Collaborative Research: PetaApps: Enabling Multiscale Modeling of Turbulent Clouds on Petascale Computers

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Reliable weather and climate prediction at local-to-global scales ultimately depends on a full and quantitatively accurate understanding of microphysical processes governing interactions of individual cloud droplets and their interplay with cloud-scale dynamics. Of those mechanisms known to affect the growth of individual droplets and evolution of cloud droplet populations within clouds, turbulence induced collision-coalescence appears to be among the most complex. Previous direct numerical simulation methods capable of resolving individual droplets have traditionally been confined to consideration of exceedingly small volumes (of order a cubic meter), as well as being restricted to relatively laminar (non-turbulent) flows. These methods have thus been incapable of addressing the impact of turbulent eddies, which typically have dimensions of 10-100 m, on cloud droplet-scale processes such as condensation, coalescence and mixing. Even very high resolution "Large Eddy Simulation (LES)" models, which have only recently achieved grid spacings routinely approaching 10 m, must approximate cloud microphysics via simplifying (and inherently limited) parameterization techniques. As such, the millions to billions of individual droplets comprised by even a small cloud cannot be individually represented and tracked via these traditional approaches.Application of advanced petascale computing methods by this investigative team will allow turbulence-to-cloud-scale (i.e. multiscale) cloud dynamics and microphysics to be treated in a seamless fashion absent unjustified simplifying assumptions. This approach will lead to a more robust understanding of shallow ice-free convective clouds, such as subtropical stratocumulus and trade-wind cumulus, which are known to play a critical role in the global climate system. Computer codes developed under this effort will be designed for ease of portability, ensuring their availability for broad use throughout the relevant research community.Broader Impact: This collaborative effort will allow advanced engineering fluid-mechanics research tools and state-of-the-art petascale computational methods to be effectively merged and applied to critical atmospheric processes within a unified numerical simulation framework. This approach will address several open questions that have faced the cloud-physics research community for many years, and will ultimately contribute to improved weather and climate forecasts. Multidisciplinary education will be enhanced through collaborative involvement of multiple graduate and undergraduate students, as well as a Postdoctoral scholar.