Improving Modeled Momentum Flux in the Atmospheric Boundary Layer

飞机上的乘客经常在起飞后不久就感觉到从颠簸到平稳飞行的过渡，因为他们飞过行星边界层（PBL）的顶部，这是一层受到表面强烈影响的大气层。 乘客感受到的上下湍流运动是地面空气与高空空气交换的主要机制，主要是通过这种交换，高空空气感受到地面的影响，反之亦然。 由于边界层中的湍流涡旋尺寸较小，因此无法在天气和气候模式中明确模拟，必须通过代表其聚集行为的参数化方案间接表示。 该奖项支持气候过程团队参与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地面风场和风场随高度变化的关键。 气候模式通常使用简化的垂直动量通量参数化，其中通量是向下梯度的，将动量从风力较强的水平输送到风力较弱的水平。 向下梯度通量通常是足够的，但不是一个很好的近似时，低空急流是存在于边界层。例如，飓风通常在地表以上有最强的风，而下降梯度近似法可能是造成地表风太弱而无法获得伴随的海平面气压最小值的原因。 大平原低空急流是美国大平原上空边界层顶部附近向北流动的夜间急流，它是另一个下梯度近似值得怀疑的情况，考虑到急流在强雷暴发展中的作用，需要更好地表示动量通量。该方案在社区地球系统模式（CESM）中发展和实施，这是一个由国家大气研究中心主持和支持的开源模型。 该计划是能够产生的梯度以及向下梯度的动量通量，因为它是预测，解决了趋势方程的动量通量封闭是通过假设高阶项基本上是随机的。高阶项的值是从采样到假设的概率密度函数中获得的，该概率密度函数的参数由小规模过程模型模拟和现场活动数据的组合确定。 该项目还包括使用该方案进行科学调查，包括使用和不使用该方案进行飓风模拟，以确定各向异性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，提高其对研究人员和决策者的价值。 CESM是一个拥有全球用户社区的社区模型，因此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）的气候模型中实施，由未获得该奖项资助的合作者实施。 此外，这项工作旨在改善与恶劣天气相关的过程的代表性。 该项目为本科生和研究生以及两名博士后研究人员提供支持和培训，从而培养下一代大气科学研究人员。该奖项反映了NSF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.