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 aircraft data. These data show that high reflectivity cores are associated with giant raindrops (D > 4 mm) present in narrow (∼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 raindrops subsequently fall through the updraft core, they are exposed to high liquid water content, allowing them to grow to large sizes, provided that updrafts are not significantly sheared. The data indicate that size sorting due to differential terminal velocities of the larger and smaller raindrops occurs initially in the updraft. As a result, the larger raindrops fall through an environment in which there is a low concentration of smaller raindrops, decreasing the probability of breakup. 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 distributions were approximately exponential, with higher concentrations of small raindrops and no giant raindrops. Consequently radar reflectivities and rainfall rates were lower. In these regions, collisional breakup played a more significant role in eliminating the large size tail of the spectra.