Broadening of Cloud Droplet Spectra through Turbulent Eddy Hopping
New turbulent eddy hopping methodology produces realistically broad cloud droplet distributions.
Clouds are made up of droplets of a range of sizes; the “spectrum” or distribution of the droplet sizes has important implications for how clouds reflect solar radiation and the development of precipitation. Two of the key parameters describing cloud droplet spectra are the mean radius and the spectrum width. Spatial variability of the mean radius in clouds developing in different environments is relatively well understood and it is predominantly determined by the characteristics of small aerosol particles on which cloud droplets formed (called cloud condensation nuclei, or CCN) and by the bulk thermodynamic properties (such as the temperature and moisture as well as cloud dilution). The width of the spectrum remains poorly understood.
A novel methodology is being developed to represent the impact of unresolved turbulent eddies on the width of the droplet spectrum. The methodology follows the concept of eddy hopping–that is, variable trajectories and growth histories of cloud droplets arriving at a given location inside a turbulent cloud. The eddy-hopping mechanism needs to be accounted for in large-eddy simulations (LES) of turbulent clouds because it has a potentially significant impact on rain formation through collision/coalescence and on radiative transfer through a cloudy atmosphere.
This study investigates spectral broadening of droplet size distributions through a mechanism referred to as “turbulent eddy hopping”. The key idea, suggested several decades ago, is that droplets arriving at a given location within a turbulent cloud follow different trajectories and thus different growth histories, and that this leads to a significant spectral broadening. In this study, a parcel model is used to contrast growth of cloud droplets with and without turbulence. As expected, parcels without turbulence produce extremely narrow droplet spectra. In contrast, the impact for the parcel model with turbulence is significant, with the width of the distribution increasing several times when compared to the spectrum without turbulence. High-resolution cloud model simulations using this methodology are ongoing.
Wojciech W. Grabowski
National Center for Atmospheric Research
This work was partially supported by the Polish National Science Center (NCN) “POLONEZ 1” Grant 2015/19/P/ST10/02596 and by the U.S. ASR Grant DE-SC0016476. The POLONEZ 1 grant has received funding from the European Union’s Horizon 2020 Research and Innovation Program under the Marie Sklodowska-Curie Grant Agreement 665778.
Grabowski, W. and G. Abade, “Broadening of Cloud Droplet Spectra through Eddy Hopping: Turbulent Adiabatic Parcel Simulations.” Journal of the Atmospheric Sciences, 74: 1485-1493 (2017). [DOI: 10.1175/JAS-D-17-0043.1].