The effects of turbulent collision–coalescence on precipitation formation and precipitation-dynamical feedbacks in simulations of stratocumulus and shallow cumulus convection

摘要

A double moment warm rain scheme that includes the effects of turbulence ondroplet collision rates has been implemented in a large-eddy model toinvestigate the impact of turbulence effects on clouds and precipitation.Simulations of shallow cumulus and stratocumulus show that differentprecipitation-dynamical feedbacks occur in these regimes when the effects ofturbulence are included in the microphysical processes. In both casesinclusion of turbulent microphysics increases precipitation due to a morerapid conversion of cloud water to rain. In the shallow convection case, thegreater water loading in the upper cloud levels reduces the buoyancyproduction of turbulent kinetic energy and the entrainment. Thestratocumulus case on the other hand shows a weak positive precipitationfeedback, with enhanced rainwater producing greater evaporation, strongercirculations and more turbulence. Sensitivity studies in which the clouddroplet number was varied show that greater number concentrations suppressthe stratocumulus precipitation leading to larger liquid water paths. Thispositive second indirect aerosol effect shows no sensitivity to whether ornot the effects of turbulence on droplet collision rates are included. Whilethe sign of the second indirect effect is negative in the shallow convectioncase whether the effects of turbulence are considered or not, the magnitudeof the effect is doubled when the turbulent microphysics are used. It isfound that for these two different cloud regimes turbulence has a largereffect than cloud droplet number and the use of a different bulkmicrophysics scheme on producing rainfall in shallow cumuli. However, forthe stratocumulus case examined here, the effects of turbulence on rainfallare not statistically significant and instead it is the cloud droplet numberconcentration or the choice of bulk microphysics scheme that has the largestcontrol on the rain water.