Improving Modeled Momentum Flux in the Atmospheric Boundary Layer

飞机上的乘客在起飞后不久就会经历从颠簸到平稳的过渡，因为他们会飞过行星边界层（PBL）的顶部，这是一层受地表强烈影响的大气。乘客感受到的上下的湍流运动是地面空气与空中空气交换的主要机制，主要是通过这种交换，空中的空气感受到地面的影响，反之亦然。由于它们的体积小，PBL中的湍流涡旋不能在天气和气候模式中明确模拟，必须通过代表其总体行为的参数化方案间接表示。该奖项支持一个气候过程团队，该团队致力于开发边界层内湍流垂直动量交换的参数化方案。垂直动量通量是确定边界层中地面风和风随高度变化的关键。气候模式通常使用垂直动量通量的简化参数化，其中通量是下梯度的，将动量从强风水平输送到风力较弱的水平。下降梯度通量通常是足够的，但当PBL中存在低空急流时，它不是一个很好的近似。例如，飓风通常在地表上方有最强的风，而下梯度近似可能是地表风太弱而无法达到伴随的最低海平面压力（SLP）的原因。大平原低空急流是美国大平原上空PBL顶部附近向北流动的夜间急流，是下梯度近似存在问题的另一个例子，考虑到急流在强雷暴发展中的作用，更好地表示动量通量是可取的。该方案是在社区地球系统模型（CESM）中开发和实施的，这是一个由国家大气研究中心主持和支持的开源模型。该方案能够产生上升梯度和下降梯度动量通量，因为它是预测的，解决动量通量的趋势方程，其中通过假设高阶项本质上是随机的来实现闭合。高阶项的值是从抽样到假设的概率密度函数中获得的，该函数的参数由小规模过程模型模拟和现场活动数据相结合确定。该项目还包括利用该方案进行科学调查，包括有和没有该方案的飓风模拟，以确定上升动量通量对地面风速的影响。这项工作的主要广泛影响是，它寻求改进气候模型，并提高它们对研究人员和决策者的价值。CESM是一个具有全球用户社区的社区模型，因此在项目中实现的模型改进可以被广泛采用。该计划还将在地球物理流体动力学实验室（GFDL）的气候模型中由未获得该奖项资助的合作者实施。此外，这项工作力求改善与恶劣天气有关的过程的表现。该项目为本科生和研究生以及两名博士后提供支持和培训，从而培养下一代大气科学研究人员。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Passengers on airplanes often feel a transition from bumpy to smooth flight shortly after takeoff as they fly through the top of the Planetary Boundary Layer (PBL), the layer of atmosphere which is strongly influenced by the surface. The up-and-down turbulent motions felt by passengers are the primary mechanism through which air at the surface is exchanged with air aloft, and it is primarily though this exchange that air aloft feels the effects of the surface and vice versa. Because of their small size, turbulent eddies in the PBL cannot be simulated explicitly in weather and climate models and must be represented indirectly through parameterization schemes that represent their aggregate behavior. This award supports a Climate Process Team engaged in the development of a parameterization scheme for the turbulent vertical exchange of momentum within the boundary layer. Vertical momentum flux is critical for determining surface winds and the change of wind with height in the PBL. Climate models commonly use simplified parameterizations of vertical momentum flux in which the flux is downgradient, transporting momentum from levels with stronger winds to levels where winds are weaker. Downgradient flux is often adequate but is not a good approximation when a low-level jet is present in the PBL. For instance hurricanes commonly have their strongest winds above the surface, and the downgradient approximation could be responsible for surface winds which are too weak for the accompanying sea level pressure (SLP) minimum. The great plains low-level jet, a northward-flowing nighttime jet near the top of the PBL over the US Great Plains, is another case where the downgradient approximation is questionable, and better representation of momentum flux is desirable given the role of the jet in the development of severe thunderstorms.The scheme is developed and implemented in the Community Earth System Model (CESM), an open-source model hosted and supported by the National Center for Atmospheric Research. The scheme is able to produce upgradient as well as downgradient momentum fluxes because it is prognostic, solving a tendency equation for the momentum flux in which closure is achieved by assuming that the higher-order terms are essentially stochastic. Values for the high-order terms are obtained from sampling into an assumed probability density function with parameters determined by a combination of small-scale process model simulations and field campaign data. The project also includes scientific investigation using the scheme, including hurricane simulations with and without it to determine the impact of upgradient momentum fluxes on surface wind speed.The primary broader impact of the work is that it seeks to improve climate models and enhance their value for researchers and decision makers. CESM is a community model with a global user community, thus model improvements achieved in the project can be widely adopted. The scheme will also be implemented in the climate model of the Geophysical Fluid Dynamics Laboratory (GFDL) by collaborators not funded under this award. Moreover, the work seeks to improve representation of processes associated with severe weather. The project provides support and training to undergraduate and graduate students as well as two postdoctoral researchers, thereby cultivating the next generation of researchers in atmospheric science.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.