Cloud droplet diffusional growth in homogeneous isotropic turbulence: bin microphysics versus Lagrangian super-droplet simulations
<p>The increase in the spectral width of an initially monodisperse population of cloud droplets in homogeneous isotropic turbulence is investigated by applying a finite-difference fluid flow model combined with either Eulerian bin microphysics or a Lagrangian particle-based scheme. The turbule...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/4059/2021/acp-21-4059-2021.pdf |
| Summary: | <p>The increase in the spectral width of an initially monodisperse
population of cloud droplets in homogeneous isotropic turbulence is
investigated by applying a finite-difference fluid flow model combined with
either Eulerian bin microphysics or a Lagrangian particle-based scheme. The
turbulence is forced applying a variant of the so-called linear forcing
method that maintains the mean turbulent kinetic energy (TKE) and the TKE
partitioning between velocity components. The latter is important for
maintaining the quasi-steady forcing of the supersaturation fluctuations
that drive the increase in the spectral width. We apply a large
computational domain (64<span class="inline-formula"><sup>3</sup></span> m<span class="inline-formula"><sup>3</sup></span>), one of the domains considered in
Thomas et al. (2020). The simulations apply 1 m grid length and are in the
spirit of the implicit large eddy simulation (ILES), that is, with
small-scale dissipation provided by the model numerics. This is in contrast
to the scaled-up direct numerical simulation (DNS) applied in Thomas et al. (2020). Two TKE intensities and three different droplet concentrations are
considered. Analytic solutions derived in Sardina et al. (2015), valid for
the case when the turbulence integral timescale is much larger than the
droplet phase relaxation timescale, are used to guide the comparison
between the two microphysics simulation techniques. The Lagrangian approach
reproduces the scalings relatively well. Representing the spectral width
increase in time is more challenging for the bin microphysics because
appropriately high resolution in the bin space is needed. The bin width of
0.5 <span class="inline-formula">µm</span> is only sufficient for the lowest droplet concentration (26 cm<span class="inline-formula"><sup>−3</sup></span>). For the highest droplet concentration (650 cm<span class="inline-formula"><sup>−3</sup></span>), an order of
magnitude smaller bin size is barely sufficient. The scalings are not
expected to be valid for the lowest droplet concentration and the high-TKE
case, and the two microphysics schemes represent similar departures.
Finally, because the fluid flow is the same for all simulations featuring
either low or high TKE, one can compare point-by-point simulation results.
Such a comparison shows very close temperature and water vapor
point-by-point values across the computational domain and larger
differences between simulated mean droplet radii and spectral width. The
latter are explained by fundamental differences in the two simulation
methodologies, numerical diffusion in the Eulerian bin approach and a relatively small number of Lagrangian particles that are used in the
particle-based microphysics.</p> |
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| ISSN: | 1680-7316 1680-7324 |