Understanding the importance of microphysics and macrophysics for warm rain in marine low clouds: Part II. Heuristic models of rain formation.

Two simple heuristic model formulations for warm rain formation are introduced and their behavior explored. The first, which is primarily aimed at representing warm rain formation in shallow convective clouds, is a continuous collection model that uses an assumed cloud droplet size distribution consistent with observations as the source of embryonic drizzle drops that are then allowed to fall through the (assumed constant) cloud accreting cloud droplets. The second, which is applicable to steady-state precipitation formation in stratocumulus, is a simple two-moment autoconversion and accretion model in which cloud liquid water is removed by drizzle formation and replenished on a externally-specified timescale that reflects the efficacy of turbulent overturning that characterizes stratocumulus. The models ’ behavior is shown to be broadly consistent with observations from the A-Train constellation of satellites, allowing us to explore reasons for changing model sensitivity to microphysical and macrophysical cloud properties. The models are consistent with one another, and with the observations, in that they demonstrate that the sensitivity of rain rate to cloud droplet concentration Nd (which here represents microphysical influence) is greates