Seminarium fizyki atmosfery

Impact of cloud-base turbulence on CCN activation

prof. dr hab. Wojciech W. Grabowski

Mesoscale and Microscale Meteorology (MMM) Laboratory, NCAR


22 października 2021 13:15, ul. Pasteura 5, B0.14 and online via ZOOM

This presentation will discuss the impact of turbulence on the cloud-based activation of cloud condensation nuclei (CCN), microscopic soluble aerosol particles responsible in formation of cloud droplets in natural clouds. Cloud-based CCN activation is the key process determining microphysical properties of warm (ice-free) clouds, shallow cumulus and stratocumulus in particular. In the classical picture, a smooth (non-turbulent) updraft of the sub-cloud air carrying CCN reaches saturation and allows CCN activation resulting in formation of cloud droplets. This process is typically considered without the impact of turbulence and focusing on such factors as the cloud-base updraft speed and properties of CCN, their concentration and chemical composition. We contrast this classical picture with an approach in which a turbulent parcel rising through the cloud base results in the formation of cloud droplets. Numerical calculations of the rising turbulent parcel apply a methodology similar to the forced direct numerical simulation of small-scale isotropic and homogeneous turbulence. The methodology – extension of the stochastic model applied in our previous studies (Abade et al. J. Atmos. Sci. 2018) – will be explained and results from a large set of simulations will be discussed. The simulations consider both single-size CCN - to contrast simulation results with turbulent CCN activation in the Michigan Tech Pi chamber - and CCN featuring a typical size distribution. The simulations document a significant impact of cloud base turbulence on CCN activation, especially when cloud-base updraft is relatively weak and turbulent velocity fluctuations are comparable to the mean updraft. Possible extensions of the modelling approach to a more realistic cloud-based activation framework will be discussed

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PRESENTATION SLIDES: Impact of cloud-base turbulence on CCN activation

Wojciech_Grabowski_turbulentCCNactivation.pdf (5 MiB) Uploaded: 28 października 2021 09:53

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