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

季风潮是指从热带地区移动过来的水汽流，在北美季风的极地边缘引发雷暴。预测这些浪潮的开始和演变是具有挑战性的。以前对浪涌机制的研究主要集中在热带强迫上。这项工作是基于另一种假设，即破碎的罗斯比波放大了对流层上层的季风高压，并导致温带和热带之间的“屏障”突然向极地移动。这为向极地涌入西南部沙漠的湿气提供了通道。据推测，罗斯比波破裂导致中尺度强迫，以两种方式驱动海湾浪涌。首先，急流山谷加热的快速向极地移动对比得到了对美国西部内陆的动力控制。这为地表附近的热空气进入大盆地创造了有利的环境。地表加热增加了对流层中下部的热对比，并导致热低压的发展。其次，破波将温带气旋空气注入亚热带，导致倒槽的形成。因此，倒转的槽向西绕着高压脊向加利福尼亚湾移动。当这个倒转的低气压槽在高层高压周围以反气旋方式传播时，对流层下层潮湿的空气被迫向极地方向靠近与大盆地上方加热高原相关的地表低气压。这些对波浪破裂的反应汇合在一起，为北美西部中纬度内陆地区打开了热带湿气向极地侵入的大门，从而导致广泛的风暴活动。这些假设将通过观察性诊断研究和模型研究进行检验。建模研究将采用自适应网格数值模型的模拟，该模型适用于捕捉波浪破碎的非线性和随后的中尺度响应。将对北美季风试验（NAME）的个案研究进行一组模拟。敏感性运行将在有无温带动力学的情况下进行，以及有无热带扰动的情况下进行，以检验温带在为海湾风暴潮事件提供有利条件方面的重要性。这项研究更广泛的影响源于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，该项目可以提高对美国西南部季风激增的操作预测。

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.