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MISU Seminar | by Wojciech W. Grabowski

Diffusional growth of cloud droplets in turbulent clouds 

by Wojciech W. Grabowski 
National Center for Atmospheric Research, Boulder, USA and Institute of Geophysics, Faculty of Physics, University of Warsaw, Warsaw, Poland

Time: Friday 25 August 2017, 11h15

Place: Rossbysalen C609, Arrhenius Laboratory, 6th floor


 Wojciech W. GrabowskiThis presentation will discuss spectral broadening of the droplet size distribution in turbulent clouds through a mechanism referred to as eddy hopping. The key idea suggested in late 1980ies by Al Cooper is that droplets arriving at a given location within a turbulent cloud follow different trajectories and thus have different growth histories, and that this leads to a significant spectral broadening.

I will discuss the development of a novel Lagrangian Cloud Model (LCM) to study the impact of eddy hopping on simulated cloud droplet spectra. LCM merges Eulerian approach for the cloud-scale flow dynamics with Lagrangian approach for the cloud droplets. The LCM’s key element is the subgrid-scale (SGS) scheme that affects droplet motion and includes prediction of the SGS supersaturation fluctuations along droplet trajectories. The latter is in addition to the supersaturation predicted by the resolved cloud-scale flow model. Flow velocity fluctuations and the SGS supersaturation fluctuations are tied to the turbulent kinetic energy (TKE) predicted by the flow model.

First, LCM is applied to a simple problem of droplet growth inside an adiabatic cloudy turbulent parcel rising at a constant velocity in the atmosphere. As expected, adiabatic parcel without turbulence produces extremely narrow droplet spectra. In the turbulent parcel, SGS supersaturation fluctuations lead to a significant spectral broadening that depends on the assumed parcel extent and the assumed turbulent kinetic energy dissipation rate.

 Subsequently, the LCM is applied to high-spatial-resolution simulations of realistically-forced small cumulus clouds. The simulations corroborate the impact seen in the rising turbulent adiabatic parcel study. These results seem to explain cloud observations that typically show broad droplet spectra in undiluted and weakly diluted cloudy volumes.



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