RII Track-4: Turbulence Effects on Cloud Microphysical Processes: Development and Testing of Subgrid-Scale Parameterizations for Large Eddy Simulation

University Of Oklahoma Norman Campus, Norman OK

Investigators

Abstract

Clouds play a critical role for many processes in the Earth system, including precipitation, atmospheric chemistry, and climate. Over the past few decades, scientists have shown that turbulence within clouds can cause large fluctuations in humidity, temperature, and water droplet concentrations, which directly impact cloud lifetimes and rain formation. Despite the importance of small-scale cloud turbulence, few studies have focused on how to represent its effects in computer models used for research and weather forecasting. In this project, the Principal Investigator (PI) will partner with experts at Michigan Technological University (MTU) to develop models that accurately represent the effects of small-scale cloud turbulence in computer models. In order to achieve this goal, the PI and a graduate student will perform laboratory experiments in the MTU cloud chamber, the nation's only turbulence chamber capable of creating and sustaining turbulent clouds in a controlled laboratory environment. This funding will establish a lasting collaboration between the PI and hosts and will support the education of a graduate student by providing access to unique laboratory facilities. By expanding the PI's expertise to encompass laboratory experiments and cloud turbulence, this project will enable future collaborations between the PI and partners in the National Weather Center, thereby strengthening the overall competitiveness of the Central Oklahoma weather enterprise. Turbulence is a key driver of cloud mixing and entrainment, causing significant fluctuations in temperature, humidity, cloud particle concentrations, and collisions or breakup. Recent studies have demonstrated that turbulent fluctuations of supersaturation can lead to a broadening of the cloud droplet size distribution, initiating rain formation significantly faster than what theory predicts for a quiescent environment. As computational power increases, large eddy simulation (LES), which directly resolves the largest scales of turbulence, is becoming a promising technique for both weather forecasting and for studying the linkages between turbulence, microphysics, and large-scale cloud properties. This project aims to develop new subgrid-scale models that can accurately represent small-scale turbulent fluctuations of supersaturation in LES. The PI and a graduate student will partner with hosts at Michigan Technological University to perform experiments in the MTU cloud chamber, where steady-state turbulent clouds can be produced through moist Rayleigh-Benard convection. An ensemble of laboratory experiments will be performed to investigate the distribution of supersaturation fluctuations across spatial and temporal scales, and to develop new models that can recover the subgrid supersaturation variance based only on knowledge of resolved-scale features of the flow. This project will allow the PI to develop research expertise in laboratory experiments and cloud turbulence, thereby enabling future collaborations with partners in the National Weather Center. The future research areas enabled by this project have significant societal benefits, including improved precipitation forecasts and radar measurements, and an improved understanding of the role of clouds in Earth's present and climate. 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.