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，在北美季风的极地边缘引发雷暴。预测这些激增的开始和演变是具有挑战性的。以前对风暴潮机制的研究主要集中在热带强迫上。这项工作基于另一种假设，即破裂的Rossby波放大了季风上对流层高压，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一条管道。假设Rossby波破裂导致中尺度强迫，从而以两种方式驱动墨西哥湾涌流。首先，喷气式山谷供暖的快速极地移动与之形成对比，以获得对美国西部内陆的动态控制。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下部的温度反差，并导致了温度低的发展。其次，波浪破碎将热带外气旋空气的细丝注入到副热带地区，导致倒槽的形成。因此，倒置的海槽向西绕着海脊向加利福尼亚州海湾卷曲。当这个倒槽以反气旋的方式围绕高层高压传播时，潮湿的对流层下层空气被迫向与大盆地上空加热的高原相关的地面低层极地移动。这些对波浪破碎的反应汇合在一起，使北美西部中纬度内陆地区受到热带湿气的侵袭，从而导致大范围的风暴活动。这些假设将通过观察性诊断研究和模型研究进行验证。建模研究将使用自适应网格数值模式的模拟，该模式是一个适合捕捉波浪破裂的非线性和随之而来的中尺度响应的框架。将对北美季风实验(NAME)的案例研究进行一系列模拟。将在有和没有温带动力以及有和没有热带扰动的情况下进行敏感性试验，以检查温带外在为墨西哥湾涌浪事件提供有利条件方面的重要性。这项研究的更广泛影响源于北美季风环流对水资源、火灾天气频率、能源供应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报。

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.