Toward elucidating the microstructure of warm rainfall: A survey

Quantitative measurement, estimation, and prediction of precipitation remains one of the grand challenges in the hydrological and atmospheric sciences with far-reaching implications across the natural sciences. Although the roots of current research activity in this topic go back to the beginning of the twentieth century, advances in radar technology and in numerical modeling have provided the impetus for prolific research in the area of cloud and precipitation physics over the last 50 years. As radar rainfall measurements progressively became the staple of hydrometeorological observing systems, cloud and precipitation microphysics emerged as increasingly preeminent areas of research. Here we present a synthesis of the state of the science with respect to the physical dynamics of hydrometeors and, specifically, the transient processes that affect the temporal evolution of rainfall microstructure and that are directly relevant to the quantitative interpretation of radar rainfall measurements and explicit numerical simulations. The focus of our survey is on raindrop morphodynamics (equilibrium raindrop shape and raindrop oscillations), drop-drop interactions (bounce, coalescence, and breakup), and the dynamical evolution of raindrop size distributions in precipitating clouds.