Potential vorticity structure of embedded convection in a warm conveyor belt and its relevance for large-scale dynamics
Potential vorticity structure of embedded convection in a warm conveyor belt and its relevance for large-scale dynamics
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温暖传送带中嵌入对流的位涡结构及其与大尺度动力学的相关性
DOI：
10.5194/wcd-1-127-2020
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发表时间：
2020
影响因子：
0.7
通讯作者：
H. Wernli
中科院分区：
文献类型：
--
作者：
A. Oertel;M. Boettcher;H. Joos;M. Sprenger;H. Wernli
摘要。暖传送带是温带气旋的重要气流。它们可以通过改变位涡在交叉等熵上升过程中的分布来影响大尺度流动的演变。虽然WCB通常被描述为倾斜上升和产生层状云的气流，但最近的研究发现，对流活动嵌入在大尺度WCB云带中。然而，这种嵌入wcb的对流的影响尚未得到详细的研究。本文系统分析了北大西洋东部西环流中嵌入对流对云和降水结构、PV分布以及大尺度气流的影响。因此，我们将在线轨迹应用于高分辨率对流模拟，并进行复合分析，比较准垂直上升的对流WCB轨迹与典型斜上升的WCB轨迹。我们发现，对流的WCB上升导致地面降水和对流层中高层霰的形成明显增强，而斜向的WCB则没有，这表明WCB内嵌对流对极端降水的关键作用。与斜向WCB轨迹相比，对流WCB轨迹的初始等效位温更高，并且对流WCB轨迹起源于一个更大的位不稳定区域，这使得云的非绝热加热更强烈，交叉等熵上升更强。此外，嵌入对流的特征在PV结构中有明显的印记。绝热产生的低层正PV异常，与气旋环流异常相关，对于对流WCB轨迹来说，明显更强。斜向的WCB轨迹导致对流层上层形成广泛的低PV空气区域（仍然具有微弱的正PV值），这与先前的研究一致。与此相反，对流的WCB轨迹在高层形成中尺度水平PV偶极子，其中一个极达到负PV值。在更大尺度上，这些个别的中尺度PV异常可以聚集成从对流上升气流区延伸出来的细长PV偶极子带，这与相干的大尺度环流异常有关。这种对流产生的PV偶极带的一个示例表明，在大约10小时内，负PV极被平流到更靠近上层波导的地方，在那里它加强了等熵PV梯度，并有助于形成射流条纹。
Abstract. Warm conveyor belts (WCBs) are important airstreams in extratropical cyclones. They can influence large-scale flow evolution by modifying the potential vorticity (PV) distribution during their cross-isentropic ascent. Although WCBs are typically described as slantwise-ascending and stratiform-cloud-producing airstreams, recent studies identified convective activity embedded within the large-scale WCB cloud band. However, the impacts of this WCB-embedded convection have not been investigated in detail. In this study, we systematically analyze the influence of embedded convection in an eastern North Atlantic WCB on the cloud and precipitation structure, on the PV distribution, and on larger-scale flow. For this reason, we apply online trajectories in a high-resolution convection-permitting simulation and perform a composite analysis to compare quasi-vertically ascending convective WCB trajectories with typical slantwise-ascending WCB trajectories. We find that the convective WCB ascent leads to substantially stronger surface precipitation and the formation of graupel in the middle to upper troposphere, which is absent for the slantwise WCB category, indicating the key role of WCB-embedded convection for precipitation extremes. Compared to the slantwise WCB trajectories, the initial equivalent potential temperature of the convective WCB trajectories is higher, and the convective WCB trajectories originate from a region of larger potential instability, which gives rise to more intense cloud diabatic heating and stronger cross-isentropic ascent. Moreover, the signature of embedded convection is distinctly imprinted in the PV structure. The diabatically generated low-level positive PV anomalies, associated with a cyclonic circulation anomaly, are substantially stronger for the convective WCB trajectories. The slantwise WCB trajectories lead to the formation of a widespread region of low-PV air (that still have weakly positive PV values) in the upper troposphere, in agreement with previous studies. In contrast, the convective WCB trajectories form mesoscale horizontal PV dipoles at upper levels, with one pole reaching negative PV values. On a larger scale, these individual mesoscale PV anomalies can aggregate to elongated PV dipole bands extending from the convective updraft region, which are associated with coherent larger-scale circulation anomalies. An illustrative example of such a convectively generated PV dipole band shows that within around 10 h the negative PV pole is advected closer to the upper-level waveguide, where it strengthens the isentropic PV gradient and contributes to the formation of a jet streak.
This suggests that the mesoscale PV anomalies produced by embedded convection upstream organize and persist for several hours and therefore can influence the synoptic-scale circulation. They thus can be dynamically relevant, influence the jet stream and (potentially) the downstream flow evolution, which are highly relevant aspects for medium-range weather forecast. Finally, our results imply that a distinction between slantwise and convective WCB trajectories is meaningful because the convective WCB trajectories are characterized by distinct properties.
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三个模拟温带气旋中温带对流层顶附近非绝热产生的位涡结构
影响因子：
3.2
作者：
Chagnon J
通讯作者：
Chagnon J
影响因子：
8.9
作者：
C. Grams;H. Wernli;M. Böttcher;J. Čampa;U. Corsmeier;S. Jones;J. Keller;C. Lenz;L. Wiegand-
通讯作者：
C. Grams;H. Wernli;M. Böttcher;J. Čampa;U. Corsmeier;S. Jones;J. Keller;C. Lenz;L. Wiegand-
影响因子：
8.9
作者：
J. Chagnon;S. Gray;J. Methven
通讯作者：
J. Chagnon;S. Gray;J. Methven
负位涡的非绝热产生及其对北大西洋急流的影响
影响因子：
8.9
作者：
Harvey B
通讯作者：
Harvey B