Two different collection kernels which include turbulence effects on thecollision rate of liquid droplets are used as a basis to develop aparameterization of the warm-rain processes autoconversion, accretion, andself-collection. The new parameterization is tested and validated with the helpof a 1-D bin microphysics model. Large-eddy simulations of the rain formationin shallow cumulus clouds confirm previous results that turbulence effectscan significantly enhance the development of rainwater in clouds and theoccurrence and amount of surface precipitation. The detailed behavior differssignificantly for the two turbulence models, revealing a considerableuncertainty in our understanding of such effects. In addition, thelarge-eddy simulations show a pronounced sensitivity to grid resolution, whichsuggests that besides the effect of sub-grid small-scale isotropicturbulence which is parameterized as part of the collection kernel also thelarger turbulent eddies play an important role for the formation of rain inshallow clouds.
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