圣克鲁斯德拉谢拉纬度附近 (17°48′) S). 定义了根据每个站点的局部最大北风速阈值定义的弱、中和强 SALLJ 类别，以分析 SALLJ 强度与大气下层热力学特性之间的关系。夜间经常观察到强 SALLJ，而一天中的任何时间都可能发生弱 SALLJ。强案例具有更深且更不稳定的夜间边界层，这可能是由于风速最大值（0600 UTC）附近的 SALLJ 暖平流。