The Evolution of Three-Peak Raindrop Size Distributions in One-Dimensional Shaft Models. Part I: Single-Pulse Rain

Abstract

Pulsed input of raindrop packages at the top of one-dimensional shafts, followed by coalescence and breakup during the fall produces, when integrated over time of the whole rainfall, three-peak drop-size distributions (3PDs) at all levels throughout the shaft. The 3PDs are different from the three-peak equilibrium distributions (3PEDs) that develop with steady sources of rain. However, for long pulse lengths, the 3PD becomes closer to the 3PED; for short pulse lengths, the large-drop peak is not very prominent for the input of Marshall-Palmer distributions. The diameters corresponding to the maximum concentrations are approximately the same as those of the 3PED. For 3PED input, the biggest differences between the number concentrations for the 3PED and 3PDs occur for pulse lengths between 240 and 600 s while the radar reflectivity concentrations of 3PDs steadily approach that of 3PEDs with increasing pulse length. All trends are the same whether a 3PED or a Marshall-Palmer distribution is used as input. In nature, the raindrops often arrive at the ground in packets, with the largest drops followed by progressively smaller ones. This, and the presence of 3PDs at the ground, when the drop-size distributions were integrated over time, were observed in Malaysia by List et al.