The Influence of Shear on Deep Convection Initiation. Part I: Theory
The Influence of Shear on Deep Convection Initiation. Part I: Theory
复制标题
切变对深层对流引发的影响。
DOI：
10.1175/jas-d-21-0145.1
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发表时间：
2022
影响因子：
3.1
通讯作者：
Nowotarski, Christopher J.
中科院分区：
文献类型：
--
作者：
Peters, John M.;Morrison, Hugh;Nelson, T. Connor;Marquis, James N.;Mulholland, Jake P.;Nowotarski, Christopher J.
本文介绍了一种新的假设，垂直风切变（“剪切”）的作用，深对流的启动（DCI）。在这一假设中，最初的潮湿上升气流，超过了宽度和剪切阈值将“根”在一个逐渐加深的转向电流随着时间的推移，增加其低层云的相对流量和流入量，扩大，并随后降低其敏感性，蒸发驱动的稀释，朝着准稳定的自我维持的状态。相比之下，初始上升气流，不超过上述阈值的经验抑制增长的剪切引起的向下压力梯度加速，将不会扎根于一个足够深的转向电流，以增加其流入，将缩小随着时间的推移，并会屈服于膨胀驱动的稀释。在后一种情况下，需要一个外部驱动的提升机制来维持深对流，如果没有这样的提升机制，深对流将不会持续。从运动方程出发，建立了一个理论模型，进一步探讨了这一假设。该模型表明，剪切一般抑制DCI，提高初始的子云上升气流的宽度，这是必要的。然而，有一个明显的分歧，在上升气流增长的模式后，对流的发病。足够宽的初始上升气流增长并最终达到稳定状态。相比之下，初始上升气流宽度不够，会随着时间的推移而收缩，最终在没有外部支持的情况下完全衰减。一个尖锐的初始上升气流半径阈值区分这两种结果。因此，与我们的假设和观察结果一致，剪切抑制DCI在某些情况下，但在其他情况下，促进它。
This article introduces a novel hypothesis for the role of vertical wind shear (“shear”) in deep convection initiation (DCI). In this hypothesis, initial moist updrafts that exceed a width and shear threshold will “root” within a progressively deeper steering current with time, increase their low-level cloud-relative flow and inflow, widen, and subsequently reduce their susceptibility to entrainment-driven dilution, evolving toward a quasi-steady self-sustaining state. In contrast, initial updrafts that do not exceed the aforementioned thresholds experience suppressed growth by shear-induced downward pressure gradient accelerations, will not root in a deep-enough steering current to increase their inflow, will narrow with time, and will succumb to entrainment-driven dilution. In the latter case, an externally driven lifting mechanism is required to sustain deep convection, and deep convection will not persist in the absence of such lifting mechanism. A theoretical model is developed from the equations of motion to further explore this hypothesis. The model indicates that shear generally suppresses DCI, raising the initial subcloud updraft width that is necessary for it to occur. However, there is a pronounced bifurcation in updraft growth in the model after the onset of convection. Sufficiently wide initial updrafts grow and eventually achieve a steady state. In contrast, insufficiently wide initial updrafts shrink with time and eventually decay completely without external support. A sharp initial updraft radius threshold discriminates between these two outcomes. Thus, consistent with our hypothesis and observations, shear inhibits DCI in some situations, but facilitates it in others.
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