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

非洲东风波(AEW)是波长为2000至2500公里的非洲东风急流的南北起伏，形成远至苏丹的东部，并向西传播穿过萨赫勒。它们是夏季萨赫勒地区大范围天气变化的主要形式，在给这一干旱多发地区带来季风雨方面发挥了关键作用。当AEW继续在大西洋上向西(或向东)传播时，它们可以发展成飓风，据估计，85%的主要大西洋飓风(包括飓风桑迪)来自AEW。然而，现有的AEW起源和演化理论并不能很好地解释AEW的许多观测特征。本项目研究了AEW的起源和演化、AEW风暴路径(AEW发展和传播的典型路径)的结构、AEW活动的间歇性以及AEW与湿对流的相互作用。这项由首席调查员(PI)在之前的工作中开发的研究的一个关键组成部分是，AEW被组织成波包，其中的能量色散可以根据经验确定的群速度进行跟踪。这种动态在某种程度上相当于在沿着中纬度风暴路径移动的天气天气系统中观察到的“下游发展”，不同的是，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系列重大后果，包括来自风暴路径西端成熟AEW的能量可能传播到风暴路径东端的AEW生成区域，从而促进新AEW的生成。在某种程度上，上下游传播的结果可以解释为有限纬向范围的不稳定急流的对流不稳定和绝对不稳定之间的差异。在对流不稳定中，在喷流不稳定上生长的波包有下游的能量传播，将波包移出不稳定区域。在绝对不稳定情况下，波包尾端的群速度在上游，因此部分波包能量保留在不稳定区域，并促进进一步增长。因此，AEW活动的间歇性可能是由于从绝对对流不稳定过渡到对流不稳定造成的，因为在后一种情况下，能量不会向上传播到AEW起源区。该项目研究的主要工具是天气研究和预报(WRF)模式，这是一个应用于赤道以北非洲和大西洋邻近部分的区域大气模式。模式模拟是使用模式的标准全物理版本和中等复杂版本进行的，其中使用外部热强迫项来产生非洲东部急流和AEW风暴路径的理想化版本。除了为研究上游和下游能量弥散的作用而设计的模拟外，还计划进行更多的数值试验，以检查在AEW风暴路径中经常观察到的双重结构，在这种结构中，风暴路径的南北分支分别位于自由大气(约650毫巴)和边界层(925毫巴)。利用该模型的全物理版本，还研究了气体波与对流的耦合。这些模拟的结果将与在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与大西洋飓风(包括登陆的美国飓风和萨赫勒季风雨)之间的联系，这项工作具有更广泛的影响。更好地了解AEW的基本动态对于预报单个AEW以及AEW活动增强或减少的周期都是有价值的。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.