Microstructures and precipitation development in cumulus and small cumulonimbus clouds over the warm pool of the tropical Pacific Ocean

In situ airborne measurements obtained in convective clouds in the vicinity of the Marshall Islands on 15 days in July and August 1999 are used to determine the microphysical structures and precipitation‐producing mechanisms in these clouds. The liquid water contents of the clouds were generally well below adiabatic values, even in newly risen turrets. This is attributed to the entrainment of ambient air and to the very efficient removal of cloud water by the collision and coalescence of drops. The formation of raindrops began when the concentration of droplets with radii <15 µm exceeded ∼3 cm−3 or, equivalently, when the effective cloud drop radius reached ∼ 13 µm. Clouds rained when their depth exceeded ∼1.5 km. Due to rainout, the effective radius of cloud droplets began decreasing ∼2–4 km above cloud base.