The development of cellular convection in orographic clouds is investigated through idealized numerical simulations of moist flow over topography using a cloud-resolving numerical model. For statically unstable clouds (N_m² < 0) in the presence of basic-state wind shear, profound differences are apparent between the behavior of 2D and 3D simulations with similar topographic profiles. In the 2D case convection is noticeably suppressed by the wind shear, while well-organized longitudinal convective roll circulations develop in 3D simulations with a uniform ridge and periodic boundaries in the cross-flow direction. These convective bands---which also develop in more realistic simulations with finite-length 3D ridges---substantially enhance the maximum rainfall rates, precipitation efficiencies, and precipitation accumulations in the numerical simulations.

Additional numerical simulations have been performed to determine the effects of various environmental factors on the structure of the convective orographic rainbands. Examples with directional shear of the basic-state wind indicate that the orientations of the rolls depend on both the mean wind and the wind shear in the cloud layer. Other cases are presented in which the depth of the unstable cloud, the degree of instability within the cloud, and the presence of frictional dissipation in the system are varied to examine the effects of these factors on the spacing and intensity of the convective bands.

To verify that these numerically-simulated convective rainbands correspond to real-world physical phenomena, observational data from an actual orographic precipitation event involving post-frontal flow over the coastal mountains of western Oregon will be presented. In this example, streamwise-oriented rainbands develop with similar features in both the observational data and a numerical simulation of the precipitation event.