Entrainment, Detrainment, and Dilution of Dry and Moist Atmospheric Thermals

Abstract This study examines the entrainment, detrainment, and dilution of dry and moist (cloud) atmospheric thermals in large-eddy simulations. In a neutrally stable environment (with respect to dry dynamics), moist thermals have an increase in radius R with thermal height z t ( α ≡ dR / dz t ) about 4 times smaller compared to dry thermals when density stratification is considered and ∼2.4 times smaller without density stratification (i.e., applying the Boussinesq approximation). An analytic expression relating α to several dimensionless parameters is derived from the thermal impulse–circulation relation to clarify the factors impacting α . This expression shows that the difference in buoyancy structure between moist and dry thermals, with buoyancy concentrated in the central cores of moist thermals owing to latent heating, explains their smaller spreading rates. Individual contributions of entrainment and detrainment are analyzed using a direct parcel-based approach in the simulations. Moist thermals have similar fractional detrainment but much smaller fractional entrainment rates compared to dry thermals, consistent with the differences in α . Despite having smaller α , moist thermals are similarly dilute (quantified by a passive tracer) as dry thermals because of their greater mixing efficiency with the environment. Thus, moist thermals are substantially dilute but expand much less in size/volume as they rise compared to dry thermals. The α values for moist thermals in (dry) neutral and statically stable environments are similar, but fractional entrainment and especially detrainment rates are greater in the stable environment. Large detrainment rates are associated with a breakdown of the broader thermal vortex ring structure, especially with low environmental relative humidity, attributed in part to evaporation and buoyancy reversal. Significance Statement Thermals—blobs of rising buoyant air—are nearly ubiquitous in cumulus clouds. However, the mechanisms by which cloud thermals mix with surrounding air and spread outward as they incorporate (entrain) this air are uncertain. Many past studies have examined dry thermals as a proxy for understanding cloud thermals. Using a numerical model, the current study shows that cloud thermals spread roughly 2–4 times less than dry thermals but have a higher mixing efficiency. As a result, cloud thermals are similarly diluted by environmental air as dry thermals despite having a smaller spreading rate. This is important because dilution generally reduces the buoyancy of cloud thermals, helping to determine how fast and how far clouds as a whole rise.