THE MICROPHYSICAL STRUCTURE AND EVOLUTION OF HAWAIIAN RAINBAND CLOUDS- PART-II - AIRCRAFT MEASUREMENTS WITHIN RAINBANDS CONTAINING HIGH REFLECTIVITY CORES

The microphysical structure of high reflectivity cores and surrounding weaker echo regions in Hawaiian rainbands is documented using aircraf t data. These data show that high reflectivity cores are associated wi th giant raindrops (D > 4 mm) present in narrow (similar to 500 m wide ) columns coincident with the core updraft. Updrafts were found to be strong enough to suspend 1-2-mm raindrops near cloud top. As these rai ndrops subsequently fall through the updraft core, they are exposed to high liquid water content, allowing them to grow to large sizes, prov ided that updrafts are not significantly sheared. The data indicate th at size sorting due to differential terminal velocities of the larger and smaller raindrops occurs initially in the updraft. As a result, th e larger raindrops fall through an environment in which there is a low concentration of smaller raindrops, decreasing the probability of bre akup. Calculations of raindrop growth rates and breakup probabilities are used to demonstrate that high reflectivity cores in the rainbands can result from simple accretional growth of 1-2-mm raindrops falling from cloud top. In regions outside of the main updraft, drop size dist ributions were approximately exponential, with higher concentrations o f small raindrops and no giant raindrops. Consequently radar reflectiv ities and rainfall rates were lower. In these regions, collisional bre akup played a more significant role in eliminating the large size tail of the spectra.