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. The increase in the spectral width of an initially monodispersepopulation of cloud droplets in homogeneous isotropic turbulence isinvestigated by applying a finite-difference fluid flow model combined witheither Eulerian bin microphysics or a Lagrangian particle-based scheme. Theturbulence is forced applying a variant of the so-called linear forcingmethod that maintains the mean turbulent kinetic energy (TKE) and the TKEpartitioning between velocity components. The latter is important formaintaining the quasi-steady forcing of the supersaturation fluctuationsthat drive the increase in the spectral width. We apply a largecomputational domain (643 m3), one of the domains considered inThomas et al. (2020). The simulations apply 1 m grid length and are in thespirit of the implicit large eddy simulation (ILES), that is, withsmall-scale dissipation provided by the model numerics. This is in contrastto the scaled-up direct numerical simulation (DNS) applied in Thomas et al. (2020). Two TKE intensities and three different droplet concentrations areconsidered. Analytic solutions derived in Sardina et al. (2015), valid forthe case when the turbulence integral timescale is much larger than thedroplet phase relaxation timescale, are used to guide the comparisonbetween the two microphysics simulation techniques. The Lagrangian approachreproduces the scalings relatively well. Representing the spectral widthincrease in time is more challenging for the bin microphysics becauseappropriately high resolution in the bin space is needed. The bin width of0.5 µm is only sufficient for the lowest droplet concentration (26 cm−3). For the highest droplet concentration (650 cm−3), an order ofmagnitude smaller bin size is barely sufficient. The scalings are notexpected to be valid for the lowest droplet concentration and the high-TKEcase, and the two microphysics schemes represent similar departures.Finally, because the fluid flow is the same for all simulations featuringeither low or high TKE, one can compare point-by-point simulation results.Such a comparison shows very close temperature and water vaporpoint-by-point values across the computational domain and largerdifferences between simulated mean droplet radii and spectral width. Thelatter are explained by fundamental differences in the two simulationmethodologies, numerical diffusion in the Eulerian bin approach and a relatively small number of Lagrangian particles that are used in theparticle-based microphysics.

Cloud droplet diffusional growth in homogeneous isotropic turbulence: bin microphysics versus Lagrangian super-droplet simulations