Long-Lasting Trade-Wind Rain Showers in a Three-Dimensional Model

Abstract

A three-dimensional warm rain model that included microphysics was used to study the reasons for ease of rainfall in Hawaiian clouds and the long-lasting rainfall from certain rainbands. It was found that drop recirculation occurs within these clouds to enhance the raindrop growth rate, and that the locations of recirculation differ with cloud types, which are determined by wind shear profiles. As has been noted in observations, rain lasts longest when the wind is parabolic with height and a strong wind blows at the middle of the trade-wind layer. In the model, a very long-lasting rainfall is calculated when the wind veers greatly below the cloud base. In Hawaiian warm clouds, dry air acts to dissipate clouds rather than to enhance them as occurs in other types of squall lines. In Hawaii, the cloud system that produces long-lasting rainfall is one where dry air does not intrude, and the drag force by raindrops creates a low-level convergence without blocking the low-level inflow of moist air. Kessler's parameterizations on cloud development and rainfall have also been examined as a part of this study.