Early drizzle formation from collision and coalescence of cloud droplets is the rate-limiting step to precipitation initiation in warm clouds. However, this process is not fundamentally understood and highly simplified in weather and climate models, and it is difficult to be isolated and investigated in real atmospheric clouds due to complex interactions and feedback under various environmental conditions. Recent theoretical studies have suggested that the collision-coalescence process can be explored and evaluated in a tall convection cloud chamber. In this study, state-of-the-art simulations are used to investigate the contribution of the collision-coalescence process on early drizzle formation. Simulations indicate that a single parameter can quantify the effect of collision-coalescence process on droplet size distributions, and the collision rate can be estimated in a simplified way. These theories may be instructive for studies of the collision-coalescence process in cloud chambers and in real atmospheric clouds.