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位涡源汇和位涡环流及其天气气候意义

Potential Vorticity Source, Potential Vorticity Circulation, and Their Weather and Climate Significances

  • 摘要: 在扼要回顾地表位涡研究进展的基础上,本文介绍了复杂地形下的位涡及位涡制造的计算及近年来关于位涡源汇和位涡环流(PVC)的研究进展,侧重介绍青藏高原表层位涡的特殊性及其对天气气候的重要影响。阐明对于绝热和无摩擦大气运动,由于位涡本身的结构重组(位涡重构)可以引起垂直涡度的发展,在夏季可以激发高原涡形成,冬季使青藏高原东部成为重要的表面涡源。基于导得的包括非绝热加热作用的、与等熵面的位移相联系的垂直运动(\omega _\rmI\textD)方程,进一步阐明青藏高原制造的正位涡沿西风气流东传会引起下游地区低空气旋性涡度、偏南风、和上升运动发展,导致位涡平流随高度增加,激发极端天气气候事件发生发展。指出青藏高原地表加热和云底的潜热释放的日变化显著地影响着地表层位涡的日变化,导致青藏高原的低涡降水系统多在午后至夜间发生发展。证明与传统的青藏高原感热加热指数相比,青藏高原地表层位涡指数能够更好地刻画关于降水的季节变化,与亚洲夏季风降水相关更密切。  本文还简单介绍了PVC的概念。指出由于区域边界面的PVC的辐散辐合的变化直接与区域位涡的变化相关联,为保持北半球位涡总量的相对稳定,跨赤道面上的PVC变化与地表PVC的变化必须相互补充,因此跨赤道面上的位涡环流的变化可以成为监测近地表气候变化的窗口。近赤道的海气相互作用能够直接造成沿赤道垂直面上的纬向风垂直切变的变化,激发跨赤道位涡环流异常,从而通过大气内部PVC的变化和青藏高原的调控影响北半球近地表的气候变化。结果表明位涡环流分析为建立热带和热带外大气环流变化的联系开辟了新的蹊径,有着广阔的应用前景。

     

    Abstract: Based on a brief review of the research progress on surface potential vorticity (PV), this study introduces the calculation of PV and its generation on complex terrain and the research progress on the source of PV and PV circulation (PVC) in recent years, focusing on the particularity of the surface PV on the Tibetan Plateau (TP) and its important influence on weather and climate. For adiabatic and frictionless atmospheric motion, the structural recombination of the PV (i.e., PV reconstruction) can cause the development of vertical vorticity, which can cause the formation of a plateau vortex in summer and make the eastern part of the plateau an important source of surface vorticity in winter. Based on the derived equation for the vertical motion associated with isentropic displacement (\omega _\rmI\textD), which includes the impact of diabatic heating, this study shows that the eastward propagation of the positive vorticity generated on the TP along the westerly wind will cause the development of cyclonic vorticity in the downstream area, southerly wind, and upward motion in the lower troposphere, resulting in the increase in PV advection with altitude, which stimulates the development of extreme weather and climate events. Notably, the diurnal variations of surface heating and latent heat release at the cloud bottom over the TP significantly affect the diurnal variation of the PV near the surface, resulting in the development of the low vortex and precipitation system over the TP from late afternoon to night. Compared with the traditional surface sensible heating index, the surface PV index of the TP can better characterize the seasonal changes of local precipitation and is more closely related to the Asian summer monsoon precipitation. The concept of PVC is also briefly introduced. Because changes in convergence of PVC across the close boundary of a region are directly related to changes in PV of the region, to maintain the relative stability of the total PV in the Northern Hemisphere, changes in PVC on the trans-equatorial plane and those in surface PVC must complement each other. Thus, changes in PVC on the trans-equatorial plane can be considered a window for monitoring near-surface climate change. The near-equatorial air–sea interaction can directly cause changes in the vertical shear of the zonal wind on the vertical plane along the equator, stimulating the trans-equatorial PVC anomaly, thereby affecting climate change near the surface of the Northern Hemisphere through the change of PVC in the atmosphere and the regulation of the TP. This study shows that PVC analysis opens up a new way for establishing the link between tropical and extratropical atmospheric circulation changes and has broad application prospects.

     

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