The spacing of orographic rainbands triggered by small-scale topography

Recent observations indicate that quasi-stationary orographic rainbands, which form in response to conditionally unstable flow over rugged topography, may add strong variability to precipitation distributions over complex terrain. While some progress has been made in understanding the dynamics behind the triggering of these bands, the spacing between adjacent bands is an unresolved problem demanding deeper investigation. We examine this issue through a hierarchy of models of increasing simplicity. Idealized simulations of moist flow over a mesoscale barrier with variable small-scale topographic forcing suggest that the banded convection is the strongest over a narrow range of horizontal scales. This range of preferred scales, which matches the inter-band spacings observed in reality, is explained by an inviscid linear model that describes the transition from stable, topographically generated lee waves to unstable moist convection inside an orographic cap cloud.