The Microphysical Structure and Evolution of Hawaiian Rainband Clouds. Part III: A Test of the Ultragiant Nuclei Hypothesis

Abstract

A Lagrangian drop-growth trajectory model, applied within dual-Doppler-derived four-dimensional kinematic fields, is used to test the hypothesis that accretion of cloud water on giant and ultragiant cloud condensation nuclei (CCN) can explain the growth of raindrops in warm maritime convective clouds. Radar data collected within offshore rainbands during the 1990 Hawaiian Rainband Project are used to provide realistic timescales and magnitudes of convective updrafts and to capture the horizontal flow variations responsible for transporting drops into and out of these updrafts. The range of conditions under which cloud droplets can grow to large raindrops during simple up?down trajectories is determined. The model results show that accretion of cloud water on giant and ultragiant nuclei can account for the formation of rain in observed timescales. Raindrops with diameters of 1?4 mm formed for the entire range of tested conditions. Maximum drop sizes ranged from 3.5 to 8.5 mm. The general tendency in the simulations was for the largest drops to fall within regions of radar reflectivity greater than 35 dBZ, and for the smaller drops to fall in parts of the rainband with weaker reflectivity. In the control runs, which the authors believe represent natural conditions, drops as large as 5 mm formed on cloud droplets whose initial diameters were comparable to deliquesced sea salt particles observed in concentrations of ?10 to 103 m?3 near the base of Hawaiian clouds. The growth rates and trajectories of 1?5-mm raindrops in these runs agreed well with the observed evolution of the reflectivity fields. The most rapid rate of drop growth in the model occurred during a near-suspension period and early fall through the upper parts of the cloud, which is in good agreement with the sharp reflectivity gradients observed near cloud top by the radars. Despite the time and space constraints placed on the results by the evolution of the updrafts, the calculations showed that the process of rain formation in warm maritime convective clouds is simple and efficient, provided that giant and ultragiant CCN are present near cloud base. While more complex processes leading to drop spectra broadening, such as mixing, stochastic condensation, stochastic coalescence, and breakup also occur in nature, they appear to be unnecessary to explain the rapid formation of rain in these warm maritime convective clouds.