The relationships between fractional entrainment rate and key microphysical quantities (e.g., liquid water content, droplet number concentration, volume mean radius, and standard deviation of cloud droplet size distributions) in shallow cumuli are empirically examined using in situ aircraft observations from the Routine Atmospheric Radiation Measurement Aerial Facility Clouds with Low Optical Water Depths Optical Radiative Observations (RACORO) field campaign over the Atmospheric Radiation Measurement Southern Great Plains site. The results show that the microphysical quantities examined generally exhibit strong relationships with entrainment rate and that the relationships collectively suggest the dominance of homogeneous entrainment mixing, which is unfavorable to the formation of large droplets and the initiation of warm rain in the clouds. The dominance of the homogeneous mixing mechanism is further substantiated by the dependency on entrainment rate of relationships among various microphysical variables and of cloud droplet size distributions. The dominance of this mechanism is also quantitatively confirmed by examining the degree of homogeneous mixing in the clouds. The dominance of homogeneous mixing may be an important reason why none of the cumulus clouds studied was drizzling.

Cloud droplet spectral relative dispersion is critical to parameterizations of cloud radiative properties, warm‐rain initiation, and aerosol‐cloud interactions in models; however, there is no consistent relationship between relative dispersion and volume‐mean radius in literature, which hinders improving relative dispersion parameterization and calls for physical explanation. Here we show, by analyzing aircraft observations of cumulus clouds during Routine AAF [Atmospheric Radiation Measurement (ARM) Aerial Facility] Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations, that the correlation between relative dispersion and volume‐mean radius changes from positive to negative as volume‐mean radius increases. With the new observation, we postulate that the sign of the correlation is determined by whether or not condensation (evaporation) occurs simultaneously with significant new activation (deactivation). The hypothesis is validated by simulations of both an adiabatic cloud parcel model and a parcel model accounting for entrainment‐mixing. A new quantity, first bin strength, is introduced to quantify this new observation. Theoretical analysis of truncated gamma and modified gamma size distributions further supports the hypothesis and reconciles the contrasting relationships between relative dispersion and volume‐mean radius, including the results in polluted fog observations. The results could shed new light on the so‐called “twilight zone” between cloudy and cloud‐free air, which in turn affects evaluation of aerosol‐cloud interactions and retrieval of aerosol optical depth.

An ongoing challenge for cloud condensation nuclei (CCN) measurements is the inclusion of unactivated particles, which affects droplet activation parameterizations and aerosol indirect effects in models. For the first time, a reciprocal relationship between the critical diameter and critical supersaturation of activated droplets from the κ‐Köhler theory is derived to accurately count unactivated particles in CCN measurements. We conducted 4‐day continuous observations to simultaneously measure the number concentration of CCN (N_CCN) and aerosol. The results show that as supersaturation (SS) decreases from 0.186% at 25 °C, the proportion of the unactivated particles in the CCN measurements increases, reaching 88% at SS of 0.07%. Owing to the N_CCN overestimation caused by N_CCN‐SS parameterizations with uncorrected N_CCN, there is significant overestimation of aerosol indirect effects, especially under low SS conditions. After removing unactivated particles, N_CCN‐SS parameterizations are adjusted accordingly, which can improve simulations of aerosol‐cloud‐radiation interactions in models.