Improving Modeled Momentum Flux in the Atmospheric Boundary Layer

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.