A Modeling Study of Easterly Waves and Their Intraseasonal Variability in the East Pacific

东风波(EWS)是一种波状大气扰动，从脊到谷的大小约为1000公里，在低纬地区从东向西传播。EWS在大部分热带地区是重要的天气预报员，在飓风的发展过程中发挥着关键作用。许多研究都集中在非洲的EWS上，它们沿着萨赫勒上空的东风急流，在北大西洋向西传播。这些波动的成因通常用东风急流特有的急流不稳定来解释，但在热带东太平洋也发现了类似的EWS，那里的大尺度气流特征截然不同。对赤道东太平洋出现EWS的一种解释是，它们起源于非洲EWS，通过与非洲东风急流有关的过程形成，随后跨越大西洋和加勒比传播到东太平洋。虽然有一些证据支持这一理论，但PI以前的工作(见AGS-1347738)表明，尽管热带东太平洋与萨赫勒地区有明显的差异，但东太平洋可以形成环流。该项目检验了东太平洋环流主要是由中美洲山脉顺风向地形产生的扰动的高尺度增长引起的假说。PI的数值模拟表明，当中美洲地形被夷为平地时，EW活动显著减少，但这些模拟是粗略的，因为地形的移除对该地区的气象产生了各种戏剧性的影响，包括平均气流、云量和降水。尽管地形和其他因素发生了变化，但这里要进行的模拟将使用一种“轻推”技术来限制背景流，以便能够在现实环境中对特定强迫机制产生的波浪进行更有针对性的研究。这项工作将在允许对流的配置中使用天气研究和预报(WRF)模型。这项工作还将解决背景流季节内振荡对EWS的调制问题。其他工作使用涡度和能量收支等诊断技术来研究观测数据中的环流产生和传播。由于环流在热带气象学和飓风发生中的关键作用，这项工作具有社会意义。如果在东太平洋发现的EWS起源于非洲东风急流，它们可能比由中美洲地形在当地产生的EWS更具可预测性，因此这项工作的结果对业务预报具有潜在的影响。考虑到季节内振荡的超前时间较长，季节内振荡对电子战活动的调制也具有实际意义。此外，该项目还为研究生提供支持和培训，从而为这一研究领域的未来劳动力提供支持。

Easterly waves (EWs) are wavelike atmospheric disturbances with a ridge-to-trough size of roughly 1,000km which propagate from east to west at low latitudes. EWs are important weather makers in much of the tropics and play a critical role in hurricane development. Much of the research has focused on African EWs, which along the easterly jet over the Sahel and propagate westward over the North Atlantic. The genesis of these waves is often explained in terms of jet instabilities particular to the easterly jet, yet similar EWs are found in the tropical eastern Pacific where the large-scale flow characteristics are very different. One explanation for the presence of EWs in the eastern equatorial Pacific is that they originated as African EWs, formed through processes associated with the African easterly jet, and subsequently propagated across the Atlantic and the Caribbean to reach the eastern Pacific. While there is some evidence to support this theory, previous work by the PI (see AGS-1347738) suggests that EWs can form in the tropical eastern Pacific despite the clear differences between this region and the Sahel.This project tests the hypothesis that east Pacific EWs are largely caused by upscale growth of topographically-generated disturbances downwind of the mountain ranges of Central America. Numerical simulation by the PI have shown that EW activity is dramatically reduced when the Central American topography is flattened, but these simulations were crude as the removal of topography has a variety of dramatic effects on the meteorology of the region including mean flow, cloudiness and precipitation. Simulations to be performed here would use a "nudging" technique to constrain the background flow despite modifications to topography and other factors so that a more focused study of wave generation by specific forcing mechanisms can be performed in a realistic setting. The work would use the Weather Research and Forecasting (WRF) model in a convection-permitting configuration. The work would also address the modulation of EWs by intraseasonal oscillations of the background flow. Additional work uses diagnostic techniques including vorticity and energy budgets to study EW generation and propagation in observational data.The work has societal relevance due to the key role of EWs in tropical meteorology and hurricane genesis. If EWs found in the eastern Pacific originated in the African easterly jet they may be more predictable than if they are generated locally by Central American topography, thus the results of this work have potential implications for operational forecasting. The modulation of EW activity by intraseasonal oscillations is also of practical interest given the long lead times of intraseasonal oscillations. In addition, the project provides support and training for a graduate student, thereby providing for the future workforce in this research area.