Abstract
The critical condition of microphysical parameters for the formation of a typical "bimodal" drop size-distribution in cumuli composed of cloud droplets and drizzle sized drops, through which cloud droplets explosively grow into raindrops by coalescence, is investigated by using an axisymmetric cumulus model. To meet this purpose, the evolutional process of droplets in the clouds with different dynamical properties is carefully examined.
The detailed analysis of the result strongly suggests the existence of a general critical condition for the formation of the typical "bimodal" drop size-distribution. The condition is simply expressed in terms of mean radius and dispersion of droplets.
In well developed clouds, dynamically active, the critical condition is met almost during the developing stage of the cloud at levels far below the highest cloud top, and the explosive growth of droplets together with the development of intense downdraft by nonlinear interaction between cloud dynamics and microphysics finally gives rise to a sudden outbreak of rainfall characterized by heavy, short-lived and localized features, large raindrops, higher rainfall efficiency and so on. On the other hand, in small-scale cumuli, which are less active, the growth of droplets is fairly gradual since the critical condition is barely satisfied near the midst of the mature stage almost in the uppermost layer of the cloud. This lower growth rate of droplets together with the high evaporation rate in the decaying stage of the cloud is an important mechanism of the precipitation process in small cumuli characterized by relatively light rainfall, smaller raindrops, smaller rainfall efficiency, and so on.
Careful consideration is given to the cumulus model.
