Modification of Cumulus Entrainment Due to Neighboring Clouds

Entrainment has been investigated extensively since the seminal paper by Henry Stommel in 1947. The major foci of previous studies were to accurately determine entrainment rates for convective clouds, ascertain the origin of entrained air, and establish a conceptual model to which the behavior of cumulus convection adheres. Relatively uninvestigated topics are the effects of interacting air circulations from neighboring clouds. In favorable conditions, such as convection over a convergence line, a cumulus cloud may spend a large portion of its lifetime in close vicinity to a neighboring cloud. In this study, we utilize the Straka Atmospheric Model to explore how the airflow circulations of neighboring cumuli mutually alter entrainment and the microphysical properties of each cloud.

In these idealized simulations, several cumulus clouds are initiated in a line along a unidirectional wind shear. The distance between cloud centers, the cloud sizes, the wind shear magnitude, and the time offset among individual cumuli are investigated. Adjacent clouds are analyzed to understand how airflow patterns are mutually perturbed, as well as the microphysics leading to precipitation. Each cumulus is also compared to a control simulation of a single isolated cloud to analyze alterations to the sizes and locations of entraining eddies. Quantitative comparisons of entrainment rates are performed using a visual display of the tetrahedron-interpolation interface-tracking scheme of Dawe and Austin (2011).