Extratropical Control of Gulf Surges: The Role of Rossby Wave Breaking and Associated Mesoscale Processes

季风潮是来自热带的水汽羽流，在北美季风的两极边缘引发雷暴。预测这些激增的开始和演变具有挑战性。以往对风暴潮机制的研究主要集中在热带强迫。这项工作是基于另一种假设，打破Rossby波放大季风对流层上层高，并导致突然极移的“障碍”之间的热带和热带。这就为向极地输送的水汽提供了一条通道，使其进入西南部的沙漠。据推测，罗斯贝波破碎导致中尺度强迫，驱动海湾涌在两个方面。首先，急流山谷加热的快速向极转移与美国西部内陆的动态控制形成对比。这创造了一个有利的环境，使地表附近的暖空气流入大盆地。地面加热增加了对流层中低层热力对比，导致热力低压的发展。其次，波浪破碎将热带气旋外的空气丝注入副热带，导致倒槽的形成。因此，反向低压槽向西围绕高压脊朝加州湾移动。当这个倒槽围绕上层高压反气旋性地传播时，对流层下层潮湿的空气被迫向极地靠近与大盆地上空加热高原相关的地面低气压。这些对波浪破碎的反应的汇合使北美西部的中纬度内陆向极地侵入热带湿气，导致广泛的风暴活动。这些假设将通过一项观察性诊断研究和一项建模研究进行检验。建模研究将采用自适应网格数值模型的模拟，该模型是一个适合于捕获波浪破碎和随后的中尺度响应的非线性的框架。将对北美季风实验（NAME）的案例研究进行一系列模拟。将在有和没有热带外动力学以及有和没有热带扰动的情况下进行敏感性试验，以检查热带外动力学在提供有利于海湾涌浪事件的条件方面的重要性。该研究的更广泛影响来自北美季风环流对水资源，火灾天气频率，能源可用性和土地利用的重要性。通过利用中纬度气流的可预测性，该项目可以改进对美国西南部季风潮的业务预测。

Monsoonal surges are plumes of moisture that move from the tropics and trigger thunderstorms over the poleward fringes of the North American monsoon. Predicting the onset and evolution of these surges is challenging. Previous research on the mechanisms of surges has focused on the tropical forcing. This work is based on an alternative hypothesize that breaking Rossby waves amplify the monsoonal upper-tropospheric high and cause an abrupt poleward shift in the "barrier" between the extratropics and tropics. This provides a conduit for poleward surges of moisture into the desert southwest. It is hypothesized that Rossby wave breaking leads to mesoscale forcing that drives Gulf surges in two ways. First, the rapid poleward shift in the jet mountain-valley heating contrasts to gain dynamical control over the interior western United States. This creates a favorable environment to flux warm air near the surface into the Great Basin. Surface heating increases the lower-middle tropospheric thermal contrasts and leads to the development of a thermal low. Secondly, wave breaking injects filaments of extratropical cyclonic air into the subtropics leading to the formation of an inverted trough. The inverted trough is consequently wrapped westward around the ridge toward the Gulf of California. As this inverted trough propagates anticyclonically around the upper-level high, moist lower tropospheric air is forced poleward toward the surface low associated with the heated plateaus over the Great Basin. The confluence of these responses to wave breaking opens the midlatitude interior western North America to poleward intrusions of tropical moisture that cause widespread storm activity. These hypotheses will be tested through an observational diagnostic study and a modeling study. The modeling study will employ simulations from an adaptive grid numerical model, a framework suited to capture the nonlinearities of wave breaking and the ensuing mesoscale response. A set of simulations will be performed for case studies from the North American Monsoon Experiment (NAME). Sensitivity runs will be conducted with and without extratropical dynamics, as well as with and without tropical disturbances to examine the importance of the extratropics in providing conditions favorable to Gulf surge events. Broader impacts of the study derive from the importance of the North American monsoon circulation for water resources, fire weather frequency, energy availability, and land use. By making use of the predictable nature of the midlatitude flow, this project could lead to improved operational predictions of monsoonal surges in the southwestern United States.