Relationships between Entrainment, Microphysics, Dynamics and Thermodynamics in Cumulus Clouds

Turbulent entrainment-mixing processes between cumulus clouds and their environments are key processes in atmospheric moist convection. Entrainment rate in growing cumulus clouds is estimated using a recently developed mixing fraction approach that is applied to in situ aircraft measurements of cumulus clouds from the RACORO field program. Examined are the relationships of entrainment rate with key microphysical quantities, vertical velocity, dissipation rate in shallow cumuli, and relative humidity in entrained dry air. First, the relationships between entrainment rate and microphysical quantities collectively suggest the dominance of homogeneous entrainment mixing (Figure 1), which is unfavorable to the formation of large droplets and the initiation of warm rain in these clouds. Second, entrainment rate is negatively correlated with vertical velocity and dissipation rate. Third, the calculated entrainment rate depends on the source of dry air that is assumed to be entrained into clouds. Entrainment rate increases with an increase in relative humidity of the dry air. These combined results highlight the essential role of entrainment in linking cloud dynamics, microphysics and thermodynamics.