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Diffusional and accretional growth of water drops in a rising adiabatic parcel: effects of the turbulent collision kernel

by W. W. Grabowski, L.-P. Wang
Atmospheric Chemistry and Physics Discussions ()


A large set of rising adiabatic parcel simulations is\n\nexecuted to investigate the combined diffusional and accretional\n\ngrowth of cloud droplets in maritime and continental\n\nconditions, and to assess the impact of enhanced droplet collisions\n\ndue to small-scale cloud turbulence. The microphysical\n\nmodel applies the droplet number density function to represent\n\nspectral evolution of cloud and rain/drizzle drops, and\n\nvarious numbers of bins in the numerical implementation,\n\nranging from 40 to 320. Simulations are performed applying\n\ntwo traditional gravitational collection kernels and two kernels\n\nrepresenting collisions of cloud droplets in the turbulent\n\nenvironment, with turbulent kinetic energy dissipation rates\n\nof 100 and 400 cm2 s−3. The overall result is that the rain\n\ninitiation time significantly depends on the number of bins\n\nused, with earlier initiation of rain when the number of bins\n\nis low. This is explained as a combination of the increase\n\nof the width of activated droplet spectrum and enhanced numerical\n\nspreading of the spectrum during diffusional and collisional\n\ngrowth when the number of model bins is low. Simulations\n\napplying around 300 bins seem to produce rain at\n\ntimes which no longer depend on the number of bins, but the\n\nactivation spectra are unrealistically narrow. These results\n\ncall for an improved representation of droplet activation in\n\nnumerical models of the type used in this study.\n\nDespite the numerical effects that impact the rain initiation\n\ntime in different simulations, the turbulent speedup factor,\n\nthe ratio of the rain initiation time for the turbulent collection\n\nkernel and the corresponding time for the gravitational\n\nkernel, is approximately independent of aerosol characteristics,\n\nparcel vertical velocity, and the number of bins used in\n\nthe numerical model. The turbulent speedup factor is in the\n\nrange 0.75–0.85 and 0.60–0.75 for the turbulent kinetic energy\n\ndissipation rates of 100 and 400 cm2 s−3, respectively

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