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    Atmospheric and hydrospheric sciences

    201508201508

    Impacts of cloud microphysics on trade wind cumulus: Which cloud microphysics processed contributes to the diversity in a large eddy simulation?

    Sato Y, Nishizawa S, Yashiro H, Miyamoto Y, Kajikawa Y, Tomita H

    Large eddy simulation, Shallow clouds

    Liquid water mixing ratio of trade wind cumuls simulated by (red) spectral bin sheme, (green) 2-moment bulk scheme, (sky blue) 1-moment bulk scheme in this study. The Black line, thick grey shade, and thin grey shade indicates the median, the range between first quatile and third quatile, and the range between maximum and minimum value of the previous model intercomparison study.

    This study investigated the impact of several cloud microphysical schemes on the trade wind cumulus in the large eddy simulation model. To highlight the differences due to the cloud microphysical component, we developed a fully compressible large eddy simulation model, which excluded the implicit scheme and approximations as much as possible. The three microphysical schemes, the one-moment bulk, two-moment bulk, and spectral bin schemes were used for sensitivity experiments in which the other components were fixed. Our new large eddy simulation model using a spectral bin scheme successfully reproduced trade wind cumuli, and reliable model performance was confirmed. Results of the sensitivity experiments indicated that precipitation simulated by the one-moment bulk scheme started earlier, and its total amount was larger than that of the other models. By contrast, precipitation simulated by the two-moment scheme started late, and its total amount was small. These results support those of a previous study. The analyses revealed that the expression of two processes, (1) the generation of cloud particles and (2) the conversion from small droplets to raindrops, were crucial to the results. The fast conversion from cloud to rain and the large amount of newly generated cloud particles at the cloud base led to evaporative cooling and subsequent stabilization in the sub-cloud layer. The latent heat released at higher layers by the condensation of cloud particles resulted in the development of the boundary layer top height.