Observed and Simulated Variability of Droplet Spectral Dispersion in Convective Clouds Over the Amazon

In this study, the variability of the spectral dispersion of droplet size distributions (DSDs) in convective clouds is investigated. Analyses are based on aircraft measurements of growing cumuli near the Amazon basin, and on numerical simulations of an idealized ice-free cumulus. In cleaner clouds, the relative dispersion (Formula presented.), defined as the ratio of the standard deviation to the mean value of the droplet diameter, is negatively correlated with the ratio of the cloud water content ((Formula presented.)) to the adiabatic liquid water content ((Formula presented.)), while no strong correlation between (Formula presented.) and (Formula presented.) is seen in polluted clouds. Bin microphysics numerical simulations suggest that these contrasting behaviors are associated with the effect of collision-coalescence in cleaner clouds, and secondary droplet activation in polluted clouds, in addition to the turbulent mixing of parcels that experienced different paths within the cloud. Collision-coalescence simultaneously broadens the DSDs and decreases (Formula presented.), explaining the inverse relationship between (Formula presented.) and (Formula presented.) in cleaner clouds. Secondary droplet activation broadens the DSDs but has little direct impact on (Formula presented.). The combination of a rather modest DSD broadening due to weak collision-coalescence with enhanced droplet activation in both diluted and highly undiluted cloud regions may contribute to maintain a relatively uniform (Formula presented.) within polluted clouds. These findings can be useful for parameterizing the shape parameter ((Formula presented.)) of gamma DSDs in bulk microphysics cloud-resolving models. It is shown that emulating the observed (Formula presented.) relationship improves the estimation of the collision-coalescence rate in bulk microphysics simulations compared to the bin simulations.