Convective clouds exchange ambient dry air through entrainment, a process that directly alters the cloud droplet size distribution by changing local humidity, temperature, and liquid water content. Observational syntheses and theoretical work show that entrainment reduces the supersaturation available for condensational growth and introduces mixing processes that reshape both the mean droplet size and the spectrum of sizes. Kerry Emanuel at Massachusetts Institute of Technology emphasizes that entrainment dilutes buoyancy and lowers peak supersaturation, limiting condensational growth rates and thus reducing mean droplet sizes in rising plumes. Peter V. Hobbs at University of Washington documented aircraft observations showing patchy evaporation and heterogeneous mixing in convective clouds, linking entrainment to measurable changes in microphysical structure.
Mechanisms
Two physical pathways control how entrainment changes droplet populations. One is the immediate reduction of supersaturation when dry air mixes with cloud air, which slows or halts condensational growth. The other is the character of the mixing itself: homogeneous mixing tends to shrink droplets more uniformly because vapor is redistributed quickly relative to evaporation timescales, while inhomogeneous mixing produces pockets where some droplets evaporate completely and others remain relatively untouched, producing more complex changes to spectral width. The net effect depends on the mixing timescale relative to droplet evaporation timescales and on droplet concentration set by aerosol loading.
Relevance and Consequences
Changes in droplet size distribution driven by entrainment affect cloud radiative properties and precipitation formation. Smaller mean droplet sizes increase cloud albedo, enhancing sunlight reflection and altering local surface energy balances, while broader or bimodal spectra can either suppress or accelerate collision-coalescence depending on details of the spectrum and turbulence. These microphysical shifts influence regional rainfall efficiency and can feed back onto land and coastal communities that depend on convective precipitation. Cultural and territorial factors matter because aerosol sources differ between maritime regions and industrialized continental areas, making clouds over some regions more susceptible to entrainment-driven microphysical change due to higher droplet concentrations from human emissions.
Representation of entrainment remains a major uncertainty in weather and climate models, as noted in theoretical reviews by established researchers, because small-scale mixing controls large-scale impacts. Accurate treatment of entrainment and its mixing regime is therefore central to predicting cloud lifetimes, radiative forcing, and precipitation in a changing environment.