Interactions between Water Vapor, Potential Vorticity, and Vertical Wind Shear in Quasi-Geostrophic Motions: Implications for Rotational Tropical Motion Systems
Interactions between Water Vapor, Potential Vorticity, and Vertical Wind Shear in Quasi-Geostrophic Motions: Implications for Rotational Tropical Motion Systems
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DOI：
10.1175/jas-d-20-0205.1
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发表时间：
2021-03
影响因子：
3.1
通讯作者：
Á. Adames
中科院分区：
文献类型：
--
作者：
Á. Adames
本文用一个线性两层模式研究了在平均热成风存在下，预报水汽对准地转（QG）运动的作用。基本方程的解揭示了两个不稳定性，可以解释湿QG系统的增长。斜压不稳定的特征是在天气尺度（水平尺度约为1000 km）的增长和系统，从这种不稳定增长的倾斜对剪切。湿涡不稳定性--一种当水汽和低对流层涡度呈现同相分量时发生的不稳定性--只存在于水汽是预报性的时候。不稳定性在天气尺度上也是最强的，但从它发展出来的系统表现出垂直堆叠的结构。当水汽为预报性偏东时，斜压不稳定性明显减弱，而水汽涡不稳定性增强。对流层低层气流的斜压（）分量使湿涡不稳定性增强，而斜压不稳定性减弱。在向西传播的系统中，低对流层西风带与东风平流异常水分和相关的对流朝低层涡旋。平流对流导致波的垂直结构从有利于斜压不稳定的结构转变为有利于湿涡不稳定的结构。另一方面，西风加强了斜压不稳定发生所必需的水分和涡度之间的相位。根据这些结果，我们推测，在东风湿润地区，如南亚和西非季风，湿涡不稳定是一个重要的不稳定。
A linear two-layer model is used to elucidate the role of prognostic moisture on quasigeostrophic (QG) motions in the presence of a mean thermal wind (). Solutions to the basic equations reveal two instabilities that can explain the growth of moist QG systems. The well-documented baroclinic instability is characterized by growth at the synoptic scale (horizontal scale of ~1000 km) and systems that grow from this instability tilt against the shear. Moisture–vortex instability—an instability that occurs when moisture and lower-tropospheric vorticity exhibit an in-phase component—exists only when moisture is prognostic. The instability is also strongest at the synoptic scale, but systems that grow from it exhibit a vertically stacked structure. When moisture is prognostic and is easterly, baroclinic instability exhibits a pronounced weakening while moisture vortex instability is amplified. The strengthening of moisture–vortex instability at the expense of baroclinic instability is due to the baroclinic () component of the lower-tropospheric flow. In westward-propagating systems, lower-tropospheric westerlies associated with an easterly advect anomalous moisture and the associated convection toward the low-level vortex. The advected convection causes the vertical structure of the wave to shift away from one that favors baroclinic instability to one that favors moisture–vortex instability. On the other hand, a westerly reinforces the phasing between moisture and vorticity necessary for baroclinic instability to occur. Based on these results, it is hypothesized that moisture–vortex instability is an important instability in humid regions of easterly such as the South Asian and West African monsoons.