Numerical Simulation of Tropical Cumulus Clouds and Their Interaction with the Subcloud Layer

Abstract

A two-dimensional numerical model suitable for simulating an ensemble of cumulus clouds has been developed. It differs from similar models in its greater emphasis on turbulent processes in the boundary layer and in clouds. In the model, cloud-scale dynamics are coupled with a third-moment turbulence closure. A turbulence-scale condensation scheme is used to parameterize the cloud water mixing ratio. We studied the response of tropical cumulus clouds to imposed large-scale vertical advection by doing two simulations: one with upward large-scale vertical velocity (the ?disturbed? case) and one without any (the ?undisturbed? case) but otherwise identical. Deep cumulus clouds formed in the disturbed case, while only shallow clouds formed in the undisturbed case. Time-averaged and horizontally averaged heat and moisture budgets from the simulations show that the subcloud layer (SCL) in the disturbed case was strongly affected by cumulus circulations and rain evaporation. Both cumulus updrafts and downdrafts were important contributors to the SCL sensible and latent heat fluxes. Seventy percent of the fluxes were due to the strongest drafts, which covered only 24% of the area at 500 m. The strongest downdrafts formed in rainshafts and carried air from the updrafts and from the environment at midlevels (1?4 km) into the SCL. These downdrafts were driven by rain water loading above the SCL; only within the SCL were they ever negatively buoyant. Such downdraft created gust fronts and cool outflow regions. In the cloud-free regions, the SCL warmed and dried due to compensating subsidence, which occurred despite the large-scale upward motion. Cumulus-scale circulations in the disturbed SCL determined where new clouds formed by creating convergence zones and moisture anomalies. These circulations also strongly affected the surface fluxes of sensible and latent heat.