ABSTRACT

Clouds play a critical but delicate role in regulating Earth's climate. They cool the Earth by reflecting about a quarter of the incoming sunlight back into space, but they also warm the Earth by trapping outgoing infrared radiation. The sensitivity of climate to cloud amount and properties poses a substantial challenge to climate modeling given the difficulty of representing clouds in climate models. In fact clouds are so difficult to simulate that climate models typically use separate cloud parameterization schemes (essentially computer subroutines) to represent different kinds of clouds. In the case of convective clouds models typically use two schemes, one for the shallow cumulus clouds which form within and on top of the of the planetary boundary layer (PBL, commonly the first kilometer or two above the surface), and another for the deep cumulus clouds which can extend through the depth of the troposphere (to heights of perhaps 10km or more). The separation of deep and shallow convection is done for practical reasons but in reality clouds come in all sizes and ultimately the same laws of physics apply regardless of size. Also, the independence of the shallow and deep convection schemes can subtly misrepresent the interaction between shallow and deep convective clouds. A unified scheme, which represents everything from small-scale convection in the PBL to deep cumulus clouds, is clearly preferable but also challenging to implement.

This award supports a Climate Process Team engaged in the development and implementation of such a unified convection scheme in the Community Earth System Model (CESM). The scheme is known as as an Eddy Diffusivity/Mass Flux (EDMF) parameterization, in which the smallest convective plumes are treated along with PBL turbulent air motions using a diffusive approximation, combined with a mass flux methodology for representing clouds which are as tall or taller than the depth of the PBL. The eddy diffusivity and mass flux components of the scheme are carefully coordinated so that they can represent the interactions between dry PBL turbulence and PBL clouds and the transition from shallow to deep convection. The EDMF scheme developed and implemented here builds on previous EDMF schemes developed by the lead PI and others, adding features including the ability to simulate an ensemble of mass flux plumes, thus allowing multiple convective regimes to exist in a single grid box, and the effects of organized convection and cold pools.

The primary broader impact of the work is that it seeks to improve the performance of climate models and their usefulness as tools for scientific research and for providing guidance for decision makers concerned with climate variability and change. CESM is an open-source model which is hosted and supported by the National Center for Atmospheric Research and has a worldwide user community. The scheme will also be implemented in the climate model of the Geophysical Fluid Dynamics Laboratory (GFDL) by collaborators not funded under this award. In addition, the award supports an annual workshop on turbulence, convection and clouds in climate models, which serves as a venue for interactions among researchers working on cloud and PBL parameterization. The project supports three postdocs, thereby developing the future workforce for climate model development.

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.

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