Droplet spectral dispersion by lateral mixing process in continental deep cumulus clouds

Abstract

In situ flight observations and theoretical analysis are used to investigate the dispersion properties of droplet size distribution (DSD) in continental deep cumulus clouds under the influence of lateral entrainment and mixing processes. The study attempts to relate the spectral width (σr) and relative dispersion (ν) of DSD with liquid water content (LWC), droplet number concentration (Nd) and mean radius (rm), which decrease from their adiabatic values due to dry air mixing.The diffusional-evaporation theory suggests that DSD spectra may broaden during the mixing as smaller droplets evaporate faster than the larger droplets under a constant saturation ratio (S < 1). However, in observations, three regimes of adiabatic fraction (AF = LWC/LWCa; subscript ‘a’ denotes adiabatic value) have been identified with different characteristics of spectral dispersion by lateral entrainment and subsequent mixing processes. Broadening of size spectra, i.e., increasing σr is seen in the highly diluted cloud edge regime (AF < 0.25) where droplet evaporation is significant. A narrowing of the size spectra is observed in the moderately diluted regime (0.65 >AF > 0.25). Whereas, in the less diluted regime (AF > 0.65), a slight spectral broadening towards the smaller size tail is noted, possibly due to reactivation of condensation nuclei. Relative dispersion (ν=σr/rm) is almost invariant (≅0.22) in the less diluted and moderately diluted regimes but increased (≅ 0.35) rapidly with decreased Nd (and LWC) in the highly diluted regime that forms the outer shell of cumulus clouds.