A two-column cumulus ensemble model of the tropical troposphere is constructed to simulate the interaction between tropical convection and the large-scale vertical circulation. The convection in each column is represented explicitly by a three-dimensional cloud-resolving model. The two columns interact so as to maintain buoyancy equilibrium between them, resulting in a ``large-scale'' circulation.

Numerical experiments starting from radiative-convective equilibrium in each column show that when one column is perturbed slightly from its initial state, a large-scale vertical circulation between the two columns develops. The column with large-scale upward motion has higher mid-tropospheric moist entropy and stronger precipitation. This result supports the instability discovered by Raymond and Zeng (2000) in which a cumulus parameterization was used in place of the cumulus ensemble model.