Atmospheric Chemistry and Physics, vol. 13, issue 16 (2013) pp. 8471-8487
This paper discusses cloud simulations aiming at quantitative assessment of the ef- fects of cloud turbulence on rain development in shallow ice-free convective clouds. Cloud fields from large-eddy simulations (LES) applying the bin microphysics with the collision kernel enhanced by cloud turbulence are compared to those with the stan- 5 dard gravitational collision kernel. Simulations for a range of cloud condensation nu- clei (CCN) concentrations are contrasted. Details of the turbulent kernel and how it is used in LES simulations are presented. Because of the disparity in spatial scales be- tween the bottom-up numerical studies guiding the turbulent kernel development and the top-down LES simulations of cloud dynamics, we address the consequence of the 10 turbulence intermittency in the unresolved range of scales on the mean collision kernel applied in LES. We show that intermittency e ff ects are unlikely to play an important role in the current simulations. Highly-idealized single-cloud simulations are used to illustrate two mechanisms that operate in cloud field simulations. First, the microphys- ical enhancement leads to earlier formation of drizzle through faster autoconversion 15 of cloud water into drizzle, as suggested by previous studies. Second, more e ffi cient removal of condensed water from cloudy volumes when a turbulent collection kernel is used leads to an increased cloud buoyancy and enables clouds to reach higher levels. This is the dynamical enhancement. Both mechanisms seem to operate in the cloud field simulations. The microphysical enhancement leads to the increased drizzle and 20 rain inside clouds in simulations with high CCN. In low-CCN simulations with significant surface rainfall, dynamical enhancement allows maintenance of the cloud water path despite significant increase of the precipitation water path and dramatically increased mean surface rain accumulation. These results call for future modeling and observa- tional studies to corroborate the findings.
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