Impacts of cloud and precipitation processes on maritime shallow convection as simulated by an LES model with bin microphysics.

This paper discusses impacts of cloud and precipitation processes on macrophysical properties of shallow convective clouds as simulated by a large-eddy model applying warm-rain bin microphysics. Simulations with and without collision-coalescence are considered with CCN concentrations of 30, 60, 120, and 240 mg^-1. Simulations with collision-coalescence include either the traditional gravitational collision kernel or a novel kernel that includes enhancements due to the small-scale cloud turbulence. Simulations with droplet collisions were discussed in Wyszogrodzki et al. (2013) focusing on the impact of the turbulent collision kernel. The current paper expands that analysis and puts model results in the context of previous studies. Despite a significant increase of the drizzle/rain with the decrease of CCN concentration, enhanced by the impact of the small-scale turbulence, impacts on the macroscopic cloud field characteristics are relatively minor. We document a clear feedback between cloud-scale processes and the mean environmental profiles that increases with the amount of drizzle/ rain. Model results show a systematic shift in the cloud top height distributions, with an increasing contributions of deeper clouds and an overall increase of the number of cloudy columns for stronger precipitating cases. We argue that this is consistent with the explanation suggested in Wyszogrodzki et al. (2013) namely, the increase of drizzle/rain leading to a more efficient condensate off-loading in the upper parts of the cloud field. An additional effect involves suppressing cloud droplet evaporation near cloud edges in low-CCN simulations as documented in previous studies. We pose a question whether the effects of cloud turbulence on drizzle/rain formation can be corroborated by remote sensing observations, for instance, from space. Although a clear signal is extracted from model results, we argue that the answer is negative due to uncertainties caused by the temporal variability of the shallow convective cloud field, sampling and spatial resolution of the satellite data, and overall accuracy of remote sensing retrievals. [ABSTRACT FROM AUTHOR]