Sensitivity of Cloud Droplet Growth to Collision and Coalescence Efficiencies in a Parcel Model

Abstract

The purpose of this study is to assess the relative importance of collision and coalescence efficiencies as reported in the literature in different drop size regimes for the development of precipitation via the condensation?coalescence process. The stochastic growth of cloud droplet distributions due to collection processes is studied using a detailed microphysical parcel model. The evolution of rainwater content (LR) and the radar reflectivity factor (Z) are plotted in order to trace the progress of transfer of cloud water into rainwater and determine the importance of droplet collection in different size ranges. The results indicate that the van der Waals forces are effective in enhancing droplet collision when the droplets are small and the distributions are narrow. Wake capture is negligible for clouds forming in a continental air mass with low liquid water contents. However, it is effective when coalescence becomes the dominant growth process and rainwater content has reached high values. When nonunity coalescence efficiencies are used, the drop growth and cloud water to rainwater conversion is reduced compared to the traditional unity coalescence efficiencies used in previous modeling studies. However, the major difference between the results using nonunity and unity coalescence efficiencies is due to the extrapolation of coalescence efficiencies measured in laboratory to size domains outside the domain of the measurements.