Gravity Wave Sources and Parameterization

这个为期三年的项目将改进全球模型中亚网格尺度重力波效应的参数化。全球尺度平均气流的重力波强迫被用来纠正模拟平流层环流中的常见缺陷：(1)冬季平流层的冷极问题与化学-气候模式中对温度敏感的臭氧化学的误差以及行星波传播和反射的误差有关；(2)春季东风的延迟出现也影响行星波的传播和早春平流层变暖的发生；(3)热带平流层下层缺乏准两年一次的振荡，这种振荡也影响行星波的传播、平流层臭氧和赤道波的传播。除了重力波在全球尺度上的平均流强迫效应外，其他尚未在全球模式中参数化的重要效应也正在显现。例如，在卷云中，波浪的垂直运动控制着晶体大小、数量密度、降水率和云的寿命，而这些又会产生全球尺度的辐射和臭氧化学效应。在对流层中，成熟对流风暴产生的向上传播的重力波可能从更高的高度反射回边界层，并进一步影响那里的对流起始。重力波强迫参数化有两个基本组成部分：(1)对波源的说明，(2)对作为高度函数的波耗散的估计。目前在全球模式中使用了几种随高度变化的波耗散参数化方法，但没有明确的区分方法。在这个项目的方法侧重于波源，并严格限制那些与观测。这不仅将使参数化更加真实，而且还可能最终区分不同的耗散方法。山地波源在全球预报和气候模式中已被具体参数化了二十年。由于山波是静止的，它们的耗散只能将平均气流拖向零风速。山波源在地理上也是有限的。来自其他来源的波是非平稳的，因此它们的耗散可能导致平均流的减速或加速。这个项目的重点是对流产生的波浪。众所周知，对流可以产生具有大范围非平稳相速度的波，并且可能是整个热带地区和夏季中纬度地区主要的重力波强迫机制。这项工作的智力价值跨越了大气科学的传统界限，利用云模拟和降水雷达观测工具来了解对流产生的小尺度波的起源和性质。通过平行线性模型的研究和与现有参数化方法的比较，将增加对重力波产生的基本物理的理解。最后，通过与全球模拟小组的合作，将评估亚网格尺度重力波强迫和全球模式反馈的参数化改进。这项工作的更广泛影响将是对从边界层顶部到高层大气的高度区域的重力波产生和重力波效应的理解的进展。除了在全球模式中改进重力波平均流强迫的参数化这一主要目标外，其结果在卷云研究和对流层对流开始的研究中也将有价值。该项目涉及培养研究生和博士后研究人员。最初，该项目将完全由女性研究人员进行，这是一种罕见的现象，因为女性在大气科学领域的代表性仍然不足。该项目还开始与两个全球气候模拟组织合作，一个在美国，另一个在欧洲。虽然亚栅格尺度的重力波效应并不是目前改善全球天气预报和气候模式的主要障碍，但许多全球模式小组认识到将模式上边界提高到中间层高度的重要性，这就加强了改进重力波效应参数化的必要性。

This three-year project will improve the parameterization of subgrid-scale gravity wave effects in global models. Gravity wave forcing of the global-scale mean flow is used to correct common deficiencies in modeled stratospheric circulations: (1) A cold-pole problem in the winter stratosphere that has links to errors in temperature-sensitive ozone chemistry in chemistry-climate models and links to errors in planetary wave propagation and reflection, (2) A delayed onset of easterly winds in the springtime that also affects the propagation of planetary waves and the occurrence of stratospheric warmings in the early spring season, and (3) the lack of a quasibiennial oscillation in the tropical lower stratosphere that also affects planetary wave propagation, stratospheric ozone, and propagation of equatorial waves. In addition to the mean-flow forcing effects of gravity waves on the global scale, other effects are emerging as important that are not yet parameterized in global models. In cirrus clouds, for instance, wave vertical motions control crystal sizes, number densities, precipitation rates, and cloud lifetimes, which in turn can have global-scale radiative and ozone chemistry effects. In the troposphere, upward-propagating gravity waves generated by mature convective storms may reflect from higher altitudes back towards the boundary layer and influence further convective initiation there.There are two essential components to gravity wave forcing parameterizations: (1) the specification of the wave source, and (2) the estimation of the wave dissipation as a function of height. There are several methods for parameterization of the wave dissipation with height in use in global models currently, but no clear way to distinguish between them. The approach in this project focuses on the wave sources, and on constraining those rigorously with observations. This will not only make the parameterizations more realistic, but it may also eventually distinguish between the different dissipation methods. Mountain wave sources have been specifically parameterized in global forecasting and climate models for two decades. Because mountain waves are stationary, their dissipation can only drag the mean flow toward zero wind speed. Mountain wave sources are also limited geographically. Waves from other sources are nonstationary so their dissipation can cause either deceleration or acceleration of the mean flow. The focus in this project is on wave generation by convection. Convection is known to generate waves with a broad range of nonstationary phase speeds and is likely the dominant gravity wave forcing mechanism throughout the tropics and in summer midlatitudes. The intellectual merit of the work crosses traditional boundaries in the atmospheric sciences, using tools of cloud-modeling and precipitation radar observations to understand the origin and nature of small-scale waves generated by convection. Through parallel linear model studies and comparison to existing parameterization methods there will be increased understanding of the essential physics of gravity wave generation. Finally, through collaboration with global modeling groups, parameterization improvements to the subgrid-scale gravity wave forcing and feedbacks in global models will be evaluated. The broader impacts of this work will be advances in understanding of gravity wave generation and gravity wave effects across altitude regions ranging from the top of the boundary layer into the upper atmosphere. In addition to the main goal of improving parameterizations of gravity wave mean-flow forcing in global models, results will also be valuable in cirrus cloud studies, and in studies of convection initiation in the troposphere. The project involves training of graduate student and postdoctoral researchers. Initially, the project will be conducted solely by female researchers, a rare occurrence because females are still under-represented in the atmospheric sciences. The project also begins partnerships with two global climate modeling groups, one in the US and and the other in Europe. While subgrid-scale gravity wave effects are not the primary obstacle to improving global weather forecasting and climate models today, many global modeling groups recognize the importance of raising their model upper boundaries to mesospheric altitudes, and this intensifies the need for improved parameterizations of gravity wave effects.