Broadening of Cloud Droplet Spectra Through Turbulent Eddy Hopping

Grabowski, W., National Center for Atmospheric Research (NCAR)

Cloud Processes

Convective Processes

Grabowski W and G Abade. 2017. "Broadening of Cloud Droplet Spectra through Eddy Hopping: Turbulent Adiabatic Parcel Simulations." Journal of the Atmospheric Sciences, 74(5), 10.1175/JAS-D-17-0043.1.


Cloud droplet spectra inside a cloudy air parcel after 1 km vertical displacement. Upper panel shows the narrow droplet spectra inside a parcel without turbulence. Lower panel shows the droplet spectra assuming that the parcel has horizontal extent of L = 50 m (similar to grid lengths of high-resolution cloud models) and turbulence intensity corresponding to the eddy dissipation rate ε = 50 cm2s-3, a typical turbulence intensity for small cumuli. The spectral with σ is shown in the panels.


Cloud droplet spectra inside a cloudy air parcel after 1 km vertical displacement. Upper panel shows the narrow droplet spectra inside a parcel without turbulence. Lower panel shows the droplet spectra assuming that the parcel has horizontal extent of L = 50 m (similar to grid lengths of high-resolution cloud models) and turbulence intensity corresponding to the eddy dissipation rate ε = 50 cm2s-3, a typical turbulence intensity for small cumuli. The spectral with σ is shown in the panels.

Science

The mean radius and the width of cloud droplet spectra are key features of clouds. 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. Often, the spectra are significantly wider than predicted by a simple model with no cloud dilution even if the observed properties suggest small or no cloud dilution. 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. The eddy-hopping mechanism needs to be accounted for in LES of turbulent clouds because it has potentially a significant impact on rain formation through collision/coalescence and on radiative transfer through a cloudy atmosphere.

Impact

The eddy-hopping methodology can be applied straightforwardly in high-resolution simulations applying a Lagrangian methodology to simulate cloud and precipitation formation--that is, applying the super-droplet method using the terminology of Shima et al. (Q. J. R. Met. Soc. 2009, p. 1307). This idea is being pursued by a follow-up study by the authors. However, application of eddy hopping to Eulerian models that are typically used in high-resolution cloud modeling requires additional study and needs to be pursued in the future. This also applies to any stochastic Eulerian cloud microphysics scheme, including representation of ice processes.

Summary

A novel methodology is being developed for a Lagrangian cloud model (i.e., applying the so-called “super-droplets” to represent formation and growth of cloud droplets) 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 thus growth histories of cloud droplets arriving at a given location inside a turbulent cloud. Application of this methodology to a parcel model suggests a significant impact on the droplet spectrum. High-resolution cloud model simulations using this methodology are ongoing.