Broadening of cloud droplet spectra through eddy hopping: Turbulent adiabatic parcel simulations

Broadening of cloud droplet spectra through eddy hopping: Turbulent adiabatic parcel simulations AbstractThis paper investigates spectral broadening of droplet size distributions through a mechanism referred to as the eddy hopping. The key idea, suggested a quarter century ago, is that droplets arriving at a given location within a turbulent cloud follow different trajectories and thus experience different growth histories, and that this leads to a significant spectral broadening. In this study, the adiabatic parcel model with super-droplets is used to contrast droplet growth with and without turbulence. Turbulence inside the parcel is described by two parameters: i) the dissipation rate of the turbulent kinetic energy ε, and ii) the linear extent of the parcel L. As expected, adiabatic parcel without turbulence produces extremely narrow droplet spectra. In the turbulent parcel, a stochastic scheme is used to account for vertical velocity fluctuations that lead to local supersaturation fluctuations for each super-droplet. These fluctuations mimic the impact of droplets hopping turbulent eddies in a natural cloud. For L smaller than a few meters, noticeable spectral broadening is possible only for strong turbulence, say, ε > 100 cm2 s−3. For L typical for grid lengths of large eddy simulation (LES) models (say, L between 10 and 100 m), the impact is significant even with relatively modest turbulence intensities. The impact increases with both L and ε. The representation of eddy hopping developed in this paper can be included in a straightforward way in the subgrid-scale scheme of a Lagrangian LES cloud model and may lead to a significant acceleration of simulated rain development through collision/coalescence. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the Atmospheric Sciences American Meteorological Society

Broadening of cloud droplet spectra through eddy hopping: Turbulent adiabatic parcel simulations

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Publisher
American Meteorological Society
Copyright
Copyright © American Meteorological Society
ISSN
1520-0469
eISSN
1520-0469
D.O.I.
10.1175/JAS-D-17-0043.1
Publisher site
See Article on Publisher Site

Abstract

AbstractThis paper investigates spectral broadening of droplet size distributions through a mechanism referred to as the eddy hopping. The key idea, suggested a quarter century ago, is that droplets arriving at a given location within a turbulent cloud follow different trajectories and thus experience different growth histories, and that this leads to a significant spectral broadening. In this study, the adiabatic parcel model with super-droplets is used to contrast droplet growth with and without turbulence. Turbulence inside the parcel is described by two parameters: i) the dissipation rate of the turbulent kinetic energy ε, and ii) the linear extent of the parcel L. As expected, adiabatic parcel without turbulence produces extremely narrow droplet spectra. In the turbulent parcel, a stochastic scheme is used to account for vertical velocity fluctuations that lead to local supersaturation fluctuations for each super-droplet. These fluctuations mimic the impact of droplets hopping turbulent eddies in a natural cloud. For L smaller than a few meters, noticeable spectral broadening is possible only for strong turbulence, say, ε > 100 cm2 s−3. For L typical for grid lengths of large eddy simulation (LES) models (say, L between 10 and 100 m), the impact is significant even with relatively modest turbulence intensities. The impact increases with both L and ε. The representation of eddy hopping developed in this paper can be included in a straightforward way in the subgrid-scale scheme of a Lagrangian LES cloud model and may lead to a significant acceleration of simulated rain development through collision/coalescence.

Journal

Journal of the Atmospheric SciencesAmerican Meteorological Society

Published: Mar 27, 2017

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