Updraft Width Implications for Cumulonimbus Growth in a Moist Marine Environment

Abstract

An idealized large eddy simulation of a tropical marine cloud population was performed. At any time, it contained hundreds of clouds, and updraft width in shallow convection emerging from a sub-cloud layer appeared to be an important indicator of whether specific convective elements deepened. In an environment with 80–90% relative humidity below the 0°C level, updrafts that penetrated the 0°C level were larger at and above cloud base, which occurred at the lifting condensation level near 600 m. Parcels rising in these updrafts appeared to emerge from boundary layer eddies that averaged ∼200 m wider than those in clouds that only reached 1.5–3 km height. The deeply ascending parcels (growers) possessed statistically similar values of effective buoyancy below the level of free convection (LFC) as parcels that began to ascend in a cloud but stopped before reaching 3000 m (non-growers). The growers also experienced less dilution above the LFC. Non-growers were characterized by negative effective buoyancy and rapid deceleration above the LFC, while growers continued to accelerate well above the LFC. Growers occurred in areas with greater magnitude of background convergence (or weaker divergence) in the sub-cloud layer, especially between 300 m and cloud base, but whether the convergence actually led to eddy widening is unclear.