Toward Building A Virtual Laboratory to Investigate Rainfall Microphysics at Process Scales

A 3D virtual microphysics laboratory (VML) has been built upon a new raindropFoam solver to simulate explicitly the evolution of cloud droplets to rainfall, and in the future the plan is to conduct end-to-end simulations of aerosol-cloud-interactions. Here we present first results relevant for rainfall. Initial tests show that the VML simulates well the classical features of raindrop dynamics in the literature. The 3D simulations enabled by VML show that large raindrops experience shape and velocity oscillations in both horizontal and vertical planes, and that the magnitude of the oscillations increase with drop size from boosted turbulence at high velocity and damped pressure by surface tension. The virtual experiments show the contrast between the classical notion of "terminal velocity" and the realistic range of velocities observed by disdrometers. That is to say, the "uncertainty" in disdrometer measurements of raindrop velocities is rooted in the physics with more prolate shapes experiencing higher vertical acceleration with smaller horizontal cross section area thus smaller drag force. The implications of prolate-oblate transitions for the interpretation of radar measurements of rainfall are discussed.