Development of giant drops and high-reflectivity cores in Hawaiian clouds: numerical simulations using a kinematic model with detailed microphysics

Cores of high radar reflectivity (>50 dBZ) and raindrops larger than 4 mm in diameter were occasionally reported in warm clouds, offshore from Hawaii. A kinematic numerical model with detailed microphysics was used to study the formation of these cores and the development of the giant drops. The role of collisional and spontaneous breakup of drops was evaluated. Our results show that spontaneous breakup of raindrops restricts the formation of giant drops (D>4 mm). This could be a result of the poor parameterization of the fragment size distribution, and the probabilities of the spontaneous breakup. The inclusion of only binary breakup mechanism explained the observed radar echoes and the drop spectra. These results corroborate the hypothesis that the updrafts in the Hawaiian clouds sort out different size drops in such a way that millimeter size drops are allowed to fall in an environment deficient of smaller raindrops. In this way, the large raindrops continue to grow by collection of small cloud droplets, but have a smaller chance for collisional breakup (the efficiency for this type of breakup is small for collisions with cloud droplets). The collisional breakup of big raindrops was also found to play a significant role in the formation of giant drops. Such drops are formed following collision–breakup of large raindrops in which one of the fragments is larger than the original drops.