Origin, Storm Track Dynamics and Convective Feedback of African Easterly Waves

非洲东风波（AEW）是非洲东风急流的南北波动，波长为2，000至2，500公里，形成于苏丹东部，并向西传播穿过萨赫勒。 它们是萨赫勒地区夏季大规模天气变化的主要形式，在为这一干旱易发地区带来季风降雨方面发挥着关键作用。 当AEW在大西洋上空继续向西（或向东）传播时，它们可以发展成飓风，据估计，85%的大型大西洋飓风（包括飓风桑迪）都是由AEW形成的。 然而，现有的AEW起源和演变理论不能很好地解释它们的许多观测特征，本项目研究了AEW的几个方面，包括AEW的起源和演变、AEW风暴道的结构（AEW发展和传播的典型路径沿着）、AEW活动的不稳定性以及AEW与湿对流的相互作用。该研究的一个关键组成部分是由主要研究者（PI）在以前的工作中开发的，其思想是将AEW组织成波包，其中可以根据经验确定的群速度跟踪能量的分散。动力学在某种程度上相当于在天气系统中观察到的“下游发展”，这些天气系统沿着中纬度风暴路径移动，除了在AEW的情况下，能量传播可以是上游或下游（由于AEW沿着非洲东风急流形成，上游方向是向东）。 上游能量传播可能会产生一些重要的后果，包括来自风暴道西端成熟AEW的能量可能传播到风暴道东端AEW生成区域，从而促进新AEW的生成。在某种程度上，上游和下游传播的结果可以解释为有限纬向范围的不稳定射流的对流不稳定性和绝对不稳定性之间的差异。在对流不稳定性中，在射流不稳定性上增长的波包具有下游能量传播，其将波包移出不稳定区域。在绝对不稳定性中，波包尾端的群速度是上游的，因此一些波包能量保留在不稳定区域并促进进一步增长。因此，空中预警活动的不稳定性可能是由绝对不稳定性向对流不稳定性的转变引起的，因为在后一种情况下，能量不会向上游传播到空中预警发生区。该项目的主要研究工具是天气研究和预报模型，这是一个应用于赤道以北非洲和大西洋邻近地区的区域大气模型。 模型模拟是使用标准的全物理模型和一个中间复杂性的版本，其中外部热力强迫项被用来产生一个理想化的版本的非洲东风急流和空中预警风暴轨迹。除了旨在研究上游和下游能量弥散作用的模拟外，还计划进行额外的数值试验，以研究在空中预警风暴道中经常观察到的双重结构，其中风暴道的南支和北方支分别出现在自由大气（约650毫巴）和边界层（925毫巴）中。耦合的AEW对流也进行了研究，使用全物理版本的模型。这些模拟的结果将与非洲季风多学科分析（AMMA）现场活动中收集的现场活动数据进行比较。由于AEW和大西洋飓风（包括登陆美国的飓风）之间的联系，这项工作具有更广泛的影响。 更好地了解AEW的基本动力学可能对预测单个AEW和AEW活动增强或减少的时期都有价值。 此外，这项工作将支持和培训两名研究生，从而为这一研究领域的未来劳动力提供支持。

African Easterly Waves (AEWs) are north-south undulations of the African Easterly jet with wavelengths of 2,000 to 2,500 kilometers that form as far east as Sudan and propagate westward across the Sahel. They are the principle form of large-scale weather variability over the Sahel in summer and play a key role in bringing monsoon rains to this drought-prone region. When AEWs continue their westward (or easterly) propagation over the Atlantic they can develop into hurricanes, and it is estimated that 85% of major Atlantic hurricanes (including hurricane Sandy) form from AEWs. However, existing theories for the origin and evolution of AEWs do not satisfactorily account for many of their observed characteristics.This project investigates several aspects of AEWs, including their origin and evolution, the structure of the AEW stormtrack (the typical pathway along which AEWs develop and propagate), intermittency in AEW activity, and interactions between AEWs and moist convection. A key component of the research, developed in previous work by the Principal Investigator (PI), is the idea that AEWs are organized into wave packets in which the dispersion of energy can be tracked according to an empirically determined group velocity. The dynamics are somewhat equivalent to the "downstream development" observed in synoptic weather systems moving along the midlatitude stormtracks, except that in the case of AEWs the energy propagation can be upstream or downstream (as AEWs form along the African Easterly Jet, the upstream direction is eastward). Upstream energy propagation could have a number of significant consequences, including the possibility that energy from mature AEWs on the western end of the stormtrack could propagate to the region of AEW genesis on the eastern end of the stormtrack, thereby promoting the genesis of new AEWs. To some extent, the consequences of upstream and downstream propagation can be interpreted as the difference between convective and absolute instabilities of a unstable jet of limited zonal extent. In convective instability, wave packets growing on the instability of the jet have downstream energy propagation which moves the packet out of the unstable region. In absolute instability, the group velocity at the trailing end of the wave packet is upstream, so that some of the wave packet energy remains in the unstable region and promotes further growth. Thus, intermittency in AEW activity could be caused by a transition from absolute to convective instability, as in the latter case the energy does not propagate upstream into the AEW genesis region. The primary tool for research in this project is the Weather Research and Forecasting (WRF) model, a regional atmosphere model applied over Africa north of the equator and the adjacent portion of the Atlantic. Model simulations are performed using both the standard full-physics version of the model and an intermediate complexity version in which external thermal forcing terms are used to produce an idealized version of the African Easterly Jet and AEW stormtrack. In addition to simulations designed to examine the role of upstream and downstream energy dispersion, additional numerical experiments are planned to examine the dual structure often observed in the AEW stormtrack, in which southern and northern branches of the stormtrack are found in the free atmosphere (around 650mb) and in the boundary layer (925mb), respectively. Coupling of AEWs to convection is also studied, using the full-physics version of the model. Results of these simulations will be compared to field campaign data collected in the African Monsoon Multidisciplinary Analysis (AMMA) field campaign.The work has broader impacts due to the links between AEWs and Atlantic hurricanes, including landfalling US hurricanes, and the monsoon rains of the Sahel. A better understanding of the fundamental dynamics of AEWs could be of value for forecasting both individual AEWs and also periods of enhanced or reduced AEW activity. In addition, the work will support and train two graduate students, thereby providing for the future workforce in this research area.