Effects of cloud turbulence on warm-rain processes in maritime shallow convection

This presentation will discuss cloud field simulations aiming at quantitative assessment of the effects of cloud turbulence on rain development in shallow ice-free convective clouds. We compare large-eddy simulations (LES) of cloud fields applying the bin microphysics scheme combined with either the standard gravitational collision kernel or the kernel that includes enhancement of droplet collisions due to cloud turbulence (i.e., the turbulent collision kernel). Simulations for a range of cloud condensation nuclei (CCN) concentrations are contrasted. Because of the disparity in spatial scales between DNS studies guiding the turbulent kernel development and the LES simulations of cloud dynamics, we first address the issue of the turbulence intermittency in the unresolved range of scales on the mean collision kernel applied in LES. We show that intermittency effects are unlikely to play a significant role. By comparing simulations applying either the gravitational or the turbulent collision kernel, we document two mechanisms that operate in cloud field simulations. First, the microphysical enhancement leads to an earlier formation of drizzle through faster autoconversion of cloud water into drizzle, as suggested by previous studies. Second, more efficient removal of condensed water from cloudy volumes when the turbulent collision kernel is used leads to an increased cloud buoyancy and enables clouds to reach higher levels. This is the dynamical enhancement. Both mechanisms lead to an impressive surface precipitation enhancement when turbulent kernel is used. Implications of these results for future modeling and observational studies of shallow convection, as well as for the parameterization of these clouds in larger-scale models will be also discussed.