Raindrop Size-Distribution in Hawaiian Rains

A brief survey of the major techniques of raindrop size-sampling is given. The filter-paper technique, finally adopted for use in this study, adapts itself admirably to the sampling of Hawaiian orographic rains. The change in the drop-size distribution of rain as it falls from cloud to ground may be considerable. It is affected by wind shear, gravity separation, evaporation and drop collision. The evaporation error alone can be appreciable. The many small drops of the Hawaiian orographic rains may completely evaporate in a sub-cloud fall of only 1000 m. The evaporation problem was eliminated, and the others minimized, by sampling all the orographic rain at cloud base or within the cloud itself. Drop-size distributions were obtained in such non-orographic rains as thunderstorms and cyclonic storms. The pertinent meteorological parameters, such as liquid-water content, median drop diameter, and radar reflectivity, agree reasonably well with the values given by other investigators. The measurements made in orographic rains from non-freezing clouds, however, lead to considerably different values of these factors. The raindrop distributions are narrow, with the largest drops rarely exceeding 2 mm in diameter. In general, the higher the intensity, the more numerous are the drops at the large end of the spectrum. At the small end of the drop spectrum (<0.4 mm), however, increased intensity is accompanied by a decrease in the drop count. Distributions of this type indicate the absence of any chain-reaction process. Concentrations of drops less than 0.5 mm in diameter often are in excess of 40,000 m−3. These large numbers of small drops give low values for median drop diameter and radar reflectivity, but high values of liquid-water content. All of the drop distributions have been put into three categories: (1) non-orographic rain, (2) orographic rain at cloud base, and (3) orographic rain within the cloud and near cloud top. In each case, regression equations have been developed to express the meteorological parameters as a function of rain intensity.