COMPARISON OF COLLISION VELOCITY DIFFERENCES OF DROPS AND GRAUPEL PARTICLES IN A VERY TURBULENT CLOUD

The motion of water drops and graupel particles within a turbulent med ium is analyzed. The turbulence is assumed to be homogeneous and isotr opic. It is demonstrated that the inertia of drops and graupel particl es falling within a turbulent flow leads to the formation of significa nt velocity deviations from the surrounding air, as well as to the for mation of substantial relative velocity between drops and graupel part icles. The results of calculations of the continuous growth of raindro ps and graupel particles moving within a cloud of small droplets are p resented both in a non-turbulent medium and within turbulent flows of different turbulence intensity. Continuous growth of a drop-collector was calculated with the coalescence efficiency E-epsilon = 1, as well as using E-epsilon values provided by Beard and Ochs [Beard, K.V., Och s, H.T., 1984. Collection and coalescence efficiencies for accretion. J. Geophys. Res., 89: 7165-7169.] ranging from 0.5 to about 0.75 for d ifferent droplet sizes. In the case of graupel-droplet interaction E-e psilon was assumed equal to 1. It is shown that in the case E-epsilon = 1 in a non-turbulent medium, the growth rates of graupel and raindro ps are close. Under turbulent conditions typical of mature convective clouds, graupel grows much faster than a raindrop. In the case E-epsil on < 1 the growth rate of a water drop slows down significantly, so th at graupel grows faster than raindrops even under non-turbulent condit ions. Turbulence greatly increases the difference between the growth r ates of graupel and drop:collectors. Possible consequences of the fast er growth of graupel in terms of cloud microphysics are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.