Warm Rain Study in Hawaii—Rain Initiation

Abstract

More than 300 hours of aircraft flights were conducted in Hawaii from 1977 to 1979 to study precipitation mechanisms in warm rain. Airborne instruments were used to measure drop size distributions over the size range from cloud droplets to raindrops. The major findings are as follows: 1) Greater broadening in the distribution of cloud droplets was observed after the modal size cloud droplet reached 30 μm in diameter during upward motion. This critical size was attained quickly when large cloud droplets occurred near the cloud base. Upper level divergence above the island of Hawaii allowed the trade wind layer to moisten and produce large droplets near the cloud base. 2) Major drop growth occurs near the cloud top in close association with motion of the cloud top cell. The updraft in the cloud top cell acts to select drops which form the observed secondary maximum in the raindrop size spectrum. 3) The growth mode of raindrops during fall varies with the cloud type which, in turn, is determined largely by the vertical wind profile. Raindrop formation at the cloud top is necessary for precipitation to occur at the cloud base.