Abstract Tornadoes were reported in south-central Chile on 30 and 31 May 2019. To better understand the mechanisms that supported the organization and severity of the storms that generated them, a series of high-resolution sensitivity simulations were conducted using the Weather Research and Forecast (WRF) model. In addition to a control simulation, three simulations were run that successively reduced the topography to 50%, 25%, and 0% of original height; three more simulations were run that increased SST by 2 K everywhere, decreased it by 2 K everywhere, and set all SSTs greater than 16 °C to exactly 16 °C; finally, two more simulations were run to explore the combined effects of SST changes and lower topography. Results indicate that as topography was lowered, a strong northerly low-level jet along the upslope of the Andes weakened and broadened in both vertical and horizontal extent. Values of 0–6-km bulk shear and 0–3-km storm-relative helicity (SRH) over the regions where the tornadoes occurred diminished with successive reduction of the topography. As a result, simulated brightness temperatures were warmer, and swaths of updraft helicity were fewer and shorter. These results indicate that on 30–31 May 2019, flow blocking by the Andes topography generated mesoscale wind shear conditions that favored tornadoes. When SSTs were increased, instability (quantified by convective available potential energy, CAPE) also increased, although primarily offshore, and updraft helicity swath length increased, indicating a positive impact on simulated storm intensity. When SSTs were decreased, CAPE also decreased along with a decline in magnitude of updraft helicity. These results suggest that SSTs also impact storm severity via an influence on atmospheric instability. When the two factors were combined, warmer SSTs were not able to overcome a reduction in wind shear associated with a decrease in the topography. This suggests that in south-central Chile, wind shear is more important than CAPE for tornadic thunderstorms, as has been found for other regions around the world).

中文翻译：

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2019 年 5 月 30 日至 31 日在智利中南部的龙卷风：对地形和海温的敏感性

摘要 2019 年 5 月 30 日至 31 日，智利中南部报告了龙卷风。为了更好地了解支持产生龙卷风的风暴的组织和严重程度的机制，使用 Wea​​ther Research and Forecast 进行了一系列高分辨率敏感性模拟(WRF) 模型。除了控制模拟之外，还运行了三个模拟，依次将地形降低到原始高度的 50%、25% 和 0%；又进行了三个模拟，将各处的 SST 增加 2 K，各处减少 2 K，并将所有大于 16 °C 的 SST 设置为恰好 16 °C；最后，还运行了两个模拟来探索 SST 变化和较低地形的综合影响。结果表明，随着地形的降低，沿着安第斯山脉上坡的一股强大的偏北低空急流在垂直和水平范围内减弱和扩大。龙卷风发生地区的 0-6 公里体积剪切和 0-3 公里风暴相对螺旋度 (SRH) 的值随着地形的连续减少而减少。结果，模拟的亮度温度更高，上升气流的螺旋度越来越小。这些结果表明，2019 年 5 月 30 日至 31 日，安第斯山脉地形造成的气流阻塞产生了有利于龙卷风的中尺度风切变条件。当 SST 增加时，不稳定性（通过可用对流势能 CAPE 量化）也增加，尽管主要是离岸，并且上升气流螺旋带长度增加，表明对模拟风暴强度产生积极影响。当 SST 降低时，CAPE 也随着上升气流螺旋度的下降而下降。这些结果表明 SST 还通过对大气不稳定性的影响来影响风暴的严重程度。当这两个因素结合在一起时，较暖的 SST 无法克服与地形减少相关的风切变减少。这表明在智利中南部，对于龙卷风雷暴，风切变比 CAPE 更重要，正如世界其他地区所发现的那样）。