A Diagnostic Study of Cold-Air Outbreaks over South America
A Diagnostic Study of Cold-Air Outbreaks over South America
复制标题
南美洲冷空气爆发的诊断研究
DOI：
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发表时间：
2000
期刊：
影响因子：
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通讯作者：
Paula K. Vigliarolo
中科院分区：
文献类型：
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作者：
C. Vera;Paula K. Vigliarolo
中纬度扰动，如强烈冷锋和截止低压是南美洲南部严重天气事件的一个非常重要的原因，特别是在南半球的冬季。在冷锋之后，来自高纬度的冷空气被迫向赤道方向流向安第斯山脉以东。偶尔，锋面可能会在热带地区产生寒潮，使巴西南部、东南部和中部的作物种植区出现冰冻温度（葡萄牙语称为GEADAS）。本文记录了南美洲冬季冷涌天气尺度波的结构和动力过程。结果表明，旋转扩展经验正交函数技术是能够提取定义良好的区域模式从850百帕高空风扰动场。在南美洲上空，对流层中、下层的波的表现非常不同：上层波在穿越安第斯山脉时向东北方向传播，其演变方式与罗斯贝波色散的概念一致，而在下层，波往往与山脉的形状一致，与地形罗斯贝波理论一致。然而，有人发现，冷涌似乎并不产生旋转捕获波。在与南美洲外热带地区的冷涌相关的天气尺度模式（非GEADA合成）和与在热带地区产生极端低温的主要冷空气爆发相关的天气尺度模式（GEADA合成）之间发现了显着差异。虽然这两个浪涌的特点是一个长波模式，包括南美洲上空的气旋扰动和南太平洋上空的反气旋扰动，在GEADA合成的早期阶段，确定了两个额外的功能：副极地和副热带短波模式。上层副热带气旋扰动的存在与副热带急流位置更北有关，这将有助于锋面系统向赤道渗透。此外，这种气旋扰动增强了亚热带和热带纬度的上升运动，这与阿根廷中部上空的一个明确的次级环流及其下降分支有关。结果表明，这种配置有利于副极地短波槽的快速和向赤道的渗透，巴西南部的温度下降，和阿根廷中部的反气旋加强。因此，副热带高空特征的存在对南美洲热带地区冷涌的发生起着关键作用。
Midlatitude disturbances such as intense cold fronts and cutoff lows are a very important cause of severeweather events over the southern part of South America, particularly during the austral winter months. Behind cold fronts, cold air from higher latitudes is forced to flow equatorward to the east of the Andes. Occasionally fronts might produce cold surges in tropical latitudes, with freezing temperatures (referred as to GEADAS in Portuguese) over crop-growing areas of southern, southeastern, and central Brazil. The structure and dynamical processes of winter synoptic-scale waves associated with cold surges over South America are documented in this paper. It is shown that the rotated extended empirical orthogonal function technique is capable of extracting well-defined regional patterns from 850-hPa meridional-wind perturbation fields. Over South America, waves behave very differently in the lower and middle troposphere: while upperlevel waves propagate northeastward as they cross the Andes, evolving in a manner consistent with the concept of Rossby wave dispersion, at lower levels, waves tend to conform to the shape of the mountain range, in agreement with the theory of topographic Rossby waves. However, it was found that cold surges do not seem to result from the generation of rotationally trapped waves. Significant differences have been found between synoptic-scale patterns associated with cold surges over extratropical South America (NO GEADA composites) and synoptic-scale patterns associated with major coldair outbreaks that produce extreme low temperatures at tropical regions (GEADA composites). While both surges are characterized by a long-wave pattern consisting of a cyclonic perturbation over South America and anticyclonic perturbation behind over the southern Pacific Ocean, at the early stages of GEADA composites, two additional features were identified: a subpolar and subtropical short-wave pattern. The presence of the upperlevel subtropical cyclonic perturbation is associated with the subtropical jetstream location farther north, which would facilitate the equatorward penetration of frontal systems. In addition, this cyclonic perturbation enhances rising motion at subtropical and tropical latitudes that is associated with a well-defined secondary circulation with its descending branch over central Argentina. It is shown that this configuration favors the rapid and equatorward penetration of the subpolar short-wave trough, the temperature decrease over southern Brazil, and the anticyclone intensification over central Argentina. Thus the presence of the subtropical upper-level feature plays a key role on the cold-surge occurrence over tropical regions of South America.