Between Broadening and Narrowing: How Mixing Affects the Width of the Droplet Size Distribution

Entrainment and mixing play an essential role in shaping the droplet size distribution (DSD), with commensurate effects on cloud radiative properties or precipitation formation. In this paper, we use a model that considers all relevant scales related to entrainment and mixing by employing the linear eddy model (LEM) as a subgrid‐scale (SGS) mixing model, coupled with a large‐eddy simulation model and a Lagrangian cloud model (LCM) for a single cumulus congestus cloud. We confirm that the DSD is broadened toward small‐size droplets during homogeneous mixing. During inhomogeneous mixing, the DSD width remains almost unchanged. The DSD width can also be narrowed after mixing. We show that this happens when DSD is broadened toward small‐size droplets, which evaporate rapidly, while larger droplets are almost unaffected. In addition, when droplets ascend during mixing, DSD narrowing is caused when the adiabatic increase in supersaturation is slower than the average droplet evaporation, allowing only the largest droplets to benefit from the newly produced supersaturation. The narrowing mixing scenario prevents clouds from having too broad DSDs and causes the DSD relative dispersion to converge around 0.2 to 0.4. As this scenario is more frequent when the LEM SGS model is used, our results indicate that adequately modeling turbulent mixing is necessary to represent a realistic DSD shape. Plain Language Summary Clouds are always in contact with the surrounding air. Because the air outside the cloud is drier than the cloud, cloud droplets tend to evaporate when it enters the cloud. The size of the cloud droplets after evaporation can vary depending on the timescales of turbulent mixing and droplet evaporation. If the dry air mixes quickly, all droplets evaporate simultaneously. If the dry air is mixed slowly, only the droplets exposed to the dry air evaporate. However, this mixing occurs on small scales that traditional cloud models cannot account for. To account for this, we use a special model capable of representing all relevant scales. We confirm previous theoretical work that when mixing is fast, all droplets evaporate and the mean droplet size decreases. When mixing is slow, some droplets evaporate completely, but the average droplet size remains constant. We also observe cases where only small droplets evaporate while large droplets barely change. This scenario happens when there are many small droplets to evaporate or when additional moisture from cloud mot