An Investigation of Subsiding Shells Associated with Deep Moist Convection in Idealized Simulations

Downdrafts play an essential role in the feedback between deep convective clouds and their surrounding environment. One downdraft-related feature found at the edge of convective clouds is a ring of descent known as a subsiding shell. Though the characteristics of the subsiding shell are well-studied in shallow cumulus, relatively little is known about the shells that surround deep convective clouds. Using CM1, two simulations modeling scattered, single-cell convection were performed utilizing weakly-sheared dry and wet season composite soundings compiled during the Green Ocean Amazon Campaign. The dry and wet season have nearly identical boundary layers and only differ in their middle and upper tropospheric relative humidities. All updrafts in the simulations that met a required vertical velocity threshold were composited and analyzed, along with their near cloud environments. It was found that subsiding shells contributed to nearly half of the middle tropospheric net downward mass flux in both simulations, though total downward mass flux was greater in the dry simulation. Such large magnitudes of subsidence in these shells encouraged a parcel trajectory analysis, which showed that downward accelerations were primarily driven by pressure perturbations and significantly enhanced by negative buoyancy. Future work aims to establish the connection between subsiding shells and deep convective surface-reaching downdrafts using parcel trajectories. Both the current and future work aim to increase understanding of and improve mass transport processes found in cumulus parameterization.